Effects of complex enzyme preparations supplementation in 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 fed to lactating dairy cows are associated with impaired nitrogen (N) use efficiency (NUE) and subsequent environmental nitrogen pollution.While low-protein, amino acid (AA)-balanced diets represent a promising alternative strategy, they carry the risk of compromising nutrient digestibility and cow productivity.Exogenous enzyme preparations have the potential to mitigate these limitations; however, their specific effects and underlying mechanisms of action in this feeding context remain not fully elucidated.Methods: A total of 90 mid-lactation Holstein dairy cows were enrolled in the study and assigned to 3 treatment groups via a randomized block design.The dietary interventions were as follows: (1) a normal protein (NP) diet (NP group; 17.30% crude protein, CP); (2) a low-protein (LP), AA-balanced diet (LP group; 16.09% CP, supplemented with rumen-protected lysine [RP-Lys] and rumen-protected methionine [RP-Met]); and (3) the LP diet supplemented with 20 g/head per day of a complex enzyme preparations (LPEP group).The total experimental duration was 75 days, comprising a 15-day adaptation period followed by a 60-day formal feeding period.Results: Compared with the NP and LP groups, cows in the LPEP group exhibited significantly better production performance, apparent nutrient digestibility, feed conversion efficiency, and milk quality.Serological analyses showed that cows in the LPEP group had significantly higher serum albumin concentrations, alongside significantly lower serum urea nitrogen and blood glucose levels.Furthermore, the LPEP diet optimized the AA profiles in both serum and milk.In the rumen, the LPEP diet significantly reduced ruminal ammonia nitrogen (NH3-N) concentrations, while increasing the concentrations of microbial crude protein (MCP), acetic acid, propionic acid, and total volatile fatty acids (TVFA), with no shift in the overall ruminal fermentation pattern.Most notably, the combination of the low-protein AA-balanced diet and complex enzyme preparations effectively reduced N excretion, while simultaneously improving NUE and N balance in dairy cows. Conclusion:In conclusion, supplementation of a low-protein, AA-balanced diet with a complex enzyme preparations exerts a marked synergistic effect with rumen-protected amino acids (RP-AAs).This combined strategy simultaneously improves production performance, milk A c c e p t e d A r t i c l e quality, and NUE, while reducing nitrogen waste output.These findings provide critical insights for the development of environmentally sustainable, efficient feeding regimens for dairy cattle.