Endocrine studies in ivermectin-treated heifers from birth to puberty.
Continuous treatment with ivermectin from birth to puberty advanced sexual maturation by 3.7 wk in Holstein heifers grazing pastures naturally infected with nematodes. Every 14 d jugular blood samples were taken from birth to 45 wk of age from all heifers. No differences in serum FSH, estradiol, or thyroxine levels were observed during the trial between the treated and untreated group. Mean LH levels were diminished in untreated heifers 4 wk before the first estrus and the amplitude of LH pulses was augmented in treated heifers when puberty was reached. Serum IGF-I levels increased from birth to 22 wk of age and then reached a plateau in both groups, but levels were consistently higher in treated heifers from 26 wk of age onward. Body weight gain was retarded in parasitized heifers and IGF-I values were positively correlated with body weight only during the first 20 wk of life. We suggest that enhanced prepubertal IGF-I levels in conjunction with increased prepubertal LH levels and pubertal LH pulse amplitude might be involved in the accelerated somatic maturation and in puberty advancement observed in ivermectin-treated heifers.