The aim of the present study was to evaluate the effects of moxidectin (MOX – 200 µg/kg/sc) administered on Day Zero of a fixed-time artificial insemination (FTAI) protocol on fecal egg counts and the reproductive efficiency of primiparous and multiparous Nellore beef cows kept in the pasture. A simulation was performed of the possible correlation between the fecal egg count and helminth load per animal (cow versus calf). An economic analysis was performed of the magnitude of the return over investment (ROI) of moxidectin treatment at the onset of the FTAI protocol. Three experiments were performed with a total of 6,320 Nellore females (primiparous and multiparous) and calves approximately 45 days old. The animals were subdivided into two groups: control and treated with MOX subcutaneously on Day 0 concomitant with the reproduction protocol adopted in the first FTAI. Egg counts per gram of feces (EPG) were performed on Days 0 and 11. Resynchronization of ovulation (RO) was performed in cows diagnosed with negative pregnancy 30 days after the first FTAI. Nellore bulls were used for natural service at a proportion of one bull per 25 Nellore cows 10 days after the second diagnosis of gestation (D92). The results of the three experiments indicated that treatment with MOX at the beginning of the FTAI protocol (Day 0) increased the pregnancy rate at the first FTAI and had no effect on the pregnancy rate at the second FTAI (RO), leading to a larger number of pregnant cows at the end of the breeding season, with an ROI of 44.9 and 19.0 for primiparous and multiparous cows, respectively. The mean parasite load (EPG) decreased significantly after treatment with moxidectin in both primiparous and multiparous cows. However, the statement that the difference in the pregnancy rate for the treated cows occurred due to the control of helminth infection should be interpreted with caution. Further studies are needed to gain an understanding the unknown physiological mechanisms of moxidectin that could be affecting the reproductive results.

本研究的目的是评估在固定时间人工授精 (FTAI) 协议的第零天施用莫昔克丁 (MOX – 200 µg/kg/sc) 对粪便卵数和初产和经产的生殖效率的影响。 Nellore 肉牛饲养在牧场上。对每只动物的粪便卵数和蠕虫负荷之间可能的相关性进行了模拟（奶牛与小牛）。在 FTAI 协议开始时，对莫昔克丁治疗的投资回报 (ROI) 幅度进行了经济分析。对总共 6,320 头 Nellore 雌性（初产和多产）和大约 45 天大的小牛进行了三个实验。将动物分为两组：对照组和在第 0 天皮下用 MOX 治疗，同时采用第一次 FTAI 中采用的繁殖方案。在第 0 天和第 11 天进行每克粪便的鸡蛋计数 (EPG)。在第一次 FTAI 后 30 天诊断为阴性妊娠的奶牛中进行排卵再同步 (RO)。在第二次诊断妊娠 (D92) 后 10 天，Nellore 公牛以每 25 头 Nellore 母牛一头公牛的比例用于自然服务。三个实验的结果表明，在 FTAI 方案开始时（第 0 天）用 MOX 治疗增加了第一次 FTAI 的妊娠率，而对第二次 FTAI（RO）的妊娠率没有影响，导致更大的繁殖季节结束时怀孕奶牛的数量，初产和经产奶牛的投资回报率分别为 44.9 和 19.0。用莫昔克丁治疗初产和经产奶牛后，平均寄生虫载量 (EPG) 显着降低。然而，应谨慎解释治疗奶牛的妊娠率差异是由于控制蠕虫感染所致。需要进一步研究以了解莫昔克丁可能影响生殖结果的未知生理机制。