Effects of dietary tryptophan: Lysine ratio on the reproductive performance of primiparous and multiparous lactating sows

The objective of this study was to determine the effects of dietary tryptophan (Trp) to lysine (Lys) ratio on the performance of lactating sows. Thirty primiparous and 195 multiparous lactating sows (Landrace × Yorkshire) were allocated on the basis of parity, body weight, and backfat to 5 dietary treatments according to a randomized complete block design with 5 blocks of farrowing date. The 5 experimental diets contained 0.16%, 0.19%, 0.23%, 0.26%, or 0.29% standardized ileal digestible (SID) Trp and SID Lys level was identical (0.87%) for all diets, with the Trp:Lys ratio being 0.18, 0.22, 0.26, 0.30, and 0.33, respectively. Sows received their assigned lactation diets from day 109 of gestation to weaning. Litter size was standardized to 10-12 piglets within 2 days after farrowing. The average lactation length was 19 days and no creep feed was provided. Changes of sow body weight and backfat during lactation, weaning-to-estrus interval, litter size and weight, litter weight gain, preweaning piglet mortality, and performance of subsequent parity (total born alive, stillborns, and mummies) were recorded. A quadratic pattern of body weight loss (P = 0.046), backfat loss (P = 0.067), and average daily feed intake (P = 0.096) with increasing Trp level was observed for the primiparous sows during lactation, and the losses were minimized with the SID Trp:Lys ratio of 0.25 (0.22% SID Trp) and average feed intake was maximized with the SID Trp:Lys ratio of 0.22 (0.19% SID Trp) according to a segmented regression analysis model. Nevertheless, preweaning piglet mortality of the primiparous sows increased linearly and quadratically (P < 0.001) as Trp level increased. On the contrary, piglet mortality of the multiparous sows decreased linearly (P = 0.031) with increasing dietary Trp and the mortality was noticeably reduced when the SID Trp:Lys ratio reached 0.26 (0.23% SID Trp). Furthermore, stillborn piglet numbers of multiparous sows in the subsequent parity decreased linearly (P = 0.035) with increasing level of Trp. Other parameters were not significantly (P > 0.05) influenced by the dietary treatment. In conclusion, our results indicate that the optimal Trp:Lys ratio for lactating sows is between 0.22 and 0.26.