Effects of dietary threonine on performance, plasma parameters and immune function of growing pigs

Abstract

Sixty-four growing pigs (Beijing Black × Landrace × Duroc), weighing an average of 17.5 ± 0.5 kg, were divided into four groups with four pens per treatment (two gilts and two castrates per pen) and fed diets containing various levels of threonine to determine its effects on performance, plasma levels of free amino acids, plasma urea nitrogen and immune function during a 4 week trial. The basal diet was based on maize and soybean meal, supplemented with rapeseed meal and cottonseed meal, and contained 9.2 g kg −1 lysine and 5.9 g kg −1 threonine. l-threonine was added to the basal diet to provide 6.8, 7.7 and 8.9 g kg −1 threonine in the remaining three diets. On day 7, all pigs were injected with either Bovine Serum Albumin (BSA) or Swine Fever Attenuated Vaccine (SFAV) to determine humoral antibody response. The addition of threonine improved weight gain with the quadratic and cubic polynomial contrasts being significant ( p < 0.05). Feed conversion was also significantly improved with the linear, quadratic and cubic polynomial contrasts being significant ( p < 0.01). The highest weight gain occurred at 6.8 g kg −1 threonine and the best feed conversion at 8.9 g kg −1 threonine. The concentration of plasma urea nitrogen decreased ( p < 0.01) and reached a plateau when dietary threonine increased from 5.9 to 6.8 g kg −1. Serum threonine increased ( p < 0.01) and serum lysine decreased ( p < 0.01) as dietary threonine increased, with both amino acids showing sharp inflection points at 6.8 g kg −1 dietary threonine. Increasing dietary threonine levels increased serum IgG concentration ( p < 0.01) and anti-BSA antibody ( p < 0.01) level. In conclusion, although maximum growth rate of 17–31 kg pigs occurred at a dietary threonine level of 6.8 g kg −1, higher threonine levels were needed to maximize humoral antibody production and IgG levels.