J. Anim. Sci. 2003. 81:217-231
© 2003 American Society of Animal Science
Effect of ambient temperature on mammary gland metabolism in lactating sows1,2
D. Renaudeau3,
J. Noblet and
J. Y. Dourmad4
Institut National de la Recherche Agronomique, 35590 Saint-Gilles, France
4 Correspondence:
Institut National de la Recherche Agronomique, UMRVP, 35590 Saint-Gilles, France (phone: 33-223-48-5000; fax: 33-223-48-5080; E-mail:
dourmad{at}st-gilles.rennes.inra.fr).
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Abstract
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Two groups of three multiparous Large White x Landrace sows were used to investigate the direct effect of ambient temperature on mammary gland metabolism. Sows from the first group were exposed to temperatures of 28°C between d 8 and 14 of lactation, and 20°C between d 15 and 21; treatments were reversed in the second group. Four to six d after farrowing, an ultrasonic blood flow probe was implanted around the right external pudic artery and catheters were fitted in the right anterior mammary vein and in the carotid artery. After surgery all sows were fed 3.8 kg/d of a lactation diet. The arteriovenous (AV, mg/L) plasma samples were obtained every 30 min between 0915 and 1545 on d 5 of exposure to ambient temperature; the same day, milk samples were collected at 1630. Additional arterial samples were obtained between 1000 and 1100 on d 1, 4, and 6 of exposure. Milk yield was estimated from the body weight gain of the litter. Elevated temperature tended to reduce BW loss (2.44 vs 1.82 kg/d, P < 0.10), but did not affect milk yield (11.0 kg/d). Glucagon and leptin arterial concentrations increased (12 and 8%, respectively; P < 0.10), but thyroxin and triiodothyronine concentrations decreased (26 and 16%, respectively; P < 0.01) between 20 and 28°C. Expressed as a percentage of total nutrients, A–V difference, glucose, amino acids, triglycerides (TG), free fatty acids, and lactate A–V differences represented 60, 20, 11, 8, and 1%, respectively. Exposure to 28°C increased the extraction rate of glucose, TG, and 
-amino acid N (13, 8, and 2.5%, respectively; P < 0.10). The extraction rates of essential and nonessential amino acids were not affected by temperature. The right pudic artery mammary blood flow increased (872 vs 945 mL/min, P < 0.05) between 20 and 28°C, whereas milk yield was unaffected by temperature. It is suggested that this apparent inefficiency of the sow mammary gland in hot conditions could be related to an increase of proportion of blood flow irrigating skin capillaries in order to dissipate body heat.
Key Words: Mammary Glands • Milk Composition • Nutrient Uptake • Sows • Temperature
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Introduction
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In highly prolific lactating sows, 75% of total energy intake and 90% of total amino acid intake are used for milk production (Dourmad et al., 2000). Few studies have been conducted on lactating sows to quantify the uptake of milk precursors from the blood by the mammary gland. This requires a simultaneous determination of the arteriovenous difference in concentrations of precursors and the measurement of mammary blood flow. By applying the Fick principle to lysine used as a marker, Trottier et al. (1997) and Dourmad et al. (2000) indirectly estimated mammary blood flow. Renaudeau et al. (2002) developed a technique for the direct measurement of mammary blood flow in sows using a transit time ultrasound flow probe. This study found that milking, postural change, and meal distribution contribute to variations of blood flow.
In hot conditions, feed intake and, to a smaller extent, milk production, decline in order to avoid an excessive increase of body temperature (Quiniou and Noblet, 1999; Renaudeau et al., 2001). The reduction of milk yield in hot conditions could be related to the decrease of voluntary feed intake and the associated reduction in nutrient availability for milk synthesis. But results of Mullan et al. (1992) and Messias de Bragança et al. (1998) suggest a direct effect of ambient temperature on milk yield. The impaired ability of lactating sows to mobilize their body reserves and to redistribute blood flow to the skin in order to increase heat loss, and/or altered endocrine function were suggested to explain the reduced nutrient supply to the mammary gland of multiparous lactating sows kept in hot conditions (Messias de Bragança et al., 1998).
The objectives of our study were 1) to measure plasma arteriovenous differences of milk precursors, 2) to quantify nutrient uptake by the mammary gland, and 3) to investigate the direct effect of elevated temperature on mammary gland metabolism.
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Materials and Methods
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Experimental Design.
Two groups of three multiparous crossbred Large White x Landrace sows were used in the experiment in a cross-over design (Cochran and Cox, 1962). Within each group, two sows were surgically operated on d 4 and one on d 5 of lactation (d 0 = day of farrowing). After surgery, sows from the first group were exposed to temperatures of 28°C between d 8 and 14, and 20°C between d 15 and 21. Sows from the second group were exposed to the same temperatures, but in reverse order (Figure 1
). The temperature gradually changed on d 7 from 24 to 28°C or 24 to 20°C within 6 h, and on d 14 changed from 28 to 20°C or from 20 to 28°C within 12 h. Between farrowing and surgery, daily feed allowance increased progressively by 1 kg/d to a maximum of 3.8 kg/d. In order to evaluate the effect of ambient temperature independent of its effect on feed intake, all sows were fed 3.8 kg/d of a lactation diet in which the level of digestible essential AA (relative to lysine) and the mineral and vitamin contents met or exceeded the recommendations of NRC (1998). The composition, chemical characteristics, and nutritional values of the diet are given in Table 1. Sows when exposed to 28°C were considered close to their ad libitum intake, whereas when exposed to 20°C, they were severely restricted (Quiniou and Noblet, 1999). The diet was offered as dry pellets. An automatic feed dispenser distributed approximately 420 g of feed every 2 h between 0600 and 2200, and 210 g every 4 h between 2200 to 0600, in order to mimic the diurnal feeding behavior of lactating sows described by Quiniou et al. (2000).
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Figure 1. Experimental design and blood sampling procedures. Punctual samples: one arterial sample per sow was obtained between 1000 and 1100. Arterial and venous (AV) samples: AV samples were obtained every 30 min between 0915 and 1545. Dotted line: group 1; continuous line: group 2.
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Animal Management.
At d 110 of pregnancy, sows were moved to farrowing crates (2.4 x 2.0 m) with slatted floors, and the ambient temperature was fixed at 24°C. A detailed description of animal management has been previously reported by Renaudeau et al. (2001). Sows had free access to water from a low-pressure nipple connected to a 55-L graduated water bank. Within the first 12 h following birth, piglets were injected with 200 mg of iron dextran and tattooed. Litter size was standardized to 12 piglets by cross fostering within 48 h after farrowing. A heating zone was provided to the piglets using an infrared lamp and a heating mat. Piglets were weaned at 22 d of age and sows were slaughtered 25 d after farrowing.
Surgical Procedure.
After a 12-h fast, sows were transferred from the farrowing unit to the surgery room, while the rest of their litter remained in the farrowing crate. The transit time ultrasonic blood flow probe and the catheters were sterilized with ethylene oxide before surgery. The 8-mm blood flow probe (Transonic Systems Inc., Ithaca, NY) was placed around the external pudic artery on the right side (Renaudeau et al., 2001). For the arterial cannulation, an indwelling Tygon catheter (Tygon Tubing, Cole Parmer Inst. Co., Vernon Hills, Illinois; 2.29 mm o.d., 1.27 mm i.d.) was inserted through the carotid artery at a distance of 40 cm. The artery was ligatured and the catheter was maintained with a suture between the cuff and the surrounding tissues. The mammary vein cannulation was performed according to Trottier et al. (1995). A 6-cm incision was made between the first and the second gland on the right side and a Tygon catheter (1.78 mm o.d, 1.02 mm i.d.) was implanted at a distance of 18 cm in the anterior mammary vein via a small branch irrigating the skin.
The cable of the probe and the catheters were tunneled under the skin and externalized on the back (10 cm behind the lumbar vertebrae) and the dorsum of the neck, respectively. Apart from measurement periods, cables of flow probe and catheters were stored in a strengthened purse sewn on the skin of the sows. The duration of surgery never exceeded 4 h. Postsurgical monitoring followed the same protocol as described previously (Renaudeau et al., 2002). The catheters were flushed every day with 10 mL of normal saline solution (154 mM NaCl) containing 50 IU/mL of heparin and 5 mg/mL of di-hydroxistreptomycine (Intervet, Angers, France).
Measurements.
The same sampling protocol was used at each sampling time. Before the blood sample was taken, 5 mL of blood was taken and thrown away in order to eliminate dilution from the heparin block, and then 10 mL of blood was collected in heparinized syringes in the artery and the vein simultaneously and subsequently transferred to heparinized tubes. In addition, 2 mL of blood was collected in heparinized syringes for blood gases analyses. Finally, 5 mL of saline solution with heparin (20 IU/mL) was injected to prevent blood clot development in the cannula.
The arterial and venous (AV) samples (see Figure 1
) were obtained every 30 min between 0915 and 1545 on d 12 and 19, corresponding to d 5 of exposure to ambient temperature. That delay was chosen from the results of Quiniou and Noblet (1999), who showed that lactating sows were adapted to ambient temperature within 5 d of exposure. A total of 14 arterial and 14 venous samples were obtained per sow daily. Immediately after sampling, blood packed-cell volume and blood gases were determined and the remaining blood was centrifuged for 3 min at 8,500 x g at 4°C. The supernatant fluid was divided into subsamples and stored at -20°C for further analysis. Additional arterial blood samples ("punctual samples," Figure 1) were collected between 1000 and 1100 on d 1, 4, and 6 of exposure to ambient temperature in order to determine plasma concentrations of cortisol, insulin, glucagon, thyroxin (T3), triiodothyronine (T4), and leptin.
The measurement of mammary blood flow in the right pudic artery (PMBF) started 1 d after surgery in order to test the probe. Thereafter, PMBF was measured from 0830 to 1700 on d 4, 5, and 6 of exposure to ambient temperature. Data were averaged for each 1-min period and stored in a computer.
Live weight and backfat thickness at the last-rib level and at 65 mm from the middle line (P2 site), were determined before farrowing and on d 8, 15, and at weaning (d 22). Sows were slaughtered on d 25, and the location of the flow probe and the catheters were checked. Piglets were individually weighed at birth and on d 8, 15, and 22. On d 12 and 19 at approximately 1600 (after AV samples), piglets were separated from the dam after suckling. Forty minutes later, sows were injected 20 IU of oxytocin (Intervet) in the arterial catheter and hand milked. A total of 300 to 400 mL of milk was collected and immediately stored at -20°C for further analysis to determine milk composition. Rectal temperature was recorded after each AV sample on d 5 and once after punctual samples on d 1, 4, and 6 of exposure to experimental ambient temperature using a digital thermometer. Skin temperature (at the P2 position) was measured only after punctual samples with a type K probe. Time spent standing or sitting was continuously recorded using an infrared barrier.
Chemical Analyses.
Glucose, insulin, lactate, triglycerides (TG), urea, calcium, and phosphorus were measured on all plasma samples. Concentrations of FFA, glycerol, and -amino acid N in plasma, and pH, O2, and CO2 contents in blood were determined every hour from 0945 to 1545. Plasma samples were pooled by day to measure arterial and venous AA concentrations. Oxygen concentration was measured on a Ciba Corning 270 co-oxymeter (Ciba Corning Diagnostics, Cergy Pontoise, France). Carbon dioxide concentration and pH were analyzed simultaneously using a Ciba Corning 768 blood gases system, which allows correction for rectal temperature. Plasma glucose, FFA, lactate, TG, glycerol, urea, -amino acids, calcium and phosphorus were analyzed using enzymatic methods adapted to a multianalyzer Cobas Mira apparatus (Roche, Basel, Switzerland). The concentration of thyroid hormones was determined using commercial kits (ICN, Costa Mesa, CA). Insulin, glucagon, and leptin concentrations were measured using RIA commercial kits (GIS Bio Int., Gif sur Yvette, France; Pharmacia, St. Quentin, France; Linco Research Inc., St. Louis, MO, respectively). Plasma cortisol concentration was quantified with an immunoassay, as previously described by Meunier-Salaün et al. (1991).
The AA contents were determined by ion-exchange liquid chromatography (Biochrom 20, Pharmacia, Saclay, France) after a 24-h hydrolysis in HCl (6 mol/L). For sulfur AA, the hydrolysis was performed by a performic oxidation. Tryptophan was hydrolyzed only for feed and milk in barium hydroxide solution (1.5 mol/L) for 20 h, separated by HPLC, and detected fluorimetrically (Waters 600E, St. Quentin en Yvelines, France). For the determination of free AA contents in plasma, the diluted samples (1:10) were analyzed by chromatography on a cation-exchange resin column (Beckman 6300 analyzer, Global Medical Instrumentation Inc., Albertville, MN).
Feed and milk were analyzed for moisture, ash, N, and fat according to AOAC (1990) methods. Feed was analyzed for crude fiber according to Van Soest et al. (1991). Gross energy in feed and milk was measured using an adiabatic bomb calorimeter. Lactose content in milk was determined using an enzymatic method (Boehringer Mannheim, reference No. 176303).
Calculations and Statistical Analysis.
The PMBF data were expressed in milliliters of blood per minute and averaged per sow and per day of exposure. Milk production was estimated between d 8 and 15, and between d 15 and 22 from BW gain of piglets and litter size (Noblet and Etienne, 1989). Time spent standing or sitting was expressed in minutes per hour and averaged per sow and per day of exposure. The extraction rates of nutrients were calculated as the ratio between A–V difference and arterial concentration. The daily total mammary blood flow rate was estimated for each temperature assuming that measured PMBF corresponded to one quarter of the whole mammary gland. Daily nutrient uptake by the mammary gland (g/d) was estimated from A–V difference concentration (g/L) measured on d 5 of exposure to ambient temperature and total plasma flow (L/d) measured on the same day. Similarly, daily nutrients output in milk (g/d) was calculated from milk concentration (g/kg) and milk yield (kg/d).
Sows performance and milk composition data were submitted to an ANOVA (PROC GLM; SAS Inst., Inc., Cary, NC) for a crossover design (Cochran and Cox, 1962) including the effects of ambient temperature (n = 2), period (n = 2; d 8 to 14, and d 15 to 21 with d 0 = farrowing day), and sow number (n = 6). Body temperatures, hormones concentrations, and PMBF were submitted to an ANOVA (PROC GLM of SAS) including the effects of ambient temperature, period, animal, and duration of exposure (n = 3; d 1, 4, and 6, with d 0 = set up of experimental temperature). Arterial concentrations, A–V differences, and extraction rates of nutrients were analyzed with an ANOVA for repeated measurements (PROC GLM of SAS) including the effect of ambient temperature, period, animal, and sampling time. A t-test procedure (PROC FREQ of SAS) was performed for testing the hypothesis that A–V differences, daily nutrients uptake, output, and retention were significantly different from zero. Linear or nonlinear regressions (SAS Inst. Inc., Cary, NC) between A–V differences or extraction rates and arterial concentrations were calculated.
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Results
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Sow and Litter Performance.
Sow and litter performance data are presented in Table 2. Average sow parity number was 3.5, and BW and backfat thickness after farrowing averaged 282 ± 14 kg and 19.4 ± 4.7 mm, respectively. There was no refusal of feed, and daily feed intake was 3.8 kg/d between d 8 and 21. Sows kept at 20°C tended to lose more BW (P < 0.10) than at 28°C (2,440 vs 1,822 g/d, respectively). However, backfat thickness loss was similar for both treatments (0.25 mm/d on average). Daily water consumption increased (8 L/d, P < 0.01) when temperature rose from 20 to 28°C. Mean litter size, growth rate, and milk production were not different (P > 0.10) at the two ambient temperatures. Rectal and skin temperatures were higher (P < 0.001) at 28°C than at 20°C (0.5 and 3.3°C, respectively). At both temperatures, rectal temperature tended to be higher (P = 0.08) on d 1 than on d 4 or 5 of exposure (38.8 vs 38.5°C). Time spent in standing or sitting positions decreased (P < 0.01) at 28°C (11.6 vs 18.0 min/h at 20°C).
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Table 2. Effect of ambient temperature on performance, physiological responses, and mammary blood flow in lactating sows (means)a
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Milk Composition.
Milk composition was not affected by temperature for most of the measurements evaluated (Table 3). On average, protein, fat, lactose, and gross energy contents were 5.3, 5.8, and 5.6 g/100 g of milk and 4.43 kJ/g of milk, respectively. The ash content averaged 0.82 g/100g of milk; calcium and phosphorus contents reached 32 and 19% of total ash concentration, respectively. Milk cystine tended to be higher at 28°C, whereas all other AA were not affected by ambient temperature. Glutamate was the major AA secreted in milk; it represented approximately 20% of the total AA found in milk. The corresponding value for lysine was 7%.
Hormone Concentrations.
Cortisol, glucagon, and insulin concentrations in plasma were not affected (P > 0.10) by ambient temperature (Table 2), whereas leptin concentration was higher (+8%, P < 0.05;) and T3 and T4 concentrations were lower at 28°C (P < 0.01; 26 and 16% less, respectively). For these latter sows, leptin concentration tended to be higher (P < 0.10) on d 1 compared to d 4 and 5 of exposure (2.05 vs 1.75 mg/L, respectively).
Mammary Blood Flow.
The implantation of the flow probe was successful in all six sows. Dissection of the mammary gland on d 25 did not indicate any local infection, bypassing of the flow probe via collateral vessels, or reduction in the artery diameter. The PMBF was higher (P < 0.05) when sows were kept at 28°C than at 20°C (945 vs 872 mL/min, respectively). At both temperatures, it decreased (P < 0.01) from d 4 to 5 and d 6 of exposure to the ambient temperature (Table 2).
Arterial Concentrations, Arteriovenous Differences, Extraction Rates.
Means of the arterial concentrations, A–V differences, and extraction rates for nutrient in plasma are presented in Tables 4 to 6. In the first group at 20°C, a venous catheter was occluded for one sow. Consequently, some mean values presented in Tables 4 and 5 (FFA, glycerol, -amino acid N, pH, O2, and CO2) were calculated from 83 samples, instead of 84 samples. According to the method used for measuring AA plasma concentrations, we could not differentiate the peaks of serine, glutamine, and asparagine in the chromatogram. All nonessential AA were then pooled in a single criterion (Table 6). Hematocrit was not affected by temperature and averaged 28.5%.
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Table 4. Effect of ambient temperature on blood arterial concentrations (A, mg/L), arteriovenous differences (A–V, mg/L) of pH and mammary extraction rate (E, %) of hematocrit, O2, and CO2a
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Table 6. Effect of ambient temperature on arterial concentration (A, mg/L), arteriovenous difference (A–V, mg/L) and mammary extraction ratea (E, %) of aminoacids
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Table 5. Effect of ambient temperature on plasma arterial concentrations A, mg/L), arteriovenous differences (A–V, mg/L) and mammary extraction ratea (E, %) of nutrients
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The A–V differences were different from 0 (P < 0.05) for all nutrients. Those of insulin and hematocrit did not differ from 0. Even though plasma arterial concentration of glucose was not affected by ambient temperature (P > 0.10; 1014 mg/L on average), the A–V difference and the extraction rate of glucose were higher (P < 0.01) at 28°C than at 20°C (+11 and 13%, respectively). On average, the A–V difference and extraction rate of TG tended to be higher (P < 0.10) at 28°C than at 20°C (+6 g/L and 3.8%, respectively). The increase of ambient temperature with the same level of feed allowance did not affect the O2 blood arterial concentration and A–V difference (Table 4). However, the A–V difference of CO2 tended to be higher in hot conditions (46 vs 38 mL/L at 20°C). The respiratory quotient (RQ) calculated from the A–V differences in O2 and CO2 was not influenced by temperature (P > 0.10) and averaged 1.10. The ranking of extraction rates, greatest to least, was TG, FFA, glucose, -amino acids N, glycerol, and lactate (42.6, 38.6, 26.5, 21.2, 20.4, and 10.5%, respectively). A significant negative and positive urea A–V difference was recorded at 20 and 28°C, respectively. Arterial concentrations and A–V differences for all essential and nonessential AA were unaffected by ambient temperature (P > 0.10), but plasma arterial concentration and mammary extraction of the -amino acid N were higher (P < 0.05) at 20°C. Figure 2 shows the relationship for both temperatures between the relative essential AA difference and their concentration in milk expressed as a percentage of total A–V difference and of total essential AA in milk, respectively. On average, the hierarchy between essential AA was maintained in their A–V difference and in their concentration in milk; leucine represented about 20.4% of total A–V difference of essential AA and 19.5% of total essential AA in milk. From this relationship, methionine was the AA with the highest efficiency of utilization by the mammary gland; lysine ranked third. Exposure to 28°C increased the arterial concentration of calcium (by 4.0 mg/L) and decreased that of phosphorus (by 2.4 mg/L) (Table 5).
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Figure 2. Relationship between concentration of essential AA expressed as a percentage of total essential AA in milk and A–V difference expressed as a percentage of total A–V difference of essential AA. Numbers in parentheses represent the ratio between percentage of A–V difference and percentage of AA concentration in milk. The line was the relationship when AA concentration in milk = AA A–V difference.
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During the sampling period (between 0915 and 1530), arterial plasma concentrations of glucose and insulin tended to increase (P < 0.10) after each meal distribution (at 1000, 1200, and 1400; results not presented). The arterial concentration and the A–V difference of lactate varied over time; both values were higher (P < 0.01) at 0915 compared to other sampling times. After meal distribution, arterial plasma concentrations of TG and FFA decreased (P < 0.10), whereas A–V difference and extraction rate decreased (P < 0.001) for FFA only.
Quantification of Daily Nutrient Uptake and Output.
The pattern of nutrient uptake from blood by the mammary gland was not affected by temperature (Table 7). Glucose was the main nutrient taken up by the mammary gland (60% of total DM uptake). Total nutrient and energy uptake from blood was higher (by 12%) when sows were exposed to a temperature of 28°C. From results of milk composition (Table 3), it can be calculated that lactose, protein, and fat represented 32, 30, and 33% of daily output of DM in milk, respectively. For every AA, daily uptake from the blood by the mammary gland was significantly different from zero (P < 0.05). The ranking of uptakes, greatest to least, for the essential AA was leucine, lysine, arginine, valine, isoleucine, phenylalanine, threonine, histidine, and methionine (Figure 3). From AA uptake and output, it is possible to estimate AA retention by the mammary gland; it was different from zero (P < 0.05) for only leucine and isoleucine (by 7 and 5 g, respectively).
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Figure 3. Daily AA uptake from blood and output in milk in lactating sow (means + SEM). Amino acid uptake from blood was calculated from the A–V differences and estimated blood flow (5,147 L/d). Amino acid output in milk was calculated from AA concentration in milk and milk yield estimated from litter body weight (Noblet and Etienne, 1989). * = Amino acid retention (output – uptake) significantly different from zero at P < 0.05.
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Discussion
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Thermoregulation
In agreement with Quiniou and Noblet (1999) and Renaudeau et al. (2001), skin and rectal temperatures increased when ambient temperature increased from 20 to 28°C. As described with growing pigs (Collin, 2000), the elevation of peripheral temperature in hot conditions is explained by an increased blood flow to the vessels in the skin for dissipation of body heat. However, the 0.5°C increase in rectal temperature at 28°C indicates that this adaptation was not sufficient and that sows were not able to maintain a constant internal body temperature in hot conditions. As a consequence, sows kept at 28°C can be considered heat stressed. Thyroid hormones are implicated in the control of heat production in homeothermic animals (McNabb, 1995). The decrease of 0.24 and 3.8 mmol/L in plasma concentrations of T3 and T4, respectively, in the sows maintained at 28°C is consistent with results obtained in lactating sows (Messias de Bragança et al., 1998), growing pigs (Collin, 2000), and lactating goats (Sano et al., 1984). Leptin concentration increased at 28°C, which is in contrast with results of Collin (2000), who reported a greater tendency for lower leptin concentrations in ad libitum-fed growing pigs at 33 than those at 23°C. Intracerebroventricular injections of leptin suppressed feed intake in pigs (Barb et al., 1998), and feed restriction during 7 d depressed the leptin concentration in lactating sows (Mao et al., 1999), illustrating the key role of leptin in the regulation of feed intake. This suggests that reduction of feed intake in sows fed ad libitum and kept in hot conditions could be mediated by the increase of leptin concentration.
Lactose Precursors.
Glucose uptake by the mammary gland represents a considerable portion of total body glucose requirement. Approximately 70% of arterial glucose is removed by the mammary gland in the sow, according to Boyd and Kensinger (1998). In agreement with results in sows (Holmes et al., 1988), cows (Rulquin, 1981), and ewes (Davis and Bickerkraffe, 1978), no relationship between A–V differences and arterial glucose concentration was found. This could be attributed to the reduced range of glucose concentration taken into account for this calculation (900 to 1,200 and 550 to 750 mg/L in sows and ruminants, respectively). In fact, using wider ranges of glucose concentration, in cows (400 to 900 mg/L; Rulquin, 1997) and sows (600 to 1,300 mg/L: Dourmad et al., 2000), significant relationships between A–V differences and arterial concentrations of glucose have been reported (r2 = 0.34 and 0.72, respectively). Although arterial concentration of glucose was not affected by ambient temperature, A–V differences, rate of extraction, and PMBF increased at 28°C. Assuming that PMBF variation reflects blood flow variation in the whole mammary gland, these results contrast with the diffusion concept, according to which the extraction rate decreases as the arterial blood flow increases (Vernon and Peaker, 1983); this may also suggest that the efficiency of the glucose transporters would increase in hot conditions. Conversely, in ad libitum-fed lactating goats exposed to severe hot conditions, glucose uptake by the mammary gland tended to decrease (by 20 and 13%, respectively, between 20 and 35°C; Sano et al., 1984) with a concomitant reduction of milk yield. They concluded that the reduction in milk production was an adaptation to prevent the development of a severe hypoglycemia in hot conditions. Moreover, mean extraction rate was higher than the value reported by Dourmad et al. (2000; 26 vs. 19%); this could be related to the higher milk production level (11.0 vs 6.7 kg/d). In conclusion, the rate of extraction of glucose seemed to be closely related to milk production level; consequently, the lack of negative effects of high temperature on glucose uptake in the present study could be explained by a similar milk yield at both temperatures.
Glucose represented 60% of the total dry matter uptake by the mammary gland (Table 7). This value is in the same range as those reported in sows by Spincer et al. (1969), Spincer and Rook (1971), and Dourmad et al. (2000) (41, 61, and 49% of total mass, respectively). From our results, it can be estimated that approximately 1,300 g of glucose was needed to support milk production of 11 kg/d. Blood glucose is the major precursor of milk lactose; 53% of glucose uptake would be used for the synthesis of lactose in sows, according to Linzell et al. (1969). This value is much lower than in cows (85%; Rulquin, 1997). About 34% of the glucose uptake by the sow mammary gland is oxidized in CO2 in order to produce energy (Linzell et al., 1969). The respiratory quotient was higher than unity (1.10) in sows, which is in agreement with Linzell et al. (1969) and Dourmad et al. (2000). This suggests that glucose is the primary source of energy for the mammary gland in lactating sows. Moreover, the increase of CO2 production and of the glucose uptake at 28°C were of a similar magnitude (+18 and 17%, respectively), whereas O2 consumption was unchanged, suggesting that surplus of glucose uptake at this temperature was used as an energy source. The remainder of glucose (approximately 14%) would be used as a lipid precursor via glycerol and fatty acids synthesis, which is consistent with the RQ close to the unity (Linzell et al., 1969). In agreement with the results of Dourmad et al. (2000), a small amount of lactate was removed from blood by the mammary gland and could also be a precursor for lactose synthesis (Rook, 1979).
Lipid Precursors.
Little information exists on the mammary gland uptake and metabolism of blood precursors of milk fat. Fatty acid (TG and FFA) uptake by the mammary gland derives from very low-density lipoproteins and chylomicrons. The TG are hydrolyzed by a lipoprotein lipase (LPL; EC 3.1.1.34) in the mammary capillaries (Neville and Picciano, 1997). The generated fatty acids, diacylglycerides, monoacylglycerides, and glycerol are taken up by the mammary epithelial cells (Veerkamp, 1995). The A–V difference of TG represents the amount of TG hydrolyzed, and consequently, is an indicator of LPL activity. At 20°C, the extraction rate of TG (41%) was similar to other values reported in sows (51%; Dourmad et al., 2000) and in cows (54%; Rulquin, 1997). Even though the plasma TG concentration decreased after meal distribution, the amount of TG removed by the mammary gland remained constant with an extraction rate adjustment (Dourmad et al., 2000). Moreover, no relationship existed between extraction rate and arterial concentration, suggesting that LPL activity is regulated according to the needs of the mammary gland. This hypothesis is supported by the review of Neville and Picciano (1997), who demonstrated that LPL activity in the mammary gland is controlled by milk synthesis rather than FFA or TG concentration.
The A–V difference of FFA (36 mg/L) was higher than the values reported in sows by Dourmad et al. (2000; 25.5 mg/L); this difference could be explained by the lower mobilization of body reserves by the sows in the latter study (0.72 vs 2.13 kg of BW loss per day in the present study). However, little glycerol was extracted from blood (1.6 mg/L) in agreement with Dourmad et al. (2000). The A–V differences of FFA and glycerol were well correlated with their arterial concentrations (Table 8). In agreement with Veerkamp (1995), this suggests that FFA were removed by diffusion following the concentration gradient between blood and the mammary cells. However, the curvilinear relationship between extraction rate and arterial concentration of FFA and glycerol (Figure 4) highlights that a saturable carrier-mediated process could be implicated in the transport of both nutrients across the mammary cell membrane. According to the results of Fielding and Frayn (1998), obtained in rats’ skeletal muscle and white adipose tissue, there were strong correlations between TG and FFA or glycerol A–V differences (P < 0.01; R2 = 0.83 and 0.66, respectively).
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Figure 4. Relationship between extraction rates and arterial concentration of FFA and glycerol. Each mark represents one plasma sample.
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From the total lipid precursors removed from plasma and lipids output in milk, it can be estimated that approximately 70% of lipids in sows’ milk came from blood. This result is consistent with the close relationship between fatty acid composition of milk and of body fat reserves when sows are in a negative energy balance (Salmon-Legagneur, 1965). This also means that 30% of lipids were synthesized de novo in the mammary gland. Using [U-14C] glucose, Linzell et al. (1969) also showed that output of total fatty acids in sow milk was greater than the uptake from the blood and that glucose was a major precursor of de novo lipid synthesis in the mammary gland. That is also consistent with the greater glucose uptake relative to lactose output in milk.
Amino Acid Metabolism.
The reduction of -amino acid N arterial concentration at 28°C (8%) was probably linked to the lower body protein mobilization in hot conditions. According to the results of Chacornac et al (1993), the -amino acid N and the total free AA arterial concentrations are well correlated (R2 = 0.72; P = 0.008). In addition, total free AA arterial concentrations showed a nonsignificant decrease (–9%, P = 0.34), which may be related to the great variability of AA plasma concentrations between sows (CV = 25%). This would suggest that the -amino acid N extraction rate increased, probably in response to its lower arterial concentration, in order to maintain milk protein synthesis in hot conditions.
The relationship between A–V difference and arterial concentration of essential AA (Figure 5) confirms that A–V difference in AA is related to their respective arterial concentration (Trottier et al., 1997; Metcalf et al., 1991). In a review, Rulquin (1997) reported that for essential AA, the A–V difference was closely dependent on the arterial concentration, suggesting that in physiological conditions, their transporters were never saturated. Furthermore, compared to A–V differences, extraction rates provided further information on the ability of the mammary gland to take up AA. In ascending order, methionine, lysine, isoleucine, phenylalanine, and leucine were extracted at a higher rate (>20%) and histidine, threonine, and valine were extracted at the lowest rate (<15%). The high efficiency in extraction rate of methionine, lysine, isoleucine, and leucine is consistent with the results of Trottier et al. (1997) in sows, Guinard and Rulquin (1994b) in cows, and Davis et al. (1978) in sheep, and may indicate their importance in milk protein synthesis. On average, the rate of extraction of essential AA was lower than the value reported by Trottier et al. (1997) (22 vs 29%, respectively). The reasons for the variation in the magnitude of mammary extraction of essential AA are probably related to the higher relative plasma AA arterial concentration in our study. For example, arterial concentration of lysine in plasma reported by Trottier et al. (1997) was threefold lower than our value (10.3 vs 34.2 mg/L), which resulted in a significant increase in extraction rate (53.0 vs 23.4%, respectively). This is probably related to the difference in dietary lysine content between the two studies (0.67 vs 0.98% in our study). Furthermore, the lysine uptake by sows’ mammary gland closely depends on its plasma concentration (Hurley et al., 2000), suggesting that the lysine carrier system should not be a limiting step for milk protein synthesis. Using duodenal infusions in cows, Guinard and Rulquin (1994a) tested the effect of the increase of arterial concentration of lysine on its extraction rate. They showed that when udder requirements for milk production were met, the extraction rate of lysine decreased. These results suggest that lysine uptake was closely related to the level of milk production.
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Figure 5. Relationship between arteriovenous difference (A–V, mg/L) and arterial concentration (mg/L) of essential AA (•).
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On average, nonessential AA extraction rate showed a large variability (CV = 52%) and was slightly lower than that of essential AA (17 vs 22%, respectively), reflecting the greater nonessential AA content in feed relative to the essential AA content (84 vs 77 g/kg). Similarly, Linzell et al. (1969), Spincer et al. (1969), and Trottier et al. (1997) reported that nonessential AA had a significantly lower extraction rate than essential AA in lactating sows. These results are also consistent with the mammary gland ability to synthesize nonessential AA from essential AA.
Figure 3 gives a quantitative representation of essential AA uptake from plasma and output in milk. Lysine was taken up at a rate of 38 g/d for sows producing 11 kg/d; from this, it can be estimated that approximately 3.5 g of lysine are required by the mammary gland to produce 1 kg of milk. The essential AA balance by the mammary gland was calculated as the difference between daily AA uptake in plasma and daily AA output in milk. For isoleucine and leucine, their uptake from blood exceeded their excretion in milk suggesting that these AA are used for maintenance requirements of the mammary gland or as a source of nonessential AA or precursors for other syntheses (glucose, lipids) (Roets et al., 1983). Using lactating sows fed ad libitum, Trottier et al. (1997) also reported that arginine, phenylalanine, and threonine were taken in excess compared to the corresponding amounts output in milk.
Mammary Blood Flow and Milk Production.
Mammary blood flow can be estimated from the Fick principle based on a direct transfer of a precursor from blood to milk without metabolism to others products. Under normal conditions (sows fed ad libitum and kept at thermoneutrality), Trottier et al. (1997) reported that lysine was utilized with a high efficiency relative to other AA. Consequently, lysine was used in that study as a marker to estimate blood flow rate. On this basis, it can be calculated that 516 L of plasma was required to produce 1 kg of milk in our experiment, independent of the ambient temperature. This is slightly lower than the corresponding values reported by Trottier et al. (1997; 541 L/kg). In fact, in order to use lysine as a marker, dietary lysine must be limiting for milk production and consequently, 100% of lysine uptake would be recovered in milk. In our experiment, lysine supply was probably less limiting and was utilized by the mammary gland with a lower efficiency than in the study of Trottier et al. (1997). Therefore, lysine uptake would not be used only for lysine deposition in milk, leading to an underestimation of the blood flow. In addition, when this method is used, it is assumed that mammary blood flow is constant over the sampling day and the period considered for the estimation of milk yield (7 d in our experiment), whereas great variations of blood flow have been measured in response to postural changes, milking and feeding (Renaudeau et al., 2002). Thus, it appears that the Fick principle used with lysine as a marker is not fully adapted to estimate the effect of ambient temperature on mammary blood flow. Alternatively, we developed a technique for direct measurement of mammary blood flow using a transit time ultrasonic flow probe (Renaudeau et al., 2002). Using this technique, we measured that exposure to 28° compared to 20°C increased mammary blood flow by approximately 5%, whereas time spent lying was increased. This suggests a postural adaptation in response to high temperature partly in order to increase conductive heat loss through the floor (Ingram, 1973). As observed by Renaudeau et al. (2002), mammary blood flow significantly increased when sows were lying. This would suggest that the increase of mammary blood flow when sows were exposed to hot conditions should be partly related to the reduction of time in standing position. With the assumption that mammary blood flow measured on the PMBF supplies one quarter of the udder, it can be calculated that mammary blood flow through the entire mammary gland averaged 3.6 L/min.
At identical feed intakes, milk production was not affected by temperature (11 kg/d on average), whereas sow BW loss tended to be lower for at 28°C. Similarly, Lough et al. (1990) did not report any reduction in milk production in pair fed cows kept at 22.5 and 29.5°C. In contrast, Mullan et al. (1992) reported a tendency for a reduced milk yield (by 1 kg/d) and Messias de Bragana et al. (1998) found a reduction in litter weight gain during the third week of lactation in pair-fed primiparous sows when ambient temperature was increased. The discrepancy between our results and these latter ones could be related to differences in parity number (multiparous vs primiparous), duration of exposure to ambient temperature (7 vs 21 or 28 d), or the range of temperature considered (20 to 28°C vs 20 to 30°C).
Even though milk production was not affected by ambient temperature when sows were pair fed, the extraction rate of the main precursors of milk synthesis (glucose, TG, and -amino acid N) and the mammary blood flow increased at 28°C. Moreover, the amount of blood required to produce 1 kg of milk by the mammary gland was higher at 28°C than at 20°C (482 vs 452 L/kg, respectively). With the assumption that blood from the external pudic artery supplies the glandular and subcutaneous mammary gland tissues, this implies that the proportion of blood flow irrigating capillaries in the skin increased in hot conditions whereas the proportion of total flow irrigating the capillaries surrounding the glandular epithelial tissue was reduced (Vernon and Peaker, 1983). The rate of heat loss is dependent on the rate of blood flow to a region; this means that the mammary gland would be involved in the dissipation of body heat in connection with the increase of time spent lying when ambient temperature increases. This would suggest that the apparent inefficiency of the sow mammary gland in hot conditions could be related to an increase of the proportion of blood flow irrigating skin capillaries in order to dissipate body heat.
In conclusion, these results show that the increase of ambient temperature from 20 to 28°C with a constant feed allowance did not directly affect milk production in multiparous sows. This conclusion partially differs from the hypothesis developed by Black et al. (1993), who suggested a redistribution of the blood flow from the mammary gland to the skin in order to increase heat loss, resulting in a lower nutrient supply for milk synthesis in hot conditions.
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Implications
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Using an arteriovenous technique in association with a direct determination of mammary blood flow, this experiment provides original information about the metabolism of the milk precursors of lactose, lipids, and protein in the mammary gland of lactating sows. Moreover, we demonstrated that, in our conditions, the elevated ambient temperature affected mammary blood flow and uptake of nutrient without changing milk yield. Furthers studies are necessary to examine the possible role of the udder in the dissipation of body heat.
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Footnotes
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1 The authors wish to acknowledge P. Bodinier, C. David, S. Dubois, M. Fillaut, J. Gauthier, G. Le Cadre, Y. Lebreton, A. M. Mounier, J. C. Hulin, Y. Jaguelin, H. Renoult, and B. Trépier for their efficient technical assistance, M. Etienne for critical evaluation of the manuscript, and Ajinomoto Eurolysine (Paris, France) for the measurement of amino acids content in plasma. 
2 This work received a grant from Institut Technique du Porc (Paris, France) for the Ph.D. of D. Renaudeau.
3 Present address: INRA, Domaine de Duclos, 97170 Petit Bourg, Guadeloupe (West French Indies), France.
Received for publication August 20, 2001.
Accepted for publication September 16, 2002.
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Literature Cited
|
|---|
AOAC. 1990. Official Methods of Analysis. 15th ed. Assoc. Offic. Anal. Chem., Washington, DC.
Barb, C. R., X. Yan, M. J. Azain, R. R. Kraeling, G. B. Rampacek, and T. G. Ramsay. 1998. Recombinant porcin leptin reduces feed intake and stimulates growth hormones secretion in swine. Domest. Anim. Endocrinol. 15:77–86.[Medline]
Black, J. L., B. P. Mullan, M. L. Lorschy, and L. R. Giles. 1993. Lactation in the sow during heat stress. Livest. Prod. Sci. 35:153–170.
Boyd, R. D., and R. S. Kensinger. 1998. Metabolic precursor for milk synthesis. Pages 71–95 in The Lactating Sow. M. W. A. Verstegen, P. J. Moughan, and J. W. Shrama, ed. Wageningen Pers. (Wageningen), The Netherlands.
Chacornac, J. P., J. Bardouin, and M. L. Houlier. 1993. Micro-dosage automatisé sur analyseur à transfert de l’azote alpha aminé circulant. Reprod. Nutr. Dev. 33:99–108.
Cochran, W. G., and G. M. Cox. 1962. Experimental Designs. 2nd ed. John Wiley & Sons, Inc., New York.
Collin, A. 2000. Effets de la température ambiante élevée sur le métabolisme energétique du porcelet. Thèse. INRA-Unité mixte de recherches sur le veau et le porc. Ecole Superieure Agronomique de Rennes, France.
Davis, S. R., and R. Bickerkraffe. 1978. Mammary glucose uptake in the lactating ewe and the use of methionine arterio-veinous difference for the calculation of mammary blood flow. Aust. J. Biol. Sci. 31:133–139.[Medline]
Davis, S. R., R. Bickerkraffe, and D. S. Hart. 1978. Amino acid uptake by the mammary gland in the lactating ewe. Aust. J. Biol. Sci. 31:123–132.[Medline]
Dourmad, J. D., J. J. Matte, Y. Lebreton, and M. L. Fontin. 2000. Influence du repas sur l’utilisation des nutriments et des vitamines par la mamelle, chez la truie en lactation. J. Rech. Porcine Fr. 32:265–273.
Eurolysine and I.T.C.F. 1995. Page 53 in Ileal Digestibility of Amino Acids in Feedstuffs for Pigs. Eurolysine and ITCF, Paris, France.
Fielding, B. A., and K. N. Frayn. 1998. Lipoprotein lipase and the disposition of dietary fatty acids. Br. J. Nutr. 80:495–502.[Medline]
Guinard, J., and H. Rulquin. 1994a. Effect of graded amounts of duodenal infusions of lysine on mammary uptake of major milk precusors in dairy cows. J. Dairy Sci. 77:3565–3576.[Abstract]
Guinard, J., and H. Rulquin. 1994b. Effect of graded levels of duodenal infusions of casein on mammary uptake in lactating cows. 2. Individual amino acids. J. Dairy Sci. 77:3304–3315.[Abstract]
Holmes, M. A., C. Maughan, A. Paterson, G. Bryant-Greenwood, G. Rice, and P. E. Hartmann. 1988. The uptake of glucose by the mammary glands of lactating sows. Proc. Nutr. Soc. Aust. 13:113. (Abstr.).
Hurley, W. L., H. Wang, J. M. Bryson, and D. B. Shennan. 2000. Lysine uptake by mammary gland tissue from lactating sows. J. Anim. Sci. 78:391–395.[Abstract/Free Full Text]
Ingram, D. L. 1973. Heat loss and its control in pigs. Page 235 in Heat Loss from Animals and Man. J. L. Monteith and L. E. Mount, ed. Butterworths, London.
Lebellego, L., J. van Milgen, S. Dubois, and J. Noblet. 2001. Energy utilization of low-protein diets in growing pigs. J. Anim. Sci. 79:1259–1271.[Abstract/Free Full Text]
Linzell, J. L., T. B. Mepham, and E. F. Annison, C. E. West. 1969. Mammary metabolism in lactating sows: Arteriovenous differences of milk precursors and the mammary metabolism of [14C]glucose and [14C]acetate. Br. J. Nutr. 23:319–332.[Medline]
Livesey, G. 1984. The energy equivalent of ATP and the energy values of food proteins and fats. Br. J. Nutr. 51:15–28.[Medline]
Lough, D. S., D. L. Beede, and C. J. Wilcox. 1990. Effects of feed intake and thermal stress on mammary blood flow and other physiological measurements in lactating dairy cows. J. Dairy Sci. 73:325–332.[Abstract]
Mao, J., L. J. Zak, J. R. Crosgrove, S. Shostak, and G. R. Foxcroft. 1999. Reproductive, metabolic, and endocrine responses to feed restriction and GnRH treatment in primiparous, lactating sows. J. Anim. Sci. 77:725–735.
McNabb, F. M. 1995. Thyroid hormone, their activation, degradation and effects on metabolism. J. Nutr. 125:1773–1776.
Messias de Bragana, M., A. M. Mounier, and A. Prunier. 1998. Does feed restriction mimic the effects of increased ambient temperature in lactating sows? J. Anim. Sci 76:2017–2024.[Abstract/Free Full Text]
Metcalf, J. A., J. D. Sutton, J. E. Cockburn, D. J. Napper, and D. E. Beever. 1991. The influence of insulin and amino acid supply on amino acid uptake by the lactating bovine mammary gland. J. Dairy Sci. 74:3412–3420.[Abstract]
Meunier-Salaun, M. C., F. Gort, A. Prunier, and W. P. G. Schouten. 1991. Behavioral patterns and progesterone, cortisol and prolactin levels around parturition in European (Large-White) and Chinese (Meishan) sows. Appl. Anim. Behav. Sci. 31:43–59.
Mullan, B.P., W. Brown, and M. Kerr. 1992. The response to the lactating sow to ambient temperature. Proc. Nutr. Soc. Aust. 17:215. (Abstr.)
Neville, M. C., and M. F. Picciano. 1997. Regulation of milk lipids secretion and composition. Annu. Rev. Nutr. 17:159–184.[Medline]
Noblet, J., and M. Etienne. 1989. Estimation of sow milk nutrient output. J. Anim. Sci. 67:3352–3359.
Noblet, J., H. Fortune, X. S. Shi, and S. Dubois. 1994. Prediction of net energy value of feeds for growing pigs. J. Anim. Sci. 72:344–354.[Abstract]
NRC. 1998. Nutrient Requirements of Swine. 11th ed. Natl. Acad. Press, Washington, DC.
Quiniou, N., and J. Noblet. 1999. Influence of high ambient temperatures on performance of multiparous lactating sows. J. Anim. Sci. 77:2124–2134.[Abstract/Free Full Text]
Quiniou, N., D. Renaudeau, S. Dubois, and J. Noblet. 2000. Influence of high ambient temperatures on feed intake and feeding behaviour of multiparous lactating sows. Anim. Sci. 70:471–479.
Renaudeau, D., Y. Lebreton, J. Noblet, and J.Y. Dourmad. 2002. Measurement of blood flow through the mammary gland in lactating sows: methodological aspects. J. Anim. Sci. 80:196–202.[Abstract/Free Full Text]
Renaudeau, D., N. Quiniou, and J. Noblet. 2001. Effects of exposure to high ambient temperature and dietary protein level on performance of multiparous lactating sows. J. Anim. Sci. 79:1240–1249.[Abstract/Free Full Text]
Roets, E., A. M. Massart-Leen, G. Peeters, and R. Verbeke. 1983. Metabolism of leucin by the isolated perfused goat udder. J. Dairy Res. 50:413–424.[Medline]
Rook, J. A. F. 1979. The role of carbohydrate metabolism in the regulation of milk production. Proc. Nutr. Soc. 38:309–314.[Medline]
Rulquin, H. 1981. Etude méthodologique sur la mesure des différences artério-veineuse mammaires chez la vache laitière. Reprod. Nutr. Dev. 21:31–46.
Rulquin, H. 1997. Règulation de la synthése et de la sécrétion des constituants du lait chez les ruminants. Renc. Rech. Ruminants 4:327–338.
Salmon-Legagneur, E. 1965. Quelques aspects des relations nutritionnelles entre la gestation et la lactation chez la truie. Annal. Zoot. 14:1–137.
Sano, H., K. Ambo, and T. Tsuda. 1984. Blood glucose kinetics in whole body and mammary gland of lactating goats exposed to heat. J. Dairy Sci. 68:2557–2564.
Spincer, J., and J. A. F. Rook. 1971. The metabolism of [U-14C]glucose, [1-14C]palmitic acid and [1-14C]stearic acid by the lactating mammary gland of the sow. J. Dairy. Res. 38:315–322.[Medline]
Spincer, J., J. A. F. Rook, and K. G. Towers. 1969. The uptake of plasma constituents by the mammary gland of the sow. Biochem. J. 111:727–732.[Medline]
Trottier, N. L. 1997. Nutritional control of amino acid supply to the mammary gland during lactation in the pig. Proc. Nutr. Soc. 56:581–591.[Medline]
Trottier, N. L., C. F. Shipley, and R. A. Easter. 1995. A technique for the venous cannulation of the mammary gland in the lactating sow. J. Anim. Sci. 73:1390–1395.[Abstract]
Trottier, N. L., C. F. Shipley, and R. A. Easter. 1997. Plasma amino acid uptake by the mammary gland of the lactating sow. J. Anim. Sci. 75:1266–1278.[Abstract/Free Full Text]
Van Soest, P. J., J. Robertson, and B. A. Lewis. 1991. Methods for dietary fiber, Neutral Detergent Fiber, and Nonstarch Polysaccharides in relation to animal nutrition. J. Dairy Sci. 74:3583–3597.[Abstract]
Veerkamp, J. H. 1995. Fatty acid transport and fatty acid-binding proteins. Proc Nutr. Soc. 54:23–37.[Medline]
Vernon, R. G., and M. Peaker. 1983. The regulation of nutrient utilization: basic principles and mechanisms. Pages 41–113 in Nutritional Physiology of Farm Animals. J. Rook and A. P. C. Thomas, ed. Longman, London.
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Abstract
Introduction
