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UBC Theses and Dissertations

Evaluation of the parameters influencing the weight of beef cows Hiley, Peter Graham

Abstract

The body weight of beef cows (cow weights) on nine ranches and farms located in the province of British Columbia were recorded each fall and spring for three years. Cow weights were also recorded on two additional occasions during the winter in the largest of these herds. The cows in these herds were all straightbred Hereford or Angus cows, or crossbreds of one or other of these breeds. During the summer five of the herds used rangeland, and four used pasture. Each herd calved in the spring after overwintering on conserved forage. The influence of breedtype, year, age, season and herd on the spring and fall weight records were determined by a least-squares analysis as outlined by Harvey (1975). Season was found to have a major influence on cow weight. Each weight change (considered as a percentage of the mean weight during the period) was included as a dependent variable in separate analyses. In addition to the parameters already mentioned, calf sex, calf age, calf weaning weight, previous weight change and the interval from calving to weighing in the spring were included where appropriate in the models for these analyses. The two mid-winter weights were included in a similar statistical analysis to determine the influence of pregnancy per se on cow weight in this herd. The parameters fitted in this model were age, year, age x year, days pregnant (DP), DP² and DP³. Age, season and herd were found to be significant sources of variation in the cow weight records; bat, within each herd, genetically different breedtypes normally had similar weights. The cows on summer rangeland were on average 119.0 lb. lighter than those on summer pasture. The increase in weight over age and the seasonal fluctuations around this mean growth curve also varied significantly between herds. However, in all but one of the herds there was a weight loss each winter and a weight gain each summer. The mean spring/fall weight difference was 114.2 lb. The mean mature age of the cows was six, and the mean mature weight was 1083.9 lb. The summer change in weight of a cow depended on age and herd, but not on breedtype. Younger cows gained more weight through the summer than older cows; the influence of herd on cow weight was not correlated with any single factor. Summer weight change was not influenced by the calf the cow suckled during the summer, but cows which lost the most weight during the previous winter gained the most the following summer. The winter weight change of a cow was not the same for all ages and breedtypes, but differed from herd to herd according to the level of winter feeding in each herd. Again there was a close correlation between this weight change and that in the previous period. The winter weight loss was found to continue through the interval from calving to weighing. The average weight loss during this period was around 0.7 lb. per day. A mean weight increase of 101.4 lb. was recorded between the 90th and 260th days of pregnancy. There was then a 6.8 lb. weight loss in the remaining 25 days through to parturition. The weight loss at parturition was 80.0 lb. or 7.3% of the cow's pre-partum weight.

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