Abstract
Low-protein (LP) diets are increasingly recognized as a cost-effective and efficient strategy in the global livestock industry. Although previous studies have demonstrated that increased valine intake during pregnancy can improve reproductive performance and mammary gland development in gilts, the effect of the standardized ileal digestible valine-to-lysine ratio (Val/Lys) in LP diets during lactation is not well understood. A total of 60 lactating sows (Landrace × Large White; 4th parity) were randomized by backfat and weighed into 6 groups (n = 10): a standard protein group (17% crude protein, Val/Lys of 0.87) and 5 LP diet groups (14.3% crude protein, Val/Lys of 0.64, 0.74, 0.84, 0.94 or 1.04). The experiment lasted from day 0 of lactation to day 21 of lactation. The results indicated that LP diets reduced (P < 0.05) weaning weight and average daily gain (ADG) in piglets, as well as sow milk yield; whereas, increasing Val/Lys in LP diets improved (P < 0.05) these parameters. A linear model determined an optimal Val/Lys ratio of 0.94 in LP diets for maximizing piglet ADG. Low-protein diets decreased jejunal trypsin levels in piglets, which increased (P < 0.05) when Val/Lys was increased. Furthermore, LP diets reduced the expression of transporters (solute carrier family 38 member 2, SLC38A2) and metabolic enzymes (branched-chain amino acid transaminase 1 and 2, BCAT1 and BCAT2) in the piglet jejunum; conversely, increasing Val/Lys enhanced these expressions. Levels of L-valine, DL-arginine and L-threonine were found to decrease (P < 0.05) in response to LP diets, as revealed by the results from sow serum metabolites. However, these concentrations rebounded (P < 0.05) when Val/Lys was increased to 0.94. Correlation analysis revealed a positive association between sow serum valine metabolites and jejunal expression of SLC38A2, BCAT1 and BCAT2 in piglets. Low-protein diets also led to a decrease in the protein levels of metabolic enzymes (P < 0.05), which were improved (P < 0.05) by increasing Val/Lys. Finally, correlation analysis revealed a positive correlation between sow serum metabolites, jejunal enzyme expression and piglet growth performance. Collectively, growth performance in piglets may be impaired by LP diets for sows due to valine transport and catabolism being disrupted, whereas growth performance may be enhanced by increasing Val/Lys in LP diets via sow milk metabolites, thereby improving piglet digestion and absorption.