Effects of ionophores on cow herd production: a review

Feeding ionophores to beef cows or replacement heifers fed harvested forages increases weight gain and improves feed efficiency. These responses are influenced by diet quality and body condition of the animal. These compounds may reduce postpartum interval to estrus, but this effect appears to be diminished if nutritional status is poor. Although ionophores do not alter fertility, they decrease age at puberty in the female. These changes in reproductive performance appear to be related to a hormonal response to an increased propionate:acetate ratio in the rumen characteristic of ionophore feeding. Responses to ionophores in milk yield and weight gain improvements in calves of ionophore-fed cows are variable. Length of gestation, incidence of dystocia, calf birth weights and percent calf crop are not altered by ionophore feeding. The potential to improve cow herd production through the use of ionophores depends primarily on forage quality and availability.