Effect of anthelmintic treatment on sexual maturation in prepubertal beef heifers

Heifers treated with ivermectin at weaning have been reported to reach pube rty at a younger age and lighter weight than untreated heifers. We tested t he hypothesis that heifers administered ivermectin would respond with earli er follicular development and a greater LH response to a 1-mg estradiol-17 beta challenge (E2C) than untreated heifers. Fall-born Angus heifers (n = 3 2) were randomly assigned on 284 +/- 9 d of age (215.5 +/- 20.8 kg) to rece ive ivermectin (IVR) or albendazole (ALB), IVR + ALB, or to remain as untre ated controls (CONT). Each group (n = 8) was housed separately in adjacent pens throughout the trial and managed to gain .8 kg/heifer on a ration cont aining 13.2% CP, 58.8% TDN, and 49.9% DM. The CONT heifers received an addi tional 2.27 kg/heifer of corn silage and 1.59 kg/heifer of corn daily to ma intain ADG at comparable levels. Individual body weight was recorded weekly , and nematode eggs per gram (EPG) of feces were measured every 21 d. Ultra sonography was performed on alternate days starting 2 wk prior to E2C to ch aracterize follicular wave patterns. Follicles were separated into classes (C1 [3 to 5 mm], C2 [6 to 9 mm], and C3 [10 mm]) and sizes (largest [LF], s econd [SLF], third [TLF], and fourth largest follicles [FLF]). The sizes of the regressing dominant follicle 1 (DF1) and the progressing dominant foll icle 2 (DF,)were also determined. Serum concentrations of LH were determine d from hourly jugular blood samples collected 8 to 24 h after injection of E2C. The IVR + ALB treatment group had more C3 follicles than ALB and CONT (P < .07). The IVR-treated heifers had larger TLF than ALB and CONT(P < .04 ). The IVR- and IVR + ALB-treated heifers had larger FLF and DF2 than ALB a nd CONT (P <,1). Least squares means for DF2 were 9.5 +/- .5, 8.0 +/- .4, 9 .5 +/- .3 and 8.3 +/- .3 mm, for IVR, ALB, IVR + ALB and CONT, respectively (P = .02 for treatment effect). The E2C-induced serum LH concentration did not differ with respect to treatment. We conclude that heifers administere d IVR display increased follicular development, supporting our earlier inve stigations regarding reduced age at puberty in heifers treated with IVR nea r weaning.