Determining an optimal lysine : energy ratio for lean growth in a modern commercial pig genotype

Abstract

Intense genetic selection for reduced back fat thickness and improved feed utilisation in pigs has resulted in modern genotypes with high lean gain potential, which now deposit a greater amount of lean tissue at faster rates than 20 years ago. As a consequence, to allow pigs to reach their genetic potential for a high capacity of lean tissue gain, higher levels of lysine relative to energy must be fed. The lysine: energy ratio can be largely influenced by genotype, sex, age and health status of the pig. Thus, continues efforts are required to characterise the effects of increasing dietary lysine in evolving modern pig genotypes reared in commercial production environments. The objective of this study was to determine the optimal lysine: energy ratio required for lean growth of a modern pig genotype (PIC337), as well as to determine the growth performance potential under typical commercial conditions. One-hundred-and-eighty PIC337 entire male pigs were used in an experiment with a 2x3 factorial arrangement, including 2 energy levels (2560 kcal NE/kg and 2161 kcal NE/kg) and 3 lysine levels (80%, 100%, 120% of PIC recommendations) in the feed. Thirty-six pens, with 5 pigs per pen, were randomly allocated to 6 treatments (n=6 replicates/treatment). The boars were 9 weeks (63 days) of age at the start of the trial and reared for a period of 17 weeks until slaughter under typical commercial conditions. Average feed intake per pen was measured weekly and all pigs were weighed bi-weekly along with P2 back fat measurements. Average daily feed intake (ADFI), average daily gain (ADG), and feed conversion ratio (FCR) were calculated. The trial continued until 26 weeks (182 days) of age after which all the boars were slaughtered to determine carcass characteristics, including hot carcass weight, cold carcass weight, drip loss %, pH initial, pH ultimate, carcass temperatures as well as back fat thickness. During the grower phase (9 to 18 weeks of age), energy had a significant effect (P <0.05) on body weight gain and FCR, but had no effect (P >0.05) on ADG or ADFI. During the same phase, standardized ileal digestible (SID) lysine had no significant effect (P >0.05) on body weight gain, ADG or ADFI, but had a significant effect (P <0.05) on FCR. Lysine: NE ratio had a significant effect (P <0.05) on FCR and ADFI. During the finisher phase (18 to 26 weeks of age), energy and lysine had significant effects (P <0.05) on body weight gain, as well as FCR, but not on ADG. However, pigs from the high energy treatments (T1, T2, and T3) and the high lysine treatments (T3 and T6) had higher body weight gains and ADG as well as reduced FCR and ADFI compared to the low energy and low lysine treatments for both the grower and finisher phases. Lysine had no effect (P >0.05) on carcass characteristics, whereas energy only had significant effects (P <0.05) on the hot and cold carcass weights as well as back fat thickness. However, the low energy and the two high lysine treatments (T3 and T6) resulted in leaner carcasses compared to the high energy treatments. Similarly, back fat deposition was reduced as the SID lysine content of the diets increased. Improvements in growth performance and feed efficiency were observed as the lysine: NE ratio increased. The optimum lysine: NE level for ADG from 9 to 18 weeks and 18 to 26 weeks of age was found to be 6.06 g of SID Lys/ MCal NE and 3.67 g of SID Lys/ MCal NE, respectively. The lysine: NE ratio required to optimise FCR for boars from 9 to 18 weeks of age and 9 to 26 weeks of age was found to be 6.06 g of SID Lys/ MCal NE and 4.4 g of SID Lys/ MCal NE, respectively. The lysine: NE ratios for optimised growth performance was found to be higher compared to other available literature. The study showed that high energy as well as high lysine levels in the diet improve growth performance but due to pig producers being paid for carcass composition as well as carcass weight, a low energy diet with high lysine levels will allow for greater return on investment as it will ultimately yield leaner as well as bigger carcasses. The study also suggests that the lysine requirements for the modern pig has increased, especially during the finishing phase.