Diurnal Variation in Weight and Rates of Shrink of Range Cows and Calves

Abstract

Cow-calf pairs were weighed on successive mornings to determine the effects of time on total weight. Early morning weights of mature Hereford/Angus crossbred cows were approximately 2.5% less than late morning weights in both the spring and summer. Weights of suckling calves were not signifilcantly different between early and late morning. Linear regression analyses indicated drylot shrink weights of cows were pnmarily a function of length of time of shrink. Rate of weight loss was approximately 1% every 3 hours after an initial 3 hour loss of 3.5%. Secondary factors were status of cow (dry or wet), relative humidity (%), season (spring or summer) and initial cow weight. Shrink rates were slightly greater for wet cows than dry cows; when relative humidity was low; during spring; and for lighter weight cows. Rates of shrink of calves were primarily related to size of calf with calves weighing less than 53 kg (117 lb) gaining weight and calves weighing more than 53 kg losing weight. Differences in liveweight of livestock are often used to evaluate the effect of various experimental treatments on livestock performance. At the Texas Experimental Ranch the magnitude of weight change over time of individual cows and the weaning weight of their calves are the principal parameters utilized to contrast the effects of stocking rate, grazing system and level of winter supplement (Heitschmidt et al. 1982). Because of the number of cattle in each treatment herd, gathering and weighing generally begins in early morning and extends into the early afternoon over a period of 3 days. Thus, weight differences between herds may reflect not only treatment effects but also the effects of the time of day the animals were gathered and weighed. The first objective of this study was to quantify differences in cow and calf weights as a function of time of day when weighed. The second objective was to examine the rate of drylot shrink of both cows and calves as a function of length of shrink, envi ronmental conditions, initial weight and time of day when the animals were gathered and penned. This objective was established to evaluate the feasibility of gathering herds and weighing after a predetermined period of shrink so as to standardize weights.