Tryptophan:Lysine ratio for pigs from 15 to 30 kg of body weight
The aim of this study was to determine the digestible Tryptophan:Lysine (Trp:Lys) ratio for starting pigs (15 to 30 kg). A previous study was carried out to determine the true ileal amino acids digestibility of the diet with low Trp and Lys levels. In the growth performance study, a total of 64 individually housed barrows (15±0,23 kg) were allotted in a completely randomized block design, with 16 treatments and four replicates. Treatments were arranged as a 4 x 4 factorial scheme (four levels of digestible Trp: 0.155, 0.185, 0.215, 0.245% and four levels of digestible Lys: 0.972, 1.112, 1.252 and 1.392%). Glutamic acid was used to keep the diets with the same nitrogen levels. At the end of the study, body weight (BW) and feed intake were recorded and feed:gain ratio (F:G) calculated. Ultrasonography was performed using an Aloka (SSD 500) and Sonograder (Renco®) equipments, and measurements of backfat thickness and loin depth were performed using the ImageJ software. Considering the regression analysis, the BW (P=0.03) and average daily gain (P=0.005) increased linearly with increased levels of digestible Trp. By surface response method, the average daily feed intake (ADFI) increased linearly (P=0.014) as the levels of digestible Trp increased in the diets, while Lys showed a quadratic effect (P=0.004). For the ADFI the optimum Lys level was estimated at 1.195%. An interaction between digestible Trp and Lys (P=0.056) was found on F:G. Considering the surface response method, there was a quadratic effect (P=0.042) of digestible Trp and Lys levels on F:G, estimating on 0.210 and 1.198% the optimum levels of digestible Trp and Lys levels, respectively, which results in a Trp:Lys ratio of 0.175. No differences (P>0.05) were observed on backfat thickness. A quadratic effect of digestible Trp (P=0.021) and Lys (0.026) levels was observed on loin depth (measured through Aloka SSD 500), which by surface response method the optimum Trp and Lys levels were 0.208 and 1.136%, respectively, providing a Trp:Lys ratio of 0.183%. Using the regression analysis we observed a quadratic effect (P=0.042) of digestible Trp on loin depth (Sonograder - Renco®), which was estimated to be optimum at 0.210%. In conclusion, the optimum digestible Trp and Lys levels for growth performance are 0.210 and 1.198%, and for loin depth are 0.208 and 1.136%, which provide a Trp:Lys ratio of 0.175 and 0.183%, respectively.
Keywords: response surface; growth performance, requirements.