Optimal standardized ileal digestible lysine levels in low-protein diets: Effects on performance of lactating sows and their offspring

The present study systematically evaluated the physiological and productive responses of lactating sows to LP diets, with particular emphasis on the optimal requirement of standardized ileal digestible lysine (SID Lys) under LP diets. A total of 84 “Landrace × Large White” sows with similar parity, expected delivery dates and body weight (BW) were selected and randomly divided into 6 groups with 14 replicates in each group and one sow in each replicate. The control diet (CON) group was formulated to 18% CP and 0.78% SID Lys. Five LP diets (15% CP) were formulated with graded SID Lys levels at 0.66%, 0.72%, 0.78%, 0.84%, and 0.90%, respectively. Dietary net energy (NE) and CP levels were consistent in all groups. The trial lasted for 28 days (from day 107 of gestation to day 21 postpartum). Reducing dietary CP by 3 percentage points to 15% did not affect sow BW loss or subsequent estrus performance (P > 0.05). LP diets increased colostrum IgM content (P < 0.05), improved apparent digestibility of dietary CP, and reduced serum urea nitrogen (SUN) and milk urea nitrogen (MUN) concentrations (P < 0.05). However, dietary CP reduction significantly decreased (P < 0.05) serum IGF-1 levels and the digestibility of dietary crude fiber (CF). When the dietary SID Lys level in LP diets was further decreased to 0.66%, both sow and piglet production performance significantly decline. However, increasing SID Lys to 0.84% in LP diets improved piglet performance, and the overall performance of both sows and piglets was not significantly different from that of CON, with the 0.84% SID Lys-LP diets group having the highest serum AA utilisation. A nonlinear fitting of sensitive indicators with linear and quadratic effects was conducted to estimate optimal SID Lys requirements for lactating sow. 14 d SUN in sows reached a breakpoint at 0.80% (broken line linear model) and 0.86% (broken line quadratic model). The SID Lys level needed for maximizing piglet benefits (weaning litter weight, individual weight) was 0.81% (broken line linear model) and 0.90% (broken line quadratic model). BW loss for sows reached a breakpoint at 0.85% (broken line linear model) and 0.90% (broken line quadratic model). In conclusion, LP diets had no negative effects on sow performance, but it affected piglet growth performance. Decreasing the dietary SID Lys in LP diets of sows significantly affects the growth and development of 21 d suckling piglets, further increasing the SID Lys in LP diets and supplementing with the crystalline amino acids (Met, Thr, Trp, and Val) could improve above problems. Broken line linear and quadratic model suggests the optimal range for SID Lys in LP diets for sows is between 0.80% and 0.90%.