Effects of complex enzyme preparation supplementation in a low-protein amino acid-balanced diet on production performance, serum biochemistry, nitrogen metabolism and nitrogen balance of mid-lactation dairy cows.

OBJECTIVE

High-protein diets for lactating dairy cows reduce nitrogen ( N ) use efficiency (NUE) and cause environmental nitrogen pollution. Low-protein (LP), amino acid (AA)-balanced diets are a promising alternative but carry the risk of impairing nutrient digestibility and cow productivity. Exogenous enzyme preparations may mitigate these limitations, but their specific effects and underlying mechanisms in this feeding scenario remain not fully elucidated.

METHODS

Ninety mid-lactation Holstein dairy cows were assigned to 3 treatment groups in a randomized block design: (1) normal protein (NP) diet (17.30% crude protein, CP); (2) low-protein (LP) AA-balanced diet (16.09% CP, supplemented with rumen-protected lysine [RP-Lys] and rumen-protected methionine [RP-Met]); (3) LP diet plus 20 g /head per day of complex enzyme preparation (LPEP group). The 75 -day trial consisted of a 15 -day adaptation period and a 60 -day formal feeding period.

RESULTS

Compared with the NP and LP groups, the LPEP group showed significantly better production performance, apparent nutrient digestibility, feed conversion efficiency and milk quality (p<0.05). Serologically, LPEP cows had significantly higher serum albumin, lower serum urea nitrogen and blood glucose, and optimized AA profiles in both serum and milk. In the rumen, the LPEP diet significantly reduced ammonia nitrogen (NH3-N) concentration, while increasing microbial crude protein (MCP), acetic acid, propionic acid and total volatile fatty acids (TVFA), with no alteration of the overall ruminal fermentation pattern. Notably, the combination of LP AA-balanced diet and complex enzymes effectively reduced N excretion and improved NUE and N balance in dairy cows.

CONCLUSION

Supplementation of complex enzyme preparation to a LP AA-balanced diet exerts a marked synergistic effect with rumen-protected AAs. This combined strategy simultaneously improves production performance, milk quality and NUE while reducing N waste, providing critical support for developing environmentally sustainable and efficient dairy feeding regimens.