Reproduction 2

摘要

Cows with reduced estrous expression have compromised fertility. Thisstudy aimed to determine if the administration of GnRH at AI affectsovulation, progesterone and fertility of cows expressing low estrousbehavior. Cows were enrolled at estrus from 3 farms (n = 1629 AI events;Farm A: 757, Farm B: 305, Farm C: 567) and randomly assigned toreceive GnRH at AI. Estrous expression was monitored by leg-mountedactivity monitors and quantified as the maximum activity occurring atestrus; using the farm median, estrous expression was categorized ashigh or low. On Farm C, cows were assessed at alert and 7d post alertfor ovulation (n = 707) using ultrasonography. On Farm B, ovulationwas assessed using plasma samples analyzed for progesterone (P4). Asubset of animals were assessed for P4 at estrus and at 7d (n = 526) andgrouped using the median. Pregnancy/AI (P/AI) was confirmed at 31± 3d post-AI. Outcome variables were tested using GLIMMIX of SASwith estrus as the experimental unit and cow within farm as a randomeffect. An interaction of treatment and estrous expression on P/AI wasfound. Without GnRH, cows with higher estrous expression had greaterP/AI than those with lower expression, whereas GnRH administrationincreased P/AI for cows with low estrous expression but not those withhigh expression (GnRH-High: 46.8 ± 2.5, GnRH-Low: 40.0 ± 2.7; NoGnRH-High: 42.3 ± 2.5, No GnRH-Low: 30.8 ± 2.7%; P < 0.01). Cowswith high expression had greater P4 7d post-alert (P = 0.04), but not atestrus. There was an interaction of treatment with P4 at alert on P/AI,cows with lower P4 receiving GnRH had greater P/AI than all othergroups (GnRH-Low P4: 58.0 ± 4.5, GnRH-High P4: 39.4 ± 4.5; NoGnRH-Low P4: 39.9 ± 4.7, No GnRH-High P4: 40.0 ± 4.7%; P = 0.01);this was consistent by estrous expression. Ovulation was not impactedby P4 at estrus but there was an interaction of estrous expression andtreatment, where cows with high estrous expression receiving GnRH hadgreater ovulation rates than all other groups (GnRH-High: 92.0 ± 1.8,GnRH-Low: 85.1 ± 2.0; No GnRH-High: 87.6 ± 1.9, No GnRH-Low:87.6 ± 2.0%; P = 0.05). In conclusion, fertility of cows with low estrousexpression may be increased using GnRH at the time of AI; however,the mechanism behind this relationship is unclear.