Increase in the reproductive efficiency of primiparous and multiparous Nellore cows following moxidectin treatment at the onset of a fixed-time artificial insemination protocol

Abstract

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.