An experiment was conducted to evaluate the response to low crude protein (CP) while maintaining equal standardized ileal digestible (SID) LYS and equal SID amino acid-to-LYS ratios (including MET+CYS, THR, TRP, VAL, ILE, and LEU) to test whether the addition of LEU and ILE allowed CP to drop below 19.5% without impacting performance when SID LYS was 1.35%. For the experiment, 288 pigs (~22d age, 6.29 ± 0.71, 8 reps/trt, 6 pigs/pen) were stratified by weight and randomly allotted to one of 6 treatments in a randomized complete block design. Treatments had a fixed SID LYS of 1.35% and CP ranging from 21 to 17.25% in 0.75% increments. Experimental diets were fed for 21 days, and body weights and feed disappearance were recorded. Visual fecal scores (1 = normal to 4 = watery) and visual pig appearance scores (1 = pale and hairy to 4 = normal color and well-rounded muscle) were assessed on ordered likert scales to determine the probabilities of observing a more normal stool quality and a worse visual appearance, respectively. Performance data was analyzed as a general linear model. Stool quality and visual appearance were analyzed as a generalized linear mixed model. Contrasts were used to test linear and quadratic effects of dietary CP. Results of the experiment are presented in Table 1. With constant SID Lys of 1.35%, reducing CP from 21.00 to 17.25% resulted in an increase then a decrease (linear P<0.005; quadratic, P < 0.005) in Body Weight, ADG, ADFI, and gain:feed. Average daily gain and ADFI began decreasing when CP decreased below 20.25%, whereas body weight and gain:feed began decreasing once CP was below 19.50%. There was a difference detected amongst treatments in stool quality, but this was not due to changes in crude protein levels of the diets. This suggests that protein fermentation was not directly impacting stool quality in these pigs. Decreasing dietary CP also linearly (P<0.05) and quadratically (P<0.05) affected the visual appearance of the pigs in a similar manner to performance. At the lowest levels of crude protein, pigs had a higher probability of having more coarse hair and paler skin suggesting that a nutritional stress response may be occurring.
These results indicate that the addition of ILE and LEU to the formula do not allow CP to decrease below 19.5% without affecting performance when SID LYS is 1.35 in young pigs.