Valine: a limiting amino acid for high-producing lactating sows Valine: a limiting amino acid for high-producing lactating sows

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Introduction
Recent research at Kansas State University, University of Kentucky, and University of Minnesota suggests that the lysine requirement of the high-producing sow is much greater than listed by NRC (1988).This change in lysine recommendations has led to questions concerning the requirement of other amino acids.We typically use an ideal amino acid ratio to determine the requirement for other amino acids based on lysine.The amino acid ratios suggested by NRC (1988) and ARC (1981) are used most often (Table 1).The ARC ratio is based on the composition of milk, whereas the NRC ratio was developed from feeding trials.These ratios are similar for all amino acids except valine.The reason that the valine to lysine ratio is important is that, according to the NRC ratio, valine is the second limiting amino acid in high protein diets for the lactating sow.If 3 lb of synthetic lysine is added to the diet, valine actually is the first limiting amino acid using the NRC ratio.However, valine is not a concern using the ARC ratio.Therefore, this experiment was designed to determine if the valine to lysine ratio suggested by ARC (1981) is too low. 1 Appreciation is expressed to Nutri-Quest, St. Louis, MO for providing amino acids for this study.The authors also wish to thank Phillips Farm, Drexel, MO for data collection and use of facilities and animals.

Procedures
On a commercial swine operation, 152 lactating sows were randomly allotted at farrowing to the experimental diets.Diets were formulated to contain .9%lysine and .75 or .9%valine giving valine to lysine ratios of .83:1 and 1:1, respectively (Table  2).L-valine replaced corn to formulate the high valine diet.All other amino acids were fortified at 105% of the ratio suggested by NRC (1988) (Table 3).Diets were corn-soybean meal-based and contained equal amounts of all synthetic amino acids except valine.Diets were calculated to contain .9%Ca and .8%P.

Discussion
Increasing dietary valine from .75% to .90%resulted in a substantial increase in litter weaning weights.These results indicate that the ideal valine to lysine ratio is greater than .83:1(.75% valine in a diet containing .90% lysine).Certainly, the ratio of .70:1proposed by ARC (1981) from the amino acid composition of milk is too low.We are currently conducting further research to more closely determine the valine requirement of the high-producing sow.
The practical aspect of this research is the impact that it has on synthetic amino acid use in sow lactation diets.Previously, most nutritionists believed that the most limiting amino acids in corn-soybean meal diets for lactating sows were lysine, threonine, methionine, and tryptophan.Thus, a common recommendation was to use .15%L-lysine HCl in sow lactation diets.The impact of synthetic lysine on the order of limiting amino acids in sow lactation diets is demonstrated in Tables 5 and 6.For these tables, the NRC (1988) and ARC (1981) ratios were used to determine the deficient amino acids in corn-soybean meal diets formulated without (Table 5) or with (Table 6) L-lysine HCl.These tables demonstrate that the order of limiting amino acids changes as lysine levels increase.Lysine is the first limiting amino acid at the lower dietary lysine levels.However, at high lysine levels (> .90%),valine becomes the first limiting amino acid using the NRC (1988) ratio (Table 5).When synthetic lysine is used in formulation (Table 6), valine becomes limiting at a relatively low lysine level (.7%).
Our research indicates that valine deficiencies limit the use of L-lysine HCl in sow lactation diets and that diets formulated on predictions of amino acid requirements based on milk production and maintenance will underestimate the valine requirement of the lactating sow.

Table 4 . Influence of Valine Level (.75 or .90%) in Sow Lactation Diet on Litter Performance
b P<.04.

Table 6 . Amino Acid Ratios of Corn-Soybean Meal Diets with 3 lb L-Lysine HCl
b Deficient according to ARC (1981) amino acid ratio.