| A two year-long study was conducted to assess the effects of ivermectin (with clorsulon) and long-acting moxidectin on the development of replacement beef heifers carrying naturally occurring parasite infections. Replacement beef heifers (n = 105) were obtained at weaning and co-grazed on permanent pastures throughout the 686 day study. Study heifers were treated with ivermectin and clorsulon (IVCL), long-acting moxidectin (LMXD), or left untreated (CON). Treatments were administered on d 0 and d 149. The calving season occurred from d 438 to 541. Study heifers were re-exposed to fertile bulls from d 546 to 615. Calves were weaned on d 686. Compared to CON, heifer bodyweights were greater (P < 0.05) for LMXD and IVCL-treated animals on d 175, 238, 287, 369, and 433. The average daily gains were greater (P < 0.01) for LMXD and IVCL-treated heifers compared to CON heifers from d 0 to d 433. Fecal, strongyle egg counts were lower (P < 0.01) for LMXD-treated heifers on d 14, 64, 149, 169, 238, and 287 compared to the IVCL and CON groups. Fecal egg counts for the IVCL group were lower (P < 0.01) than those seen in the CON group only on d 14 and d 169; the samplings subsequent to animal treatments. During the pre-breeding period, no differences were noted between treatment groups for reproductive tract score, pelvic area, hip height or pregnancy rate. Subsequent to the breeding season, no differences were exhibited in pregnancy rates related to treatment. Pre-calving BCS tended (P = 0.09) to be greater for LMXD and IVCL-treated heifers compared to CON-heifers. Calf birth weights were similar (P = 0.25) among all treatment groups. Calves born to LMXD-treated cows exhibited heavier (P < 0.05) adjusted-205 d BW compared to calves born to CON group, but calves born to IVCL-treated cows were similar (P > 0.10) to both the LMXD-treated group and to the CON group. Calves from LMXD-treated cows had greater (P < 0.05) ADG compared to calves from CON cows while calves from IVCL-treated cows were similar to calves born to CON-cows. Calf muscle score, weaning weight, hip height, and BW:hip height ratio were determined based on measurements taken at weaning (d 686). Muscle score was greater (P = 0.0004) for LMXD and IVCL groups compared to CON. Hip height was greatest for calves born to LMXD-treated cows and tended (P = 0.09) to be greater than calves born to IVCL-treated cows, and calves from both LMXD and IVCL-treated cows exhibited greater (P < 0.05) hip heights compared to calves from CON cows. The BW:hip height ratio was greater (P < 0.01) for calves weaned from LMXD and IVCL-treated cows compared to calves weaned from CON cows. Performance of calves born to the study heifers was greatest for calves born to LMXD-treated heifers, intermediate for calves born to IVCL-treated heifers, and lowest for calves born to untreated heifers. Using calving dates for each calf, an estimated days to conception was determined for all study heifers. Heifers in the LMXD and IVCL groups tended (P = 0.06) to have a shorter number of days to conception when compared to heifers in the CON group (18, 18 and 29, respectively). We conclude that anthelmintic treatment has a positive influence on fecal egg count suppression, precalving heifer growth performance and offspring growth performance, but treatment had limited effect on reproductive performance in this study. |
