School of Agriculture and Food Science, University College Dublin Lyons Research Farm, Celbridge, Naas, Co. Kildare, Ireland, W23 ENY2.
School of Veterinary Medicine, University College Dublin, Belfield, Dublin 4, Ireland, D04 V1W8.
J Dairy Sci. 2022 Jan;105(1):347-360. doi: 10.3168/jds.2021-20743. Epub 2021 Oct 9.
The objectives of this study are to evaluate the effects of (1) a potential interaction between supplement crude protein (CP) concentration and differing cow genotypes on milk production, (2) differing cow genotypes on milk production, and (3) decreasing the supplement CP concentration on milk production and N excretion during the main grazing season within a spring-calving herd. A 2 × 2 factorial arrangement experiment, with 2 feeding strategies [14%; n = 30 (lower CP; LCP) and 18%; n = 28 (higher CP; HCP) CP concentrate supplements] offered at varying levels according to pasture availability and days in milk (DIM) was conducted over the main grazing season from April 3 to September 3, 2019, at University College Dublin Lyons Farm. Cows were also grouped into 2 genotype groups: lower milk genotype; n = 30 [LM; milk kg predicted transmitting ability (PTA): 45 ± 68.6 (mean ± SD); fat kg PTA: 10 ± 4.9; and protein kg PTA: 7 ± 2.3] and higher milk genotype; n = 28 [HM; milk kg PTA: 203 ± 55.0; fat kg PTA: 13 ± 3.8; and protein kg PTA: 10 ± 2.4]. A total of 46 multiparous and 12 primiparous (total; 58) Holstein Friesian dairy cows were blocked on parity and balanced on DIM, body condition score, and Economic Breeding Index. Cows were offered a basal diet of grazed perennial ryegrass pasture. The N partitioning study took place from August 25 to 30, 2019 (187 ± 15.2 DIM). No interactions were observed for any milk production or milk composition parameter. No effect of supplement CP concentration was observed for any total accumulated milk production, daily milk production, or milk composition parameter measured. The HM cows had increased daily milk yield (+1.9 kg), fat and protein (+0.15 kg), and energy-corrected milk (+1.7 kg), compared with the LM cows. Furthermore, HM cows had decreased milk protein concentration (-0.1%) compared with LM cows. For the N partitioning study, cows offered LCP had increased pasture dry matter intake (PDMI; +0.9 kg/d), dietary N intake (+0.022 kg/d), feces N excretion (+0.016 kg/d), and decreased N partitioning to milk (-2%), and N utilization efficiency (-2.3%). In conclusion, offering cows LCP had no negative influence on milk production or milk composition over the main grazing season where high pasture quality was maintained. However, any potential negative effects of offering LCP on milk production may have been offset by the increased PDMI. Furthermore, offering cows LCP decreased N utilization efficiency due to the higher PDMI and feed N intake associated with cows on this treatment in our study.
（1）补充粗蛋白（CP）浓度与不同奶牛基因型之间的潜在相互作用对产奶量的影响，（2）不同奶牛基因型对产奶量的影响，以及（3）在春季产犊牛群的主要放牧季节内降低补充 CP 浓度对产奶量和氮排泄的影响。2019 年 4 月 3 日至 9 月 3 日，在都柏林大学学院莱昂斯农场进行了一项 2×2 析因安排实验，根据牧场可用性和泌乳天数（DIM），提供了 2 种饲养策略[14%；n = 30（低 CP；LCP）和 18%；n = 28（高 CP；HCP）浓缩饲料]。奶牛还根据产奶量和产奶量分为 2 种基因型组：低产奶量基因型；n = 30 [LM；牛奶预测传递能力（PTA）：45 ± 68.6（平均值±标准差）；脂肪 kg PTA：10 ± 4.9；蛋白质 kg PTA：7 ± 2.3]和高产奶量基因型；n = 28 [HM；牛奶 kg PTA：203 ± 55.0；脂肪 kg PTA：13 ± 3.8；蛋白质 kg PTA：10 ± 2.4]。共有 46 头经产奶牛和 12 头初产奶牛（总计；58 头）荷斯坦弗里生奶牛按胎次和泌乳天数、体况评分和经济繁殖指数进行分组。奶牛提供基础日粮为放牧多年生黑麦草牧场。氮分配研究于 2019 年 8 月 25 日至 30 日进行（DIM 为 187±15.2）。没有观察到任何产奶量或牛奶成分参数的相互作用。没有观察到补充 CP 浓度对任何总累积产奶量、日产量或牛奶成分参数的影响。与 LM 奶牛相比，HM 奶牛的日产量增加（+1.9kg），脂肪和蛋白质（+0.15kg），以及能量校正奶（+1.7kg）。此外，HM 奶牛的牛奶蛋白浓度比 LM 奶牛低（-0.1%）。对于氮分配研究，与低 CP 组相比，高 CP 组的奶牛增加了牧场干物质摄入量（PDMI；+0.9kg/d）、膳食氮摄入量（+0.022kg/d）、粪便氮排泄量（+0.016kg/d），并降低了牛奶中的氮分配（-2%）和氮利用率（-2.3%）。总之，在维持高牧场质量的主要放牧季节，为奶牛提供低 CP 对产奶量或牛奶成分没有负面影响。然而，由于与我们研究中接受这种治疗的奶牛相关的更高的 PDMI 和饲料氮摄入量，为奶牛提供低 CP 可能对产奶量产生的任何负面影响都可能被抵消。此外，为奶牛提供低 CP 由于与这种处理相关的奶牛 PDMI 和饲料氮摄入量较高，降低了氮利用率。
