Effects of graded levels of true ileal digestible threonine on performance, serum parameters and immune function of 10–25 kg pigs

Abstract

Two experiments were conducted to determine the effects of feeding graded levels of true ileal digestible (TID) threonine on the performance, serum parameters and immune function of 10–25 kg pigs. In the first experiment, four crossbred (Large White×Landrace) castrates, weighing approximately 35 kg, were surgically fitted with simple T-cannula in the distal ileum. The cannulated pigs were used in a 4×4 Latin-square design experiment to determine the coefficient of ileal true digestibility (CITD) for the amino acids in the test ingredients. Four semi-purified diets (based on maize starch, sucrose and soybean oil) were formulated using maize, peanut meal, enzymatically-hydrolyzed casein and a combination of maize and dried whey as the sole sources of dietary protein. The enzymatically-hydrolyzed casein diet was used to evaluate the flow of the endogenous amino acids. Each diet was fed for 7 days. Titanium oxide (1.0 g kg −1) was added to all diets as an indigestible marker. Ileal digesta was collected for a 12 h period during each of the last 2 days of the four feeding periods. The CITD for lysine and threonine in maize (0.85 and 0.89), peanut meal (0.88 and 0.92), and dried whey (0.87 and 0.92) were calculated from this data and used to formulate the diets fed in the follow-up performance experiment. In this experiment, 90 weaned piglets (Large White×Landrace) were fed diets containing one of five levels of true ileal digestible threonine (5.3, 5.8, 6.5, 7.5 and 8.5 g kg −1). The pigs were housed in groups of three with each diet fed to six pens of pigs. The pigs were fed the experimental diets for 28 days. On Day 14 of the experiment, all pigs were injected with ovalbumin at a rate of 1 mg kg −1 body weight (BW). Serum IgG concentrations were determined on Days 14, 21 and 28, while serum amino acid and urea nitrogen concentrations were determined on Days 14 and 28. Over the entire experimental period, incremental increases in true ileal digestible threonine intake increased weight gain (P=0.047) and improved feed conversion (P=0.002), but had no effects on feed intake (P=0.822). Both weight gain and feed conversion were maximized at an intake of 5.9 g day −1 of true ileal digestible threonine. Serum urea nitrogen concentrations decreased (P=0.009) as the intake of true ileal digestible threonine increased from 4.1 to 5.0 g day −1 and then appeared to increase as the true ileal digestible threonine intake increased from 5.0 to 6.6 g day −1 (quadratic effect, P=0.015). Serum threonine increased (P=0.001) while levels of histidine, isoleucine and valine decreased (P=0.001) as the intake of true ileal digestible threonine increased from 4.1 to 6.6 g day −1. Serum IgG concentrations increased (P=0.002) in response to increased intake of true ileal digestible threonine, with the highest serum IgG concentration observed in pigs ingesting 6.6 g day −1 of true ileal digestible threonine. Based on the results of the performance trial, we conclude that the intake of true ileal digestible threonine required to maximize the weight gain and feed conversion of 10–25 kg pigs is 5.9 g day −1. To optimize immunity, pigs should consume 6.6 g day −1 of true ileal digestible threonine.