Effects of Reducing the Digestible Lysine and Tryptophan to Effects of Reducing the Digestible Lysine and Tryptophan to Lysine Ratio on Growth Performance of Grow-Finish Pigs Lysine Ratio on Growth Performance of Grow-Finish Pigs

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Introduction
During the COVID-19 outbreak in 2020, the swine industry was forced to find dietary strategies to decrease growth rate of pigs due to plant closures and slowdowns. Lysine is the first limiting amino acid in corn soybean-meal-based diets. Decreasing the level of Lys in the diet will decrease protein deposition of the pig and, consequently, reduce overall growth rate. The second limiting amino acid in diets high in corn protein is Trp. Previous research has shown that reducing Trp in the diet from 19 to 16% of Lys will reduce feed intake, weight gain, and worsen feed efficiency (F/G). 4 Therefore, reducing Lys along with Trp could be a potential approach to further slowdown growth rate of pigs to increase the amount of time needed to reach market weights.
The objective of this study was to determine the effects of reducing Lys and Trp on growth performance of grow-finish pigs. The hypotheses were: 1) reducing the level of Lys in diets will impair growth performance, and 2) reducing Lys in diets with a further reduction in the Trp:Lys ratio will decrease weight gain and slowdown growth of pigs in the grow-finish period.

Materials and Methods
The Kansas State University Institutional Animal Care and Use Committee (IACUC) approved the protocol used in this experiment. The experiment was conducted at a commercial research facility in southwest Minnesota. The barn was naturally ventilated and double-curtain-sided and pens had completely slatted flooring and deep pits for manure storage. Each pen was equipped with a 3-hole stainless steel dry self-feeder (Thorp Equipment, Thorp, WI) and a cup waterer for ad libitum intake of feed and water. The facility was equipped with a computerized feeding system (FeedPro; Feedlogic Corp., Willmar, MN) that delivered and recorded daily feed additions.

Animals and diets
A total of 1,080 pigs (327 × 1050, PIC; initially 71.3 ± 1.91 lb) were used in a 119-d growth trial arranged in a randomized complete block design with 27 pigs per pen and 10 pens per treatment. Dietary treatments were fed in 7 phases and consisted of a control regimen (100% of the estimated SID Lys requirement for pigs in this facility) formulated to contain 1.10, 1.01, 0.91, 0.83, 0.79, 0.71, or 0.67% SID Lys from 70 to 90, 90 to 112, 112 to 159, 159 to 187, 187 to 216, 216 to 246, and 246 to 286 lb, respectively. Two additional regimens contained 90 or 80% of the SID Lys estimate. These 3 regimes were formulated to a SID Trp:Lys ratio of 19% except for the last dietary phase that contained 17% SID Trp:Lys ratio. The fourth regimen contained 80% of the SID Lys estimate with 16% SID Trp:Lys in all phases.
Pens of pigs were weighed, and feed disappearance measured approximately every 2 weeks throughout the experiment to determine ADG, ADFI, and F/G. Two weeks before the end of the experiment, 2 pigs per pen were removed and marketed. Consequently, the remaining pigs in the pen were weighed and marketed on October, 2020, final weights were obtained, and no carcass data were collected because of the ongoing outbreak of COVID-19.

Statistical analysis
Pen was the experimental unit for all data. Response variables were analyzed using a general linear mixed model. Linear and quadratic contrasts were used to evaluate the effect of SID Lys reduction (treatments 100, 90, and 80) and pairwise comparisons were used to compare the 80% SID Lys treatment with the 80-16% SID Trp:Lys treatment. Heterogeneous residual variances as a function of the response variables were fitted as needed. Model assumptions were checked and considered to be appropriately met. The experimental data were analyzed using the lme4 package in R program (version 3.5.2, R Foundation for Statistical Computing, Vienna, Austria). Results were considered significant at P ≤ 0.05 and a tendency at P ≤ 0.10.

Results and Discussion
For the overall period, ADG decreased (linear, P < 0.001) and a tendency (P = 0.087) for poorer F/G was observed with decreasing SID Lys in the diet. There were no treatment differences (P = 0.176) observed in ADFI. Within the 80% SID Lys treatments, ADG was decreased (P < 0.05) and F/G tended (P < 0.10) to worsen when a further reduction of the SID Trp:Lys ratio was applied.
In conclusion, as expected, decreasing SID Lys decreased ADG and worsened F/G. Pigs fed 90 and 80% of the estimated Lys requirement for these pigs were approximately 9.5 and 18.9 lb lighter than pigs fed at their estimated requirement. When pigs were fed 80% of the Lys requirement and were also restricted in the Trp:Lys ratio, a further reduction in growth performance and final BW was observed. These pigs were 25.8 lb lighter in final BW compared with those fed 100% of the Lys requirement estimate. This study presents useful information for producers regarding dietary changes to slowdown growth of pigs during the grow-finish period.
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