Masters Theses

Date of Award

6-1986

Degree Type

Thesis

Degree Name

Master of Science

Major

Animal Science

Major Professor

J. C. Waller

Committee Members

J. B. McLaren, R. O. O'Brian

Abstract

Increasing cow herd productivity is a major concern of the cow-calf producer. Research has indicated that 25-30% of the beef cows placed in breeding pastures fail to concieve and wean a calf the following year. This decrease in weaning percentage equates to a 20% increase in the cost of producing a live calf at weaning. This increase in production costs justifies research with the purpose of evaluating possible alternative selection criteria for replacement animals.

Past selection of replacement animals has been based on such traits as hip height, weaning weight or yearling weight yeilding a heavier product at weaning. This method of increasing production has been associated with increased birth weight,frequency of dystocia and decreased rebreeding efficiency, thus reducing the overall herd proformance.

This study was designed to determine the effect of using voluntary feed intake, measured during a 60-day postweaning period, as a criterion for replacement heifer selection. One hundred sixty-eight Angus, Here ford and reciprocal cross heifers were utilized in this study. During each of the six years, 30 heifers were grouped and fed by sire. Their diet consisted of ad libitum corn silage, supplemental protein provided by .454 kg. cottonseed meal/head/day, and a free-choice mineral mix. Total feed intake of each pen was partitiioned into intake of individual heifers using both the ratio of mean heifer weight to mean pen weight and the net energy system. Feed required for maintenance and gain was estimated from mean empty body weight and empty body gain during the feeding trial. An equation estimating daily feed intake of yearling heifers was also em ployed. Equations were adjusted so that the sum of estimated feed intake for individual heifers within each pen would total actual feed intake for the pen. Daily feed intake of beef cattle is extremely variable and in fluenced by numerous factors. Rumen capacity, low digestibility, and rate of passage limit the amount of forage consumed. Forage availability, use of supplements, climatic conditions, animal size, reproductive status, and genetic potential also affect daily feed intake. Upon completion of the feed intake trial, heifers were returned to the herd where production data were obtained.

Heifers ranged in initial weight from 170 kg. to 310 kg. and in final weight from 225 kg. to 364 kg. Average daily gain of the heifers ranged from .22 kg./day to 1.40 kg./day. Daily feed intake (DFI) ranged from 4.5 kg. to 7.0 kg. Selection of the top 25% of the replacement hiefers based on DFI produced positive secondary selection differentials for most probable producing ability when contrasted with the remainder of the re placement population (p<.05) and when contrasted with the bottom 25% (p<.05). Positive secondary selection differentials were also observed for mean weaning weight ratio when the top 25% were contrasted with the remainder of the replacement population (p<.05) and when contrasted with the bottom 25% (p<.05).

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