Effects of continuous ivermectin treatment from birth to puberty on growthand reproduction in dairy heifers

The effect of continuous ivermectin treatment from birth to puberty on grow th and reproductive performance was studied in Holstein heifer calves grown on pastures in comparison to naturally nematode-infected, untreated animal s. Ivermectin effectively abated the presence of nematode eggs in feces. Eg gs per gram (EPG) in parasitized animals increased rapidly from wk 12 to 18 of age and then decreased. Animals treated with ivermectin grew faster tha n untreated ones, and differences in body weight became significant at 6 wk of life, even before eggs appeared in the feces of either treatment group. Ivermectin-treated heifers reached puberty 3 wk earlier than infected ones as assessed with serum progesterone concentrations (ivermectin, 30.4 +/- . 8 vs untreated, 33.7 +/- 1.3 wk of age). This delay was not directly relate d to body weight. In addition, pelvic area at 39 wk and at 15 mo of age was increased in treated heifers (8 and 11%, respectively) compared with paras itized animals. No differences in the wither heights were observed. We conc lude that ivermectin treatment in dairy heifers may increase growth rate du ring development, advance the onset of ovarian function, and positively aff ect yearling pelvic area.