Effect of competition on gain in feedlot bulls from Hereford selection lines

Authors: Van Vleck, Lloyd; Cundiff, Larry; KOCH, ROBERT - RETIRED-UNIV. OF NEBR.

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/12/2007
Publication Date: 7/1/2007
Citation: Van Vleck, L.D., Cundiff, L.V., Koch, R.M. 2007. Effect of competition on gain in feedlot bulls from Hereford selection lines. Journal of Animal Science. 85(7):1625-1633.

Interpretive Summary: Animals in a pen can interact to increase or decrease individual performance. This study examined competition effects on average daily gain in the feedlot of 1,882 Hereford bulls from a selection experiment. For each of eight years, bulls from each of four lines were assigned to two pens with about 30 bulls in each pen. Gains were recorded for up to eight periods of 28 days. Models for analyses included pen effects (fixed or random), fixed effects such as year and line, and random direct genetic, competition genetic (and in some analyses competition environmental effects) and environmental effects. Each pen mate as a competitor affects records of all others in the pen so that even a small effect may be important when multiplied by the number of pen mates. All lines traced to common foundation animals so the numerator relationships among and within pens were the basis for separating direct and competition genetic effects and pen effects. Competition effects seemed to have affected average daily gain for the first 28 days in the feedlot for these Hereford bulls. Although the estimate of the fraction of variance due to competition effects was small, the large economic weight for the competition effects relative to that for direct genetic effects (number in pen minus one vs one) makes even such a small fraction of variance important. The competition effects in later periods, however, were nil. Thus, over the full time in the feedlot for these bulls, genetic competition effects do not seem to have been important. Inclusion of repeated environmental competition effects in the model even with small estimates for that component of variance did account for nearly all of the variation that traditionally is attributed to pen effects. Models with pen effects rather than environmental competition effects, however would be much easier to analyze.

Technical Abstract: For this population and pen conditions, major results were: 1) competition genetic effects seemed present for the first 28 d period but not for the following seven periods, 2) models with pens considered fixed effects could not separate variances and covariance due to direct and competition genetic effects, 3) models without competition effects had large estimates of the variance component due to pen effects for gain through eight periods, and 4) models with both genetic and environmental competition effects accounted for nearly all of the variance traditionally attributed to pen effects (even though estimates of the competition variance component were small, the estimates of pen variance were near zero). The purpose of the current study was to estimate variance and (co)variance components for direct and competition genetic effects for average daily gain for eight time periods for 1,882 Hereford bulls from three selection lines and one control line. An added goal was to document problems with such analyses with different statistical models.