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Dietary lysine and threonine requirements of the pregnant sow estimated by nitrogen balance^1,2

Institut National de la Recherche Agronomique, Unité Mixte de Recherches sur le Porc et le Veau,35590 St-Gilles, France

³ Correspondence:
(Phone: (33) 223-48-5000; fax: (33) 223-48-5080; E-mail:
dourmad{at}st-gilles.rennes.inra.fr.

	Abstract

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Two experiments were conducted to determine the lysine and threonine requirements of gestating sows. In the first experiment, four levels of lysine (0.34, 0.42, 0.48, and 0.56% crude lysine, and 0.24, 0.31, 0.38, and 0.45% standardized ileal digestible lysine) were compared in eight multiparous Large White sows. Each sow received successively the four diets according to a Latin-square experimental design. Nitrogen balance was measured over 11 d after a 10-d period of adaptation to the experimental diet. In the second experiment, four threonine/lysine ratios (0.63, 0.73, 0.80, and 0.89 on a crude basis and 0.61, 0.71, 0.77, and 0.87 on a standardized ileal digestible amino acid basis) were compared in 16 multiparous sows, according to a Latin-square experimental design. The standardized ileal digestibility of amino acids in the experimental diets was determined with ileo-rectal anastomized growing pigs. In the first experiment, nitrogen retention was affected by lysine supply (linear, P < 0.001; quadratic, P < 0.04). Nitrogen retention was lowest for treatment 1 (8.0 g/d) and highest for treatments 3 and 4 that did not differ. Nitrogen retention plateaued at 14.7 g/d in sows consuming 10.5 g/d of digestible lysine. The maintenance requirement for digestible lysine was calculated to be 27 mg/kg BW^0.75 with an efficiency of utilization of digestible lysine above maintenance at 59%. In the second experiment, nitrogen retention was affected (P < 0.03) by the threonine:lysine ratio. It was lower for the lowest threonine:lysine ratio (0.63) than for the other three treatments that did not differ among each other. These results indicate that the optimal standardized digestible threonine:lysine ratio appears to be about 0.71 for multiparous gestating sows.

Key Words: Lysine • Nitrogen Retention • Pregnancy • Sows • Threonine

	Introduction

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Present-day sows are more productive, leaner, and heavier at maturity than those raised 20 yr ago. During pregnancy, their nutrient requirements for maintenance and uterine growth are higher than in the past, due to the heavier mature body weight and the increased prolificacy. Whereas many studies have been conducted during the last decade on amino acid requirements of sows, only very few concerned pregnant sows (NRC, 1998). The factorial approach is often used to estimate amino acid requirements of pregnant sows (Pettigrew, 1993; NRC, 1998; Dourmad et al., 1999). But, due to limited data in pregnant sows, most of these estimated requirements were derived from data from growing pigs. This lack of information explains the wide range of recommendations found in the literature for crude lysine requirement of pregnant sows (from 7.7 to 11.0 g/d) or for optimal threonine:lysine ratio (from 0.70 to 0.84) (ARC, 1981; INRA, 1989; NRC, 1998). In the past, this had little practical consequences, because usual supplies of amino acids during pregnancy were generally higher than the requirement. The situation, however, is changing. Low-protein diets may have to be fed to pregnant sows in order to reduce nitrogen excretion for environmental purposes (Adeola, 1999).

Except in the case of extreme restriction (Pond et al. 1992), protein supply during pregnancy has no measurable effects on litter size or piglet birth weight (Duée and Sève, 1978; Pettigrew, 1993). It may however reduce milk production and litter growth rate during the following lactation (Baker et al., 1970; Mahan and Mangan, 1975; Kusina et al., 1999) or decrease subsequent reproductive performance (Svajgr et al., 1972; King, 1987), especially when protein intake is also limited during lactation (Mahan, 1979).

The purpose of the present study was to determine the effects of dietary lysine supply and threonine:lysine ratio on N retention of pregnant sows.

	Materials and Methods

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Experimental Design

Two experiments were conducted with pregnant Large White multiparous sows (parity 3.8 ± 0.9) inseminated with Piétrain sperm. In Exp. 1, four dietary treatments were compared in 8 sows, in two replicates of a 4 x 4 Latin-square arrangement of treatments and time periods, balanced for carryover effects. The treatments differed by their lysine content only. In order to obtain 0.34, 0.42, 0.48, and 0.56% crude lysine in diets 1, 2, 3, and 4, respectively, increasing levels of L-lysine•HCl were added to a basal diet, in substitution for glycine. Composition and analysis of the experimental diets are given in Table 1.

For each experiment, ileal digestibility of dietary amino acids was measured in four growing pigs (50 kg BW on average) with a 4 x 4 Latin-square arrangement, using ileo-rectal anastamosis technique (Laplace et al., 1985) and routine procedures (Sève and Hess, 2000). Standardized ileal digestibility of amino acids (Table 2) was calculated according to Sève and Hess (2000) by taking into account a basal endogenous loss of amino acid proportional to DM intake.

Housing and Feeding

The sows were housed individually in gestation crates (0.6 x 2.5 m) on a flat deck with rubber bedding. The temperature within the room was controlled (22 ± 2°C) and above the lower critical temperature. During the digestibility measurements, sows were kept individually in metabolism cages equipped for collection of feces. In both experiments, feeding level amounted to 2.75 kg per day and water was available ad libitum. Feed was offered in dry pellets and divided into two equal meals (0830 and 1530). Feed spillage and eventual refusal were collected and analyzed for dry matter content.

Measurements

During each balance trial, a urinary catheter was inserted into the bladder, and urine was collected daily under sulfuric acid (40 mL of 3.6 N H₂SO₄ per L of urine), pooled, and, at the end of the period, weighed and sampled for analysis. Feces were also collected daily, pooled, and, at the end of the period, weighed, mixed, subsampled, and freeze-dried for analysis. Feed and feces were analyzed for DM, ash, and N according to AOAC (1975) methods. Gross energy was measured using an adiabatic bomb calorimeter. Nitrogen in urine was measured on fresh material, whereas energy content of urine was obtained after freeze-drying approximately 50 mL in small polyethylene bags. Amino acid analyses were performed on the eight diets and on ileal-collected material for the determination of ileal digestibility. Analyses were performed by ion-exchange liquid chromatography. Sulfur amino acids were determined after performic oxidation, and tryptophan content was measured after alkaline hydrolysis (Mossé et al., 1985).

The sows were weighed at the beginning and at the end of each balance trial. They were slaughtered at about 112 d of pregnancy. Their uterus was dissected, and the number and weight of piglets were determined.

Statistical Analysis

Data were analyzed according to a Latin-square arrangement, with main effects of animal, period and treatment. Statistical analysis was computed using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). Treatment means were separated using polynomial contrasts. When quadratic responses were found in the data (P < 0.05), amino acid requirement and efficiency were estimated by using broken line analysis, with NLIN procedure of SAS.

	Results

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Average litter size at 112 d of gestation was 12.8 and 12.1, with a 1.45 kg and 1.54 kg mean piglet birth weight, in Exp. 1 and 2, respectively. Body weight of sows during N balance was not affected (P > 0.10) by dietary treatment (249 ± 0.8 kg and 239 ± 0.6 kg, in Exp. 1 and 2, respectively) but increased linearly (P < 0.001) with gestation stage, from 228 kg at period 1 to 265 kg at period 4 in Exp. 1, and from 219 kg at period 1 to 259 kg at period 4 in Exp. 2.

Experiment 1.

No feed refusal was observed during the experimental period. Apparent digestibility of energy (86.7 ± 0.2%) and nitrogen (88.4 ± 0.2%) was not affected by dietary treatments or period (P > 0.10). Nitrogen, DE, and ME intakes averaged 62.1 (±0.1) g/d, 9.43 (± 0.02) and 9.02 (± 0.03) MCal/d, respectively, and were not different (P > 0.10) among the four treatments (Table 3). As expected, daily lysine intake increased linearly from treatment 1 to treatment 4 (Table 3). Nitrogen retention (Table 3) increased with increasing lysine supply (linear, P < 0.001; quadratic, P < 0.04). N retention was least for treatment 1 (8.0 g/d) and highest for treatments 3 and 4 that did not differ (14.6 g/d on average). Similarly, the N retention coefficient (N retained/[N intake - N feces]) increased from 14.5% for treatment 1, to 26.6% for treatments 3 and 4. Nonlinear regression analyses determined that a linear-plateau relationship existed between lysine intake and N balance. A plateau value of 14.7 g N/d occurred at an intake of 10.5 g/d digestible lysine. The linear response (Figure 1) to lysine intake is described in the following equation:

View larger version (13K): [in this window] [in a new window]   Figure 1. Effect of daily supply of standardized digestible lysine (g/d) on N retention (g/d) in multiparous gestating sows

[1]

Experiment 2.

As in Exp. 1, no feed refusal was observed during the experimental period. Apparent digestibility of energy (89.6 ± 0.2%) and nitrogen (80.3 ± 0.3%) was not affected (P > 0.10) by dietary treatments, or period. Nitrogen, DE, and ME intakes averaged 40.4 (±0.2) g/d, 8.70 (±0.03), and 8.43 (± 0.03) Mcal/d, respectively, and were not different (P > 0.10) among the four treatments (Table 4). Daily intake of crude and standardized digestible lysine amounted to 11.5 and 9.0 g, respectively, and did not differ among treatments (P > 0.10). As expected, daily intake of threonine increased linearly (P < 0.001) from treatment 1 to treatment 4 (Table 4).

	Discussion

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In Exp. 1, N retention was maximized with 0.48% crude lysine and 0.38% digestible lysine in the diet. This is slightly higher than the 0.40 to 0.43% crude lysine levels recommended for gestating sows by ARC (1981), NRC (1988), and INRA (1989). Different criteria have been used to determine the lysine requirement of gestating sows. Measurement of N balance was the most frequent (Rippel et al., 1965; Allee and Baker, 1970; Salmon-Legagneur and Duée, 1972; Woerman and Speer, 1976), but piglet performance or weight gain of sows has also been used (Baker et al., 1966; Duée and Rérat, 1974). Those studies reported requirements varying from 0.42 to 0.49% crude lysine in the diet, whereas a higher recommendation was suggested when blood urea or plasma lysine was used (Sohail et al., 1978). When calculated on a daily basis, the requirement derived from the present work (13.2 g/d crude lysine, 10.5 g/d standardized digestible lysine) is higher than that obtained in previous studies (8 to 10 g/d crude lysine) or calculated from ARC (1981), NRC (1988), and INRA (1989) recommendations (8.2, 8.6, and 10 g/d, respectively). The higher crude lysine requirement found in the present study is partly related to the low-lysine digestibility of the experimental diet (79%). However, the main reason for the higher daily requirement is the increased N retention relating higher reproductive performance and maternal weight gain in the present study. As shown by Dourmad et al. (1996) in multiparous sows, and Étienne (1991) and King and Brown (1993) in primiparous sows, N retention during pregnancy is generally limited by the energy supply. Applying the NRC (1998) model to the animal characteristics (245 kg BW) and the energy feeding level (9.4 Mcal DE/d) used in the present experiment resulted in a predicted standardized digestible lysine requirement of 11.3 g/d and 0.40% of the diet, which is somewhat higher than that measured in the present study (10.5 g/d and 0.38%, respectively).

It is possible to calculate the requirement of standardized digestible lysine for maintenance by extrapolating Eq. 1 for a zero N retention. A daily requirement of 1.66 g, corresponding to 27 mg/kg BW^0.75, is then obtained. This is in good agreement with the value measured by Baker et al. (1966) in adult nonpregnant gilts, but is lower than the 36 mg/kg BW^0.75 value reported by Fuller et al. (1989) in growing pigs at about 45 kg body weight. Stein et al. (1999) showed that the endogenous loss of lysine expressed per kg of DM intake (DMI) was similar in 112 kg BW growing pig and gestating 234 kg BW sows consuming feed ad libitum, with 428 and 413 mg/kg DMI, respectively. This corresponds to 29.2 and 26.4 mg lysine/kg BW^0.75, respectively. However, when the pregnant sows were fed restrictively, the endogenous loss increased when expressed per kg DMI (522 vs 413 mg/kg DMI) but decreased when expressed per kg BW^0.75 (16.5 vs 26.4 mg lysine/BW^0.75). Thus, the lower lysine requirement for maintenance per kg BW^0.75 found in the present study compared with the value measured by Fuller et al. (1989) might be caused by the lower daily endogenous loss resulting from the feed restriction in the pregnant sows. This also suggests that extrapolating maintenance requirements expressed per kg BW^0.75 from young growing pigs consuming feed ad libitum to adult sows fed restrictedly can lead to an overestimation of maintenance requirement. Stein et al. (1999) found a higher ileal digestibility of amino acids in sows than in growing pigs. Thus, the actual digestible lysine requirement may be lower than estimated from our results, because in the present study ileal digestibility of diets was measured on growing pigs. However, from a practical basis, standardized digestibility coefficients developed with growing pigs will still be used to formulate sow gestation diets, because specific data on amino acid digestibility for sows are not available.

Nitrogen retention increased by 1.66 g/d for each gram of supplemental standardized digestible lysine. However this efficiency can be overestimated because the N balance technique is known to overestimate N retention by about 15% when compared with comparative slaughter technique (Just et al., 1982; Quiniou et al.,1995). When this correction is applied to our data, the increase of N retention for 1 g/d standardized digestible lysine amounts to 1.41 g. This means that 1 g of increase of accreted protein (N x 6.25) requires about 0.113 g/d increase of standardized digestible lysine. This is slightly less but comparable with the value 0.129 g used for pregnant sows by NRC (1998). Assuming an average lysine content of 6.7% in the protein deposited during pregnancy (Everts, 1994), it can be calculated from our data that the marginal lysine efficiency amounts to 59% when the maintenance requirement is fixed as 27 mg/kg BW^0.75, and to 65% when the maintenance requirement is fixed as 36 mg/kg BW^0.75. However, in practice, using either one or the other relationship has little effect on the calculation of the total requirement.

The present results can be used to evaluate the digestible lysine requirement of pregnant sows or gilts, according to the factorial approach, in a way similar to that used by NRC (1998). Using data on maximal N retention reviewed by Dourmad et al. (1999), it can be calculated that the standardized digestible lysine requirement increases from 1.13 g/Mcal DE at 30 d of pregnancy to 1.69 g/Mcal DE at 105 d of pregnancy in first litter sows, and from 0.86 g/Mcal DE at 30 d to 1.35 g/Mcal DE at 105 d in multiparous sows.

In Exp. 2, N retention was not improved above a standardized digestible lysine:threonine ratio of 0.71. This ratio is similar to that recommended for pregnant sows by NRC (1988), but lower than that (0.84) recommended by ARC (1981). The NRC (1998) amino acid recommendations were calculated according to the factorial approach. As a consequence, the optimal threonine:lysine ratio varies with the relative contributions of maintenance and protein accretion. It is higher in heavy sows (0.84) than in lighter sows (0.75). A true digestible threonine:lysine ratio of 0.78 is calculated when the NRC (1998) approach is applied to our experimental conditions. This is higher than the 0.71 value obtained in the present experiment for maximal N retention. In NRC (1998), the requirement for maintenance was derived from measurements on growing pigs (Fuller et al., 1989) and extrapolated on the BW^0.75 basis. As previously discussed for lysine, this can lead to an overestimation of the maintenance requirement. When the requirement is recalculated with lysine and threonine maintenance requirements of 27.0 mg/kg BW^0.75 and 36.6 mg/kg BW^0.75, respectively, the ratio is decreased to 0.72, which is in good agreement with the value obtained in the present study. When expressed on an apparent digestible basis, the optimal threonine:lysine ratio is lower than that expressed on a crude or a standardized digestible basis (0.67 vs 0.71 and 0.72, respectively), because endogenous losses, which have a relatively high threonine content, are not considered in the calculation.

From the two experiments, it is possible to evaluate the crude threonine requirement for this kind of sow, when N retention is maximized. The calculated requirement (0.35% crude threonine and 0.27% standardized digestible threonine) is within the range of literature results (0.30 to 0.36% crude threonine: Duée, 1977; Leonard and Speer, 1983; and Cuaron et al., 1983). However, as discussed for lysine, the daily requirement is higher than in previous recommendations because of a higher daily N retention in present conditions.

	Implications

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The results obtained in the present study can be used to improve the factorial determination of amino acid requirements of gestating sows. For lysine, the values obtained are close to those calculated in NRC (1998), although the partition between maintenance and retention is modified. The optimal threonine:lysine ratio of 0.71 found for multiparous gestating sows in the present study is lower than the ratio calculated by the factorial approach used in NRC (1998), primarily because the daily maintenance requirement is overestimated when data from growing pigs are extrapolated to pregnant sows on the basis of metabolic body weight.

	Footnotes

 
¹ The authors wish to acknowledge A. Roger, Y. Lebreton, and F. Le Gouevec for taking care of the animals, R. Vilboux for the preparation of the experimental diets, V. Beaumal and C. David for laboratory analyses, and M. Eudaimon for amino acid analyses.

² This work received financial support from Ajinomoto Eurolysine.

Received for publication July 9, 2001. Accepted for publication April 19, 2002.

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