The Optimal Dietary Threonine to Lysine Ratio for Growing Pigs

The growth of 64 pigs was monitored over grower and finisher periods of 26-58 and 58-96 kg respectively, to see if the optimum dietary ratio of threonine to lysine (T/L) under ad Zibitum feeding was higher than the generally accepted ratio of 0.6. Five basal (Bl-B5) diets were used to establish the pig’s response to lysine (in the range 6.0 to 11.24 and 5.59 to 10.5 g kg-1 for grower and finisher periods respectively) when dietary T/L was held constant at 0.6 and other essential amino acids were supplied relative to lysine at least 20% above accepted specificatioris. A further three response (RI-R3) diets, matching the lowest three lysine diets of the B series (except for the inclusion of free I-threonine to increase T/L to 0.7) were fed to quantify response to threonine. Measurements were made of average daily gain (ADG), feed conversion ratio (FCR) and protein deposition rate (PDR). PDR was estimated from relationships between whole body protein content and P2 backfat measurements on the live animal. The metabolism of five diets representative of the range of B (3) and R (2) diets in the grower growth assay was measured to document nutrient digestibility and nitrogen (N) retention. In the metabolism study, differences in nutrient digestibility and N retention between companion B and R diets were not significant. Percent N retained and nutrient digestibility (except for dry matter) improved (PcO.05) with increasing dietary lysine content, but the magnitude of the response was small (viz: N retention, 50.2 to 56.6%; N digestibility, 86.4 to 89.7%; and digestible energy, 14.52 to 14.69 MJ kg-l). For the B series diets of the growth assay, ADG, FCR and PDR improved linearly (PcO.01) with increasing lysine content up to the second highest level (9.62 and 8.95 g lysine kg-1 for grower and finisher periods respectively). Increasing the dietary T/L ratio from 0.6 to 0.7 significantly (PcO.05) improved ADG and PDR but not FCR in the finisher period with similar trends being apparent in the grower period. Based on the proportionality of the response to the higher T/L ratio and that to lysine for ADG and PDR, the optimum T/L ratios derived for the grower and finisher diets were 0.64 and 0.68 respectively.