Abstract：
The present study compared 2 strategies to initiate a progesterone (P4)-based timed artificial insemination (TAI) protocol for lactating dairy cows: only GnRH or estradiol benzoate (EB) plus GnRH (EB+GnRH). Lactating Holstein cows (n = 487; 184 primiparous and 303 multiparous) from 2 commercial dairy herds were used for their second or greater services postpartum. Each week, cows that were nonpregnant at the pregnancy diagnosis 32 d after a previous AI were randomly assigned to 1 of 2 experimental groups that differed only in the strategy to initiate (d 0) the TAI protocol. On d 0, every cow received a 2.0-g P4 implant; in the EB+GnRH group, cows were treated with 2.0 mg i.m. of EB and 16.8 µg i.m. of the GnRH analog buserelin acetate, whereas in the GnRH group, cows received only 16.8 µg i.m. of GnRH. On d 7 after the initial treatment, 0.530 mg i.m. of cloprostenol sodium (PGF) was administered in all cows, followed by a second dose on d 8, concomitant with 1.0 mg i.m. of estradiol cypionate and P4 implant withdrawal. The TAI was performed on d 10 (48 h after P4 device withdrawal) in both experimental groups. Only conventional Holstein semen was used throughout the study. The percentage of cows with corpus luteum (CL) on d 0 (73%) and overall ovulation rate after d 0 (54%) did not differ between groups. The CL regression between d 0 and the first PGF treatment was greater in the EB+GnRH group than the GnRH group (42% vs. 31%). Consequently, the proportion of cows with CL at PGF was greater when only GnRH was used on d 0 compared with EB+GnRH (86% vs. 82%), and the mean number of CL at PGF was greater (1.23 vs. 1.11). The expression of estrus near TAI was greater in GnRH group (84% vs. 77%), and cows showing estrus had greater (44% vs. 10%) pregnancy per AI (P/AI) on d 32 for both treatments. We found no effect of the presence of CL on d 0 or at PGF, nor of ovulation after d 0 or CL regression between d 0 and d 7 on fertility. However, fertility was critically impaired when cows did not have CL at both times, d 0 and at PGF treatment. We did not observe any interaction between treatment and other variables, and the P/AI was similar in cows receiving EB+GnRH or only GnRH on d 0 (37.8% vs. 36.6%). In summary, although there was no detectable difference in P/AI between treatments, this study demonstrated potential negative physiological outcomes caused by EB treatment on d 0 (greater incidence of luteolysis after d 0 and fewer cows with CL at PGF treatment). Overall, we found no benefit of adding EB at the initiation of a P4-based TAI protocol on fertility compared with using GnRH alone, despite differences in ovarian dynamics and expression of estrus.
摘要：
本研究比较了两种策略，以启动基于孕酮（P4）的定时人工授精（TAI）方案用于泌乳奶牛：仅GnRH或苯甲酸雌二醇（EB）加GnRH。来自2个商业奶牛群的泌乳荷斯坦奶牛（n=487；184初产和303多产）被用于产后第二次或更长时间的服务。在先前的AI之后32天，每周诊断为未怀孕的母牛被随机分配到2个实验组中的1个，这些实验组仅在启动（d0）TAI方案的策略上有所不同。在第0天，每头牛都接受了2.0克P4植入物，在EB+GnRH组中，母牛用2.0mgi.m.EB和16.8µgi.m.醋酸布塞林（GnRH）处理，而在GnRH组中，奶牛只接受了16.8µgi.m的GnRH。7d后（d7），在所有奶牛中施用0.530mgi.m.的氯前列醇钠（PGF），随后在第8天服用第二剂，并同时服用1.0mgi.m.的环戊酸雌二醇（EC）和P4植入物撤药。在两个实验组中,在第10天(P4装置取出后48小时)进行TAI。在整个研究中只使用传统的荷斯坦精液。第0天黄体（CL）母牛的百分比（73％）和第0天之后的总体排卵率（54％）在组间没有差异。EB+GnRH组的d0与首次PGF治疗之间的CL回归大于GnRH组（42vs.31%）。因此,与EB加GnRH相比，在d0时仅使用GnRH时，PGF的CL奶牛的比例更高（86vs.82%），PGF时CL的平均数更大(1.23vs.1.11).GnRH组TAI附近发情期表达较高(84vs.77%），和母牛发情期更大(44vs.两种治疗在第32天每AI（P/AI）的10%）妊娠。CL的存在对d0或PGF没有影响，d0后排卵或d0和d7之间的CL回归对生育力的影响也没有。然而,当奶牛在两个时间都没有CL时，生育能力受到严重损害，d0和在PGF处置。治疗和其他变量之间没有相互作用，并且在d0时接受EB加GnRH或仅接受GnRH的母牛的P/AI相似（37.8vs.36.6%）。总之,尽管治疗之间的P/AI没有可检测到的差异，这项研究显示了d0时EB治疗可能导致的负面生理结果(d0后黄体溶解发生率更高,PGF治疗时CL的奶牛数量更少).总之,与单独使用GnRH相比，在基于P4的TAI方案开始时添加EB对生育没有益处，尽管卵巢动力学和动情表达存在差异。
