Effect of Monensin on Growth, Reproductive Performance, and Estimated Body Composition in Holstein Heifers

and decreases the age at puberty in beef heifers (12,15).These effects of monensin potentially could dc:crease the age at first calving.Although little research has been conducted utilizing dairy heifers, the potential benefit of feeding monensin to dairy heifers may be decreased age at breeding and subsequent age at calving.Steen et al. (25) and Levy et al. (II) reported that monensin increased carcass fat content of beef steers.This effect of monensin may not be desirable for dairy heifers because excess body fat deposition during prepubertal period can adversely affect lactation (24) and late gest.ation.It has been shown that large breed heifers have a critical period of development from 90 to 300 kg at which point excessive dietary energy can permanently reduce the amount of parenchyma in the mammary gland (5).After this period.however, any increased growth rate caused by level of nutrition appears to enhance mammary size and improve body condition (27).Consistent with these findings is a need to be able to enhance growth rates of heifers as well as to monitor the body composition of these gains.This experiment was carried out to investigate the effects of feeding monensin on growth, reproductive performance, and body composition of Holstein heifers. MATERIALS AND METHODS. Forty Holstein heifers from The Pennsylvarna State University dairy farm were divided into two weight groups: heavy (330, SE =17.0 kg) and light (217, SE =13.1 kg).Within each weight group, heifers were assigned at random to two treatments, 0 (control) or 200 mg of monensinjhead daily (Elanco Products Co. Indianapolis, IN) fed via a grain mixture.Average initial age was 380.2 d (SE = 33.1)and 230.0 d (SE = 52.9)for heavy and light heifers, respectively.The trial lasted for 450 d.All heifers were fed com silage from January to April and from 257