Effect of Ad Libitum or Force-Feeding of Two Rations on Lactating Dairy Cows Subject to Temperature Stress

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Abstract
A comparison of dairy cows which received a constant intake of feed by means of rumen fistulas with cows receiving feed ad libitum demonstrated the following. The major decrease in milk production at high ambient temperatures is due to reduced feed intake. High ambient temperature per se causes a decrease in production. Part of this effect may be due to decreased rate of feed passage through the rumen. Differences in fiber or molasses content of rations at low or high temperatures showed no significant influence on amount or composition of milk produced. Increase in temperature caused a significant decrease in production. High temperatures caused a significant decrease (P<.05) in efficiency of energy utilization for milk production. Significant interaction of ration and temperature on efficiency of energy utilization for milk production indicates that ambient temperature should be considered in evaluating ration effect.