EFFECT OF MONENSIN ON MILK-PRODUCTION BY HOLSTEIN AND JERSEY COWS

Effects of the administration of monensin via concentrates to dairy co ws were studied in two trials. In one trial, 64 Holstein cows were ass igned to four groups that received 0, 150, 300, or 450 mg/d of monensi n from 5 to 24 wk postpartum. Milk production tended to increase (4.0, 3.3, and 5.4%, respectively) for the three groups of treated cows. Fa t content was decreased by 0.09, 1.89, and 4.09 g/kg, respectively, fo r these same three groups. The effect on protein content was small and nonsignificant. Feed intake was reduced in treated cows, although not significantly, and feed efficiency was improved by monensin. In a con firmatory trial, 58 Holstein and 22 Jersey cows were allocated either to a control group or to a treatment group that received 300 mg/d of m onensin from 5 to 36 wk postpartum during the first lactation and from 2 wk before calving to 36 wk postpartum during a subsequent lactation . During the first lactation, cows in the treatment group showed a 7% increase in milk production, a relative decrease (1.4 g/kg) in milk fa t content, and equal protein content compared with cows in the control group. Body weight gain and body condition scores near the end of the treatment period were higher for cows in the treatment group. A decre ase in blood ketone concentrations was found between 7 and 56 d of lac tation. Treatment effects on milk production differed between breeds a nd within genetic lines. Jersey cows were less responsive than were Ho lstein cows, and Holstein cows with a high ratio of breeding values fo r protein and fat showed larger milk production responses. Results fro m the second lactation showed similar differences between the two grou ps as did those from the first lactation.