Use of a standard forage to reduce effects of animal variation on estimates of mean voluntary intake

Sixty test forages (alfalfa, timothy, bromegrass, and orchardgrass mixtures), of differing cuttings and maturities, were harvested as hay in each of 2 yr (30/yr) from three locations. Each of the 60 hays was chopped and fed to four growing sheep to determine voluntary intake. The duration of the trial was 2 yr with five experimental periods per year. In each period, immediately prior to feeding the test forages, intake of the same standard alfalfa hay (standard forage) was measured for every sheep. Use of intake of the standard forage as a covariate reduced mean square error by 38%. Regression of least squares means of intake of the test forages on chemical composition uniformly yielded higher coefficients of determination when means were generated from an analysis of variance that included intake of the standard forage as a covariate. This procedure can be used to increase the accuracy of estimates of mean voluntary intake or to reduce the number of animals needed to attain the same accuracy that would be achieved without use of the covariate.