Correlated Response in Feed Efficiency Accompanying Selection for Milk Yield in Jerseys

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Abstract
Gross feed efficiency (milk energy output divided by energy consumed) was measured in 232 Jersey cows resulting from direct selection for milk yield and a control line of 121 daughters of 20 Jersey bulls taken randomly from commercial artificial insemination sampling programs in 1967. Data were collected between 1971 and 1982 from primiparous cows in middle to late lactation in 21 feeding trials of 28 d. Cows were fed hay and silage ad libitum plus grain according to production at the rate of 1 kg grain/3 kg milk. Models for least squares analyses included trial, line, days in milk, and either body weight change (model 1) or energy balance (model 2), estimated as the difference between energy requirements and actual energy intake. Body weight change was an insignificant source of variation, and gross feed efficiency did not differ between genetic lines using model 1. Significant line differences in gross efficiency were obtained after adjusting for energy balance; however, problems of multicollinearity invalidates meaningful inference. Results indicate genetic lines of Jerseys did not differ in gross feed efficiency when fed according to yield and suggest selection has not affected efficiency of direct conversion of nutrients into milk.