热应激奶牛蛋白质营养需要量研究OA

[Objective]This study aimed to investigate the effects of crude protein(CP)levels on production performance,serum biochemistry,and milk composition of dairy cows under heat stress,and to determine the op-timal protein requirement for late-lactation cows under such conditions.[Method]A total of 240 healthy Holstein cows with similar milk yield(41.03±0.5)kg·d-1,similar lactation days(193±15)d,the same parity(primipa-rous),and body weight(600±30)kg were randomly assigned to three groups of 80 cows each.Each group was fed one of three diets differing in CP concentration:16.67％(LCP group),17.74％(MCP group),and 18.66％(HCP group).The experiment included a 1-week acclimatization period followed by a 6-week formal trial.Daily feed intake and milk production were recorded throughout the trial.[Result]The daily temperature-humidity index(THI)in the bam remained above 68 throughout the trial,indicating that cows experienced persistent heat stress.Dietary CP level had no significant effect on respiratory rate or rectal temperature of dairy cows under heat stress(P＞0.05).However,increasing dietary CP level significantly increased dry matter intake(DMI),milk yield,milk protein per-centage,lactose percentage,and lactose yield(P＜0.01).Milk urea nitrogen content increased significantly with the increase of dietary CP level(P＜0.01).The urine nitrogen content in the LCP group was significantly lower than that in the other two groups(P＜0.01).No significant differences were observed in fat-corrected milk(FCM),milk fat percentage,total solids,or somatic cell count among groups(P＞0.05).The HCP group showed significantly lower DMI and apparent digestibility of CP compared with the other groups(P＜0.01).Dietary protein level had no signif-icant effect on the apparent digestibility of ether extract(EE),acid detergent fiber(ADF),or neutral detergent fiber(NDF)(P＞0.05).Additionally,the HCP group had significantly increased serum total protein(TP),and glutamic pyruvic transaminase(ALT)levels(P＜0.05),and markedly elevated blood urea nitrogen(BUN)and blood glucose(GLU)concentrations(P＜0.01).In contrast,the LCP group exhibited significantly enhanced total antioxidant ca-pacity(T-AOC),superoxide dismutase(SOD)activity,and glutathione peroxidase(GSH-Px)activity,along with significantly reduced malondialdehyde(MDA)concentrations(P＜0.01).Based on a comprehensive evaluation of nitrogen metabolism and other indicators of dairy cows under heat stress conditions,the optimal dietary protein con-centration for Holstein cows in the late lactation period under heat stress conditions was determined to be 16.67％.[Conclusion]Under heat stress conditions,feeding a diet containing 16.67％CP optimizes nitrogen utilization effi-ciency,reduces nitrogen pollution,and improves resource utilization efficiency in dairy cows,thereby achieving an economically viable and environmentally sustainable production outcome.