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Effects of postpartum dietary fat and body condition score at parturition on plasma, adipose tissue, and milk fatty acid composition of lactating beef cows¹

Department of Animal Science, University of Wyoming, Laramie 82071-3684

	Abstract

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Two experiments were conducted with lactating Angus x Gelbvieh beef cows to determine the effects of postpartum lipid supplementation, BCS at parturition, and day of lactation on fatty acid profiles in plasma, adipose tissue, and milk. In Exp. 1, 36 pri-miparous cows (488 ± 10 kg of initial BW; 5.5 ± 0.02 initial BCS) were given ad libitum access to hay and assigned randomly to a low-fat (control) supplement or supplements with cracked, high-linoleate safflower seeds (linoleate) or cracked, high-oleate safflower seeds (oleate) from d 3 to 90 of lactation. Diets were formulated to be isonitrogenous and isocaloric; safflower seed diets provided 5% of DMI as fat. Plasma and milk samples were collected on d 30, 60, and 90 of lactation. Adipose tissue biopsies were collected near the tail-head region of cows on d 45 and 90 of lactation. In Exp. 2, 3-yr-old cows achieving a BCS of 4 ± 0.07 (479 ± 36 kg of BW) or 6 ± 0.07 (580 ± 53 kg of BW) at parturition were used in a 2-yr experiment (n = 36/yr). Beginning 3 d postpartum through d 61 of lactation, cows were fed diets similar to those of Exp. 1. Adipose tissue and milk samples were collected on d 30 and 60, and plasma was collected on d 31 and 61 of lactation. Responses to postpartum dietary treatment were comparable in both experiments. Cows fed linoleate and oleate had greater (P < 0.001) total fatty acid concentrations in plasma than cows fed control. Except for 15:1, milk fatty acids with <18 carbons were greatest (P 0.01) for cows fed control, whereas milk from cows fed linoleate had the greatest (P 0.02) 18:1trans-11, 18:2n-6, and cis-9, trans-11 CLA. Milk from cows fed oleate had the greatest (P < 0.001) 18:1cis-9. In Exp. 1, total fatty acid concentrations in adipose tissue samples decreased at d 90 compared with d 45 of lactation, but the fatty acid profile of cow adipose tissue was not affected (P = 0.14 to 0.80) by dietary treatment. In Exp. 2, the percentage of cis-9, trans-11 CLA in adipose tissue of cows with a BCS of 6 decreased (P = 0.001) from d 30 to 60 of lactation. Plasma and milk fatty acid composition reflected alterations in postpartum diet. Less medium-chain fatty acids and more 18-carbon fatty acids in milk were indicative of reduced de novo fatty acid synthesis in the mammary gland of beef cows fed lipid supplements; however, the metabolic demands of lactation prevented the deposition of exogenously derived fatty acids in adipose tissue through d 90 of lactation.

Key Words: adipose tissue • beef cow • body condition score • fatty acid • lipid supplementation • milk

	INTRODUCTION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
In lactating cattle, decreased adipose tissue lipoprotein lipase activity (Bauman and Currie, 1980; McNamara et al., 1987) coupled with the energy demands of milk production (Bauman and Currie, 1980) direct nutrients toward mammary function and milk production, resulting in little exogenous lipid reaching the adipocyte for storage. Research in lactating beef (Bottger et al., 2002) and dairy (McNamara et al., 1995) cows, however, has implicated dietary lipids as nutrient partitioning nutraceuticals. Williams and Stanko (2000) defined nutraceuticals as nutrients having physiological effects outside of their generally accepted role.

Bottger et al. (2002) attributed maintenance of greater BCS during lactation in beef cattle to supplementation with linoleic acid, whereas dietary oleic acid increased milk fat synthesis. Additionally, beef cows in BCS of 4 at parturition maintained BCS (Lake et al., 2005) and had the metabolic proclivity to increase adipose tissue lipid accretion (Lake et al., 2006b) during the first 60 d of lactation. We hypothesized that the aforementioned nutrient partitioning effects of dietary lipids and BCS are associated with changes in fatty acid composition of tissues involved with nutrient partitioning.

Our objectives were to evaluate the effects of supplementation with higholeic acid or high-linoleic acid cracked safflower seeds and BCS at parturition on the fatty acid composition of plasma, adipose tissue, and milk from beef cows during early lactation.

	MATERIALS AND METHODS

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Experiment 1
General.
The University of Wyoming Institutional Animal Care and Use Committee approved all procedures involving animals for the following study. Cows were managed as described by Bottger et al. (2002). Briefly, 36 spring-calving, primiparous, Angus x Gelbvieh beef cows [488 ± 10 kg of initial BW; 5.5 ± 0.02 initial BCS (1 = emaciated, 9 = obese; Wagner et al., 1988)] were blocked by day of parturition and assigned randomly to 1 of 3 supplemental treatments. Beginning 3 d postpartum, cows were fed a low-fat (control) supplement or supplements with cracked, high-linoleate saf-flower seeds (linoleate) or cracked, high-oleate safflower seeds (oleate) in individual feeding stanchions until d 90 of lactation (Table 1). Cows were housed in 1 of 6 pens based on calving date, with 6 cows per pen, and had ad libitum access to native grass hay provided in large round bales. Supplements were fed once daily at 0630.

Sampling and Laboratory Analyses.
Plasma was harvested from preprandial, whole blood samples collected in 10-mL, EDTA-coated, glass Vacutainer tubes (Becton, Dickson and Co., Franklin Lakes, NJ) on d 30, 60, and 90 postpartum. Blood samples were immediately refrigerated for 12 h, centrifuged at 1,300 x g for 30 min, and plasma was harvested and stored at –20°C for analysis of fatty acids. Beginning at 0500 on d 30, 60, and 90, all cows were separated from their calves, administered 20 USP of oxytocin (Vedco Inc., St. Joseph, MO), and milked using a mechanical milking device, with the remaining milk hand-stripped. Milk collected into the mechanical milking device was thoroughly mixed before a 20-mL subsample, representative of the entire milking from each cow, was stored at –20°C for fatty acid analysis. Additionally, on d 45 and 90, each cow was injected s.c. with approximately 400 mg of lidocain hydrochloride (Vedco Inc.) as a local anesthetic to desensitize a 10-cm² area between the ischium and coccygeal vertebrae in the caudal portion of the tailhead region. Adipose tissue biopsies (5 g) were removed (Rule and Beitz, 1986) and stored at –20°C for fatty acid analysis.

Plasma samples were lyophilized (Genesis SQ 25 Super ES Freeze Dryer, The Virtis Co., Gardiner, NY), ground with a mortar and pestle, and fatty acid methyl esters were prepared as described by Lake et al. (2006c). Milk samples were lyophilized and ground with a mortar and pestle. Milk and adipose tissue were subjected to direct transesterification for preparation of fatty acid methyl esters (Murrieta et al., 2003). Tridecanoic acid (Sigma Aldrich, St. Louis, MO) was used as the internal standard for plasma, milk, and adipose tissue samples.

Separation of fatty acid methyl esters was achieved by GLC (Model 6890 series II, Hewlett-Packard, Avondale, PA) with a 100-m capillary column (SP-2560, Supelco, Bellefonte, PA), with He as the carrier gas at 0.5 mL/min. The oven temperature was maintained at 175°C for 40 min and ramped to 240°C at 10°C/min. Injector and detector (flame ionization) temperatures were 250°C. Identification of peaks was accomplished with purified standards (Nu-Check Prep, Elysian, MN; Matreya, Pleasant Gap, PA). Identification of the 18:1trans-10 isomer was putative and based on the position of a peak between those identified as 18:1trans-9 and 18:1trans-11 (Molkentin and Precht, 1995). Concentrations of individual fatty acids were reported if the total concentration of fatty acids was affected by a treatment, whereas the total concentration of fatty acids and weight percentages of individual fatty acids were reported if a treatment did not affect the total concentrations of fatty acids.

Statistical Analyses.
Data were analyzed as repeated measures for a randomized complete block design using the MIXED procedure (SAS Institute, Cary, NC). Pen (assigned by day of parturition) was the block, and the model included the additional effects of dietary treatment, day of sampling, and the treatment x day interaction. Cow within dietary treatment was the random variable used as the SUBJECT, and day of sampling was included in the REPEATED statement. Using likelihood ratio testing, an AR-1 structure was deemed most appropriate for the effects associated with day of lactation.

Experiment 2
General.
The University of Wyoming Institutional Animal Care and Use Committee approved all procedures involving animals for the following study. Cows were managed as described by Lake et al. (2005). Briefly, in a 2-yr experiment (n = 36/yr), 3-yr-old, Angus x Gelbvieh beef cows (n = 72) were managed nutritionally to achieve a BCS of 4 ± 0.07 (479 ± 36 kg of initial BW) or 6 ± 0.07 (580 ± 53 kg of initial BW) at parturition. Beginning 3 d postpartum, cows were assigned randomly within BCS to be fed hay and a low-fat (control) supplement or supplements with cracked, high-linoleate safflower seeds (linoleate) or higholeate safflower seeds (oleate) in individual feeding stanchions until d 61 of lactation.

Diets (Table 2) were formulated to be similar to those of Exp. 1 and to provide equal quantities of N and TDN within each year. Amounts fed were designed to meet the energy requirement for maintenance of cows with 544-kg BW plus production of 9 kg of milk/d. However, cows were in slight negative energy balance and lost 5.9 (BCS 4) to 10.4 (BCS 6) kg of BW over the course of the study (Lake et al., 2005). Dietary ingredients were analyzed as described previously for Exp. 1. Dietary CP was greater in yr 2 because of differences in CP of the hay (8.5% CP in yr 1; 10.6% CP in yr 2). Dietary TDN was similar between years. The lipid-supplemented diets were formulated to be isolipidic and provided 5% of DMI as fat.

Statistical Analyses.
Data were analyzed as repeated measures for a 2 x 3 arrangement of treatments in a randomized complete block design using the MIXED procedure (SAS Inst. Inc., Cary, NC). Year was used as the block, and the model included the additional effects of BCS at parturition, dietary treatment, the BCS x diet interaction, day of sampling, and all possible interactions among sampling day and treatments. Cow within BCS at parturition x dietary treatment was the random variable used as the SUBJECT, and day of sampling was included in the REPEATED statement. Using likelihood ratio testing, an AR-1 structure was deemed most appropriate for the effects associated with day of sampling. One calf died during yr 1; however, the cow was mechanically milked twice daily to enable her to remain on the experiment. Observations from this cow were tested for normality (PROC UNIVARIATE) to ensure that her observations were not outliers. During yr 2, 1 cow-calf pair was removed from the study due to death of the calf. Necropsies performed at the Wyoming State Veterinary Laboratory revealed that the deaths of the calves were not attributed to the study; consequently, least squares means were reported.

	RESULTS

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Experiment 1
Plasma Fatty Acids.
Postpartum dietary treatment x day of lactation interactions (P 0.01) were detected for plasma concentrations of 17:1 and 18:3n-3 (data not shown); however, this did not preclude evaluation of main effects because interactions were due to minor changes in magnitude rather than ranking of treatments. Dietary treatment x day of lactation interactions (P < 0.001) were also noted for plasma concentrations (mg of fatty acid per g of freeze-dried plasma) of 18:1cis-9 (Figure 1), 18:1trans-11 (Figure 2), and 18:2n-6 (Figure 3). Cows fed oleate had the greatest concentrations of 18:1cis-9 in plasma, whereas plasma concentrations of 18:1 trans-11 and 18:2n-6 were greatest in cows fed linoleate throughout the experiment. Cows fed linoleate had greater concentrations of 18:1cis-9 in plasma than cows fed control on d 30 and 90 of lactation. Concentrations of 18:1cis-9 in plasma from cows fed oleate decreased substantially from d 60 to 90, but a decrease in plasma 18:1cis-9 from d 60 to 90 was not observed in cows fed control or linoleate. Concentrations of 18:1 trans-11 and 18:2n-6 in plasma did not differ between cows fed oleate and control at d 30 and 60, but at d 90, cows fed oleate had greater plasma 18:2n-6 than cows fed control.

View larger version (9K): [in this window] [in a new window]   Figure 1. Postpartum dietary treatment x day of lactation interaction (P < 0.001) for concentration of 18:1cis-9 in plasma in Exp. 1. Diets (Table 1) were hay and a low-fat (control) supplement or supplements with cracked high-linoleate safflower seeds (linoleate) or cracked high-oleate safflower seeds (oleate). a–eData points lacking a common superscript differ (P < 0.05; pooled SEM = 0.17).

View larger version (10K): [in this window] [in a new window]   Figure 2. Postpartum dietary treatment x day of lactation interaction (P < 0.001) for concentration of 18:1trans-11 in plasma in Exp. 1. Diets (Table 1) were hay and a low-fat (control) supplement or supplements with cracked high-linoleate safflower seeds (linoleate) or cracked high-oleate safflower seeds (oleate). a–dData points lacking a common superscript differ (P < 0.05; pooled SEM = 0.06).

View larger version (9K): [in this window] [in a new window]   Figure 3. Postpartum dietary treatment x day of lactation interaction (P < 0.001) for concentration of 18:2n-6 in plasma in Exp. 1. Diets (Table 1) were hay and a low-fat (control) supplement or supplements with cracked high-linoleate safflower seeds (linoleate) or cracked high-oleate safflower seeds (oleate). a–dData points lacking a common superscript differ (P < 0.05; pooled SEM = 0.32).

Milk Fatty Acids.
Dietary treatment x day of lactation interactions were noted for concentrations (mg of fatty acid per g of freeze-dried milk) of 18:1cis-9 (P = 0.01; Figure 4), 18:1trans-11 (P = 0.003; Figure 5), cis-9, trans-11 CLA (P = 0.05; Figure 6), and 18:2n-6 (P = 0.02; Figure 7) in milk. Concentration of 18:1cis-9 in milk was greatest in cows fed oleate at d 30, 60, and 90, and cows fed linoleate had greater milk 18:1cis-9 than cows fed control at d 30. Milk 18:1cis-9 decreased from d 30 to 60 then remained unchanged for cows fed oleate or linoleate, whereas concentration of 18:1cis-9 in milk from cows fed control did not change from d 30 to 90. Likewise, concentrations of 18:1trans-11 and cis-9, trans-11 CLA in milk from cows fed control did not change from d 30 to 90. Cows supplemented with linoleate had the greatest concentrations of 18:1trans-11 in milk at d 30 and 60. Concentrations of 18:1trans-11 in milk did not differ between oleate and control on d 30 and 90, but on d 60, cows fed oleate had greater milk 18:1trans-11 than cows fed control. Because of a precipitous decline in milk 18:1trans-11 for cows fed linoleate, 18:1trans-11 did not differ between cows fed linoleate or oleate on d 90; however, cows fed linoleate had greater milk 18:1trans-11 than cows fed control on d 90. Concentrations of cis-9, trans-11 CLA in milk were greatest for cows fed linoleate followed by cows fed oleate, but unlike cows fed control or oleate, milk cis-9, trans-11 CLA decreased from d 60 to 90 in cows fed linoleate. Concentrations of 18:2n-6 in milk ranked linoleate > oleate > control on d 30. Although concentrations were less at d 60 and 90 compared with d 30, 18:2n-6 in milk was greatest for cows fed linoleate throughout the experiment. Concentrations of 18:2n-6 in milk did not differ between control and oleate on d 60 or 90 of lactation.

View larger version (9K): [in this window] [in a new window]   Figure 4. Postpartum dietary treatment x day of lactation interaction (P = 0.01) for concentration of 18:1cis-9 in milk in Exp. 1. Diets (Table 1) were hay and a low-fat (control) supplement or supplements with cracked high-linoleate safflower seeds (linoleate) or cracked high-oleate safflower seeds (oleate). a–dData points lacking a common superscript differ (P < 0.05; pooled SEM = 5.5).

View larger version (10K): [in this window] [in a new window]   Figure 5. Postpartum dietary treatment x day of lactation interaction (P = 0.003) for concentration of 18:1trans-11 in milk in Exp. 1. Diets (Table 1) were hay and a low-fat (control) supplement or supplements with cracked high-linoleate safflower seeds (linoleate) or cracked high-oleate safflower seeds (oleate). a–dData points lacking a common superscript differ (P < 0.05; pooled SEM = 5.3).

View larger version (9K): [in this window] [in a new window]   Figure 6. Postpartum dietary treatment x day of lactation interaction (P = 0.05) for concentration of cis-9, trans-11, CLA in milk in Exp. 1. Diets (Table 1) were hay and a low-fat (control) supplement or supplements with cracked high-linoleate safflower seeds (linoleate) or cracked high-oleate safflower seeds (oleate). a–dData points lacking a common superscript differ (P < 0.05; pooled SEM = 0.24).

View larger version (10K): [in this window] [in a new window]   Figure 7. Postpartum dietary treatment x day of lactation interaction (P = 0.02) for concentration of 18:2n-6 in milk in Exp. 1. Diets (Table 1) were hay and a low-fat (control) supplement or supplements with cracked high-linoleate safflower seeds (linoleate) or cracked high-oleate safflower seeds (oleate). a–dData points lacking a common superscript differ (P < 0.05; pooled SEM = 0.26).

Adipose Tissue Fatty Acids.
Dietary treatment x day of lactation interactions were not detected (P = 0.19 to 0.99) for fatty acids in cow adipose tissue. Dietary treatment did not affect concentration of total fatty acids (P = 0.49) or fatty acid weight percentages (P = 0.14 to 0.80) in adipose tissue (Table 5). Concentration of total fatty acids and percentage of 17:0 in adipose tissue, however, were greater (P 0.02) at d 45 compared with d 90.

View larger version (15K): [in this window] [in a new window]   Figure 8. Body condition score at parturition x postpartum dietary treatment (P = 0.03) for weight percentage of 18:2n-6 in adipose tissue in Exp. 2. Cows were nutritionally managed beginning in midgestation to achieve a BCS of 4 or 6 at parturition. Diets (Table 2) were hay and a low-fat (control) supplement or supplements with cracked high-linoleate safflower seeds (linoleate) or cracked high-oleate safflower seeds (oleate). a–cData points lacking a common superscript differ (P < 0.05; pooled SEM = 0.04).

View larger version (7K): [in this window] [in a new window]   Figure 9. Body condition score at parturition x day of lactation interaction (P = 0.001) for weight percentage of cis-9, trans-11, CLA in adipose tissue in Exp. 2. Cows were nutritionally managed beginning in midgestation to achieve a BCS of 4 or 6 at parturition. a,bData points lacking a common superscript are different (P < 0.05; pooled SEM = 0.03).

View larger version (8K): [in this window] [in a new window]   Figure 10. Postpartum dietary treatment x day of lactation interaction (P = 0.01) for weight percentage of 18:2n-6 in adipose tissue in Exp. 2. Diets (Table 2) were hay and a low-fat (control) supplement or supplements with cracked high-linoleate safflower seeds (linoleate) or cracked high-oleate safflower seeds (oleate). a,bData points lacking a common superscript differ (P < 0.05; pooled SEM = 0.04).

	DISCUSSION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Plasma Fatty Acids
Response to Postpartum Dietary Treatment.
Greater plasma fatty acid concentrations have been reported in dairy cows supplemented oil at 3% of DM (Loor et al., 2002). We, therefore, expected greater total fatty acid concentrations in plasma from cows fed lipid supplements (Table 3 and 6) because these cows consumed a diet with more than 3% of DM added lipid. Increases in 18:0, 18:1cis-9, 18:1trans-11, and 18:2n-6 in plasma of cows fed lipid supplements were also expected due to increased dietary supply of C18 fatty acids. Noble et al. (1972) reported that concentrations of dietary or ruminally derived fatty acids in plasma were directly proportional to the amount of fatty acids absorbed from the small intestine. Our previous experimental results (Scholljegerdes et al., 2004) demonstrated that duodenal flow of 18:0 averaged 288.5 g/d more in cattle fed linoleate and oleate than in cattle fed control; we also observed increases in duodenal flow of 18:2n-6 and 18:1trans-11 in cattle fed linoleate and increased 18:1cis-9 in cattle fed oleate.

The dietary treatment x day of lactation interaction for 18:1cis-9 (Figure 1) and 18:1trans-11 (Figure 2) in plasma (Exp. 1) was likely a reflection of fatty acid uptake by a variety of tissues. However, as lactation progressed, the decrease in plasma 18:1trans-11 in linoleate cows and 18:1cis-9 in oleate cows was not a result of increased secretion into milk because a similar dietary treatment x day of lactation interaction was observed for milk 18:1trans-11 and 18:1cis-9. Likewise, no change was evident in adipose tissue 18:1trans-11 and 18:1cis-9. Neither main effects nor interactions among main effects were significant for milk yield (Bottger et al., 2002); therefore, decreased concentration of 18:1trans-11 and 18:1cis-9 in plasma was due to something other than secretion into milk or deposition into adipose tissue. In a related experiment, we observed that concentrations of these 2 fatty acids were greatest in oviductal tissue of beef cows at 37 d postpartum and 18:1trans-11 was detected in medial basal hypothala-mic tissue (Scholljegerdes et al., 2007). We also reported that lambs fed diets containing 5% added lipid from high-linoleate or high-oleate safflower seeds had greater 18:1trans-11 in muscle tissues than lambs fed a basal diet without supplemental fat (Bolte et al., 2002). These results suggest that selective deposition of 18:1trans-11 and 18:1cis-9 in tissues not studied in our experiment could account for the decrease in plasma over time.

Response to BCS at Parturition.
The general lack of effects of BCS at parturition on plasma fatty acids in Exp. 2 was not surprising considering cows in a BCS of 4 and 6 were fed diets equal in caloric density per unit of BW after parturition (Lake et al., 2005).

Response to Day of Lactation.
Less total fatty acid concentrations in plasma during early lactation likely reflected greater utilization of circulating fatty acids by the mammary tissue, which was supported by greater BW loss during the early phases of the experiments (Bottger et al., 2002; Lake et al., 2005). Additionally, greater plasma NEFA concentrations observed in cows at d 31 vs. 61 of lactation (Lake et al., 2006a) suggests that the metabolic demands of lactation were greater during early lactation.

Milk Fatty Acids
Response to Postpartum Dietary Treatment.
Differences in milk fatty acid profiles among dietary treatments (Table 4 and 7) were consistent with results reported for dairy cows fed vegetable oil (Kalscheur et al., 1997; DePeters et al., 2001; Loor et al., 2005). As expected, cows fed linoleate had the greatest plasma concentrations of 18:2n-6, as well as the biohydrogenation intermediates 18:1trans-11 and cis-9, trans-11 CLA, directly leading to a similar increase in proportions of these fatty acids in milk. In a separate experiment (Murrieta et al., 2006), we observed increased percentages of 18:2n-6, 18:1trans-11, and cis-9, trans-11 CLA in milk fat collected from beef cows fed linoleate on d 37 of lactation. The dietary treatment x day of lactation interactions for 18:1trans-11 (Figure 5), cis-9, trans-11 CLA (Figure 6), and 18:2n-6 (Figure 7) in milk (Exp. 1) may be reflective of changes in mammary gland metabolism throughout the lactation cycle. Bottger et al. (2002) attributed a nutrient partitioning effect to dietary fatty acids where cows fed oleate had increased milk fat at the expense of BCS, whereas cows fed linoleate maintained BCS at the expense of milk energy. Interestingly, milk from linoleate-supplemented cows decreased in 18:1trans-11, cis-9, trans-11 CLA, and 18:2n-6 as lactation progressed. Eknaes et al. (2006) reported that concentrations of 18:1trans-11 in milk decreased when goats attained positive energy balance. Unlike milk, concentrations of 18:2n-6 in plasma of cows fed linoleate did not differ at d 30 and 90. We suggest that this fatty acid was available to other tissues besides the mammary gland as the cows reached a positive energy balance. Collectively, changes in fatty acid profile in milk and plasma over time may be indicative of increases in energy balance.

Increasing cis-9, trans-11 CLA concentrations in meat and milk is of interest due to potential health benefits associated with these fatty acids (NRC, 1996; Bauman et al., 2000; McGuire and McGuire, 2000). The relevance of 18:1trans-11 is that it can be desaturated to CLA at the tissue level by -9 desaturase. Griinari et al. (2000) estimated that up to 64% of milk CLA is derived through endogenous synthesis from 18:1trans-11 in the mammary gland. Chilliard et al. (2003) reported a very strong relationship (r = 0.99) between percentage of cis-9, trans-11 CLA and 18:1trans-11 in goat milk. The percentage of milk cis-9, trans-11 CLA and 18:1trans-11 in the current study also was correlated (P < 0.001; r = 0.89; data not shown), which supports the relationship described above. Moreover, adipose tissue of calves suckling cows fed linoleate had greater percentages of 18:1trans-11 and cis-9, trans-11 CLA than calves suckling cows fed control or oleate (Lake et al., 2006c).

Despite dietary effects on milk fatty acid profile, concentration of total fatty acids in milk was not affected by dietary treatment. Lack of dietary treatment effects on milk fat yield, BCS, or BW loss during the first 60 d of lactation (Lake et al., 2005) suggests that dietary sources contributed milk fatty acids or fatty acid precursors equally among treatments. Because our previous results (Lake et al., 2006a) indicated that apparent mobilization adipose tissue was comparable among dietary treatments, we assumed that all fatty acids from 10:0 to 16:1 were derived from de novo synthesis. Based on this supposition, cows fed control utilized de novo synthesis of fatty acids to contribute 55% of the total milk fatty acids. Fatty acids likely derived from de novo synthesis in milk from cows fed linoleate and oleate comprised 30 and 32% of the total fatty acids, respectively (estimates were from Exp. 2 only). Palmquist and Mattos (1978) concluded that 51 to 75% of absorbed fatty acids may be taken up by the mammary gland depending upon stage of lactation and dietary supply of fatty acids. Consequently, increased exogenously derived fatty acids in milk from cows fed supplemental lipid was not surprising. A decrease in milk fatty acids synthesized de novo by the mammary gland of cows fed supplemental lipid was expected because of the increase in dietary fatty acids. In agreement with our study, supplementing lactating dairy cows with long-chain fatty acids decreased proportions of medium-chain fatty acids and increased proportions of long-chained fatty acids in milk (Palmquist et al., 1993; Ward et al., 2002). A balance seemed to occur between de novo synthesis by the mammary gland and secretion of exogenously derived fatty acids into milk to provide equal quantities of total fatty acids among dietary treatments. In a similar experiment, Murrieta et al. (2006) noted that dietary effects on fatty acid profile of the milk fat suggested that linoleate supplementation might have decreased de novo lipogenesis while increasing uptake of dietary fatty acids, and the latter observation was consistent with a trend toward greater lipoprotein lipase mRNA in mammary tissue from cows fed linoleate. Decreased de novo synthesis may be attributable to inhibition of acetyl-CoA carboxylase activity associated with increased PUFA in cows supplemented with lipid (Palmquist et al., 1993). A concomitant increase in mammary lipoprotein lipase activity would increase uptake of exogenously derived fatty acid for triacylglyceride production within the mammary tissue (Chilliard et al., 2003). It is also possible that reduced mammary de novo synthesis of fatty acids occurred because lipid replaced dietary components that would have supported ruminal production of acetate. In support of this contention, we reported that ruminal disappearance of NDF in cattle fed linoleate and oleate averaged 423 g/d less than in cattle fed control (Scholljegerdes et al., 2004).

Response to BCS at Parturition.
Cows that have a low BCS rely more on exogenously derived fatty acids than adipose tissue reserves for milk lipid synthesis (Pedron et al., 1993). Increased percentages of 12:0 and 14:0 in milk from BCS 6 cows could suggest greater de novo synthesis of fatty acids by the mammary gland. Greater percentages of 18:0 and 18:1trans-10 in milk from BCS 4 cows suggests that the increase in C18 fatty acids in milk of BCS 4 cows was likely of dietary origin.

Response to Day of Lactation.
As peak lactation approaches and energy deficit becomes more severe, ruminal supply of acetate and liver production of glucose falls short of the mammary and body demands for these substrates causing less synthesis of short-chain fatty acids by mammary tissue and increased mobilization of adipose tissue fatty acids (Palmquist et al., 1993). Although the NRC (2000) estimates that peak lactation in beef cows producing 9 kg of milk occurs on about d 60, cows in our study seemed to have reached peak lactation between d 30 and 60. Cows in both experiments produced the same quantity of milk on d 30 and 60 (Bottger et al., 2002; Lake et al., 2005); however, BW loss was 1.5 x greater from d 3 to 30 vs. d 30 to 60 (Bottger et al,. 2002), and cows lost BCS from d 3 to 30 but maintained BCS between d 30 and 60 of lactation (Lake et al., 2005). Using circulating NEFA concentrations as an indicator of lipolysis, Lake et al. (2006a) demonstrated that cows mobilized more body lipid at d 30 than on d 60 of lactation. Collectively, our results suggest that cows were in a significant energy deficit during the first 30 d of lactation. The mammary gland uses plasma NEFA released by adipose tissue as a source of long-chain fatty acids for milk lipid synthesis when energy balance is negative (Chilliard and Ferlay, 2004). Thus, increased percentage of milk fatty acids originating from de novo synthesis as lactation progressed indicates greater availability and utilization of acetate and less reliance on fatty acids mobilized from adipose tissue.

Adipose Tissue Fatty Acids
Response to Postpartum Dietary Treatment.
It was not surprising to observe that dietary treatment had little influence on fatty acid profile of adipose tissue (Table 5 and 8) during early lactation because few dietary fatty acids were expected to reach the adipocyte for storage due to the nutrient demands associated with early lactation (Chilliard, 1993). Lack of dietary effects on adipose tissue fatty acid profile in Exp. 1 may have occurred because not all cows had attained positive energy balance by d 90 of lactation (Bottger et al., 2002).

Response to BCS at Parturition.
Concentrations of adipose tissue fatty acids were greater in BCS 6 cows; however, directional changes in proportions of fatty acids in adipose tissue could be indicative of fatty acid storage or mobilization. The BCS at parturition x postpartum dietary treatment interaction for percentage of 18:2n-6 in adipose tissue (Figure 8) was consistent with the hypothesis that the linoleate diet would increase palpable adipose tissue reserves of thin beef cows during early lactation (Lake et al., 2005). Although this dietary management strategy did not seem to alter nutrient partitioning (Lake et al., 2005), we suspected that greater percentages of 18:2n-6 in adipose tissue samples from linoleate cows with BCS of 4 were attributable to increased proportions of membrane lipids associated with connective tissue in those samples. The greater percentage of 18:0 in adipose tissue of BCS 4 cows was consistent with increased lipogenic enzyme activity (Lake et al., 2006b), decreased mobilization of body lipids (Lake et al., 2006a), and maintenance of BCS (Lake et al., 2005) in cows with BCS of 4 at parturition.

Response to Day of Lactation.
Decreased total fatty acid concentrations in adipose tissue samples at d 90 compared with d 45 in Exp. 1 were most likely similar to the effects of BCS on total adipose tissue concentrations (Exp. 2), where an average loss of 0.5 BCS over the course (Bottger et al., 2002) resulted in decreased proportions of lipid associated with adipose tissue samples. The decrease in adipose tissue cis-9, trans-11 CLA for cows in BCS of 6 at parturition from d 30 to 60 of lactation (Figure 9) was consistent with greater apparent mobilization of fatty acids from adipose tissue (Lake et al., 2006a) to support milk fat production (Lake et al., 2005).

Apparent repartitioning effects of dietary fat were detected at d 90 of lactation in the study of Bottger et al. (2002). Experiment 2 targeted early lactation to examine the effects of dietary lipid supplementation on the fatty acid profile of tissues associated with potential energy partitioning before the breeding season. Although the postpartum dietary treatment x day of lactation interaction noted for 18:2n-6 in adipose tissue (Figure 10) indicates that cows fed the lipid supplements may have been able to maintain deposition of this fatty acid into their adipose tissue, the endocrine system likely superseded dietary influences on nutrient partitioning to support lactation (Komaragiri et al., 1998).

In conclusion, dietary lipid supplementation and day of lactation affected plasma and milk fatty acid composition in beef cows during early lactation. Cows in a BCS of 4 appeared to deposit greater amounts of exogenously derived fatty acids in adipose tissue. However, the metabolic demands associated with lactation seemed to divert nutrients away from adipose tissue in beef cows during early lactation.

	Footnotes

 
¹ This project was supported by National Research Initiative Competitive Grant No. 2002-35206-11632 from the USDA Cooperative State Research, Education, and Extension Service.

² Current address: Dep. Anim. Sci. Purdue Univ., West Lafayette, IN 47907.

³ Current address: USDA-ARS, NGPRL, Mandan, ND 58554.

⁴ Corresponding author: brethess{at}uwyo.edu

Received for publication June 1, 2006. Accepted for publication October 6, 2006.

	LITERATURE CITED

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 


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