Ruminants rely on microbial protein as a significant source of amino acids (AA), yet ambitious production goals may increase AA requirements beyond supplies of AA provided by microbial protein. Providing growing cattle with adequate quantities of post-ruminal, absorbable amino acids (AA) is essential for optimizing cattle performance. Two trials were conducted to determine which AA were limiting for Angus × Holstein crossbred steers fed a corn-based diet. In trial 1, seven ruminally cannulated steers (192 ± 13 kg) were used in an experiment with a 6 × 6 Latin square design. Treatments were infused abomasally across 6-d periods and included: 1) control (water infusion); 2) 6 g/d lysine (6Lys); 3) 2 g/d methionine (2Met); 4) 6 g/d lysine plus 2 g/d methionine (6Lys2Met); 5) 12 g/d lysine plus 4 g/d methionine (12Lys4Met); 6) 12 g/d lysine plus 4 g/d each of methionine, histidine, leucine, isoleucine, valine, threonine, arginine, phenylalanine, and 1 g/d tryptophan (MIX). The additional steer received the same treatment sequence as another steer. Steers were fed 4.5 kg/d (DM basis) of a corn-based diet (57% dry-rolled corn, 23% corn gluten feed, and 16% grass hay). Periods included 2-d adaptations and 4 d for total fecal and urine collections to measure nitrogen retention. Data were analyzed as a Latin square with fixed effects of treatment and period and random effect of steer. Means were separated with pairwise t-tests. MIX led to the greatest (P< 0.01) nitrogen retention (33.2 g/d), followed by 6Lys2Met (29.5 g/d, P=0.01) and 12Lys4Met (29.3 g/d, P=0.02) when compared to control (26.0 g/d). Trial 2 was similar to Trial 1 and used the same six crossbred steers (238 ± 12 kg) fed 5.3 kg/d (DM basis) of diet. The six abomasally infused treatments included MIX and five treatments that were MIX with deletions of lysine (-LYS); methionine (-MET); histidine (-HIS); leucine, isoleucine, and valine (-BCAA); and threonine and tryptophan (-Thr/Trp). As expected, nitrogen retention (42.2 g/d) was numerically greatest for steers receiving MIX. Both -LYS (35.1 g/d; P=0.11) and -MET (35.6 g/d; P=0.11) tended to have lower nitrogen retention than MIX. Deletion of AA other than lysine and methionine from MIX did not affect nitrogen retention (P>0.17). These data illustrated significant improvements in nitrogen retention when lysine and methionine were supplied post-ruminally as well as numerically decreased nitrogen retention when lysine and methionine were removed from post-ruminal supply. Lysine and methionine appeared to be the first- and co-limiting AA for growing cattle fed our corn-based diet, and the extent of their limitation appears to be similar.

Davies, J., J. Mehalic, M. Grant, M. Miesner, G. Ducharme and E. Titgemeyer. 2025. Limiting amino acids for growing steers fed diets based on corn and corn gluten feed.  2025 American Society of Animal Science Midwest Section meeting, Abstract 61.