The ideal protein (IP) concept represents the ratio of digestible essential amino acid (AA) to lysine where all essential AA are equally limiting. However, sub-clinical enteric challenges may increase specific AA requirements for immune responses and AA malabsorption compensation. Here, we assessed effects of threonine (Thr), arginine (Arg) and glutamine (Gln) supplementation to low crude protein (CP) diets on bird resilience to sub- clinical enteric challenges, organ weights and digestible CP intake. Ross308 day-old male broilers (14 birds/pen) underwent two challenge treatments factorially combined with four feeding treatments (n=9 pens per challenge-diet combination). Birds were either unchallenged (placed on clean litter; gavaged with water) or challenged (placed on reused litter; gavaged with 2500 Eimeria maxima oocysts at d14). Diets were high-CP (D1, 22.3% CP; Thr 0.86%; Arg 1.37%; Gln: 4.36%), low-CP (D2, 20.3% CP; Thr 0.86%, Arg 1.37%; Gln 3.67%), D2 with glycine (Gly) (D3, 21.7% CP; Thr 0.86%; Arg 1.37%; Gln 3.72%; Gly 2.23%), and D3 supplemented with extra Thr, Arg and Gln at the expense of Gly (D4, 21.9% CP; Thr 1.08%; Arg 1.70%; Gln 4.82%; Gly 0.72%). Birds were fed starter mash (d0 to d10) and grower pellets (d10 to d28), with D1 and D2 formulated to IP concept. On day 28, empty gizzard weight and ileal CP digestibility were assessed to calculate digestible CP intake (d25-28). Treatment effects were assessed via 2×4 ANOVA using d10 body weight (BW) as a covariate for d10-28 data; Tukey-adjusted multiple comparisons located effects at P<0.05. Feeding and challenge treatments did not interact. By d10, challenge increased BW (205 vs 212 g; s.e.d. 3) and reduced feed conversion ratio (FCR; 1.37 vs 1.31; s.e.d. 0.03), both P=0.018), whilst D1 birds were heavier than D2, D3 and D4 birds (219, 207, 204 and 202 g, respectively; s.e.d. 4; P<0.01). By d28, subsequent E. maxima challenge reduced final BW (1480 vs 1242 g; s.e.d. 17), increased d10 to d28 FCR (1.30 vs 1.40; s.e.d. 0.02) and reduced gizzard weight (29.5 vs 26.3 g; s.e.d. 0.68), all P<0.001. D1 birds were heavier than D2, D3 and D4 birds (1430, 1363, 1333 and 1318 g, respectively; s.e.d. 24) and had greater digestible CP intake (26.8, 23.9, 24.8 and 23.1 g/d, respectively; s.e.d. 0.71), all P<0.001.
In our study, AA supplementation to IP based reduced CP diets did not improve resilience to sub-clinical enteric challenges, potentially due to AA imbalances. The reduced CP ration may have underperformed due to limiting non-essential AA, subclinical acidosis due to dietary electrolyte imbalance, or reduced ability to apply IP concept successfully during the starter phase.
