Body composition and weight changes of mature grazing Angus cows

In vivo body composition was estimated by both direct and indirect methods 33 times on 18 mature Angus cows grazing either high or low quality pasture. The direct method consisted of deuterium oxide in a deuterium oxide dilution technique employing a two compartment open model,

Body composition variables estimated were: empty body weight (EBWT); empty body water CEBW); empty body fat (EBFAT); percent water (PERH₂O); percent fat (PERFAT); gross energy (GE); and gross energy per body weight (GEPBW). Water kinetics variables estimated were: flow rate of water between gut and nongut compartments (FAB); flow rate of water between nongut and gut (FBA); and, flow rate of water from the body (FOA),

As cow weight increased, water flow rates between gut and nongut compartments decreased for cows grazing high quality pastures (P < .04) but not for cows grazing low quality pasture type (P > .23). Flow rate from the body was not related to weight and fatness. Cows grazing high quality pastures had larger PERH₂O (P < .05), PERFAT (P < .05), GE (P < .03) and GEPBW (P < .05).

Forty mature lactating Angus cows were selected to represent the variation in mature fall weight and milk production of Angus cows.

Cows were allotted to either high or low quality pasture. Weight and ultrasonic estimate of fat thickness were taken every two weeks from the time of calving until a low weight was attained and then once per month until weaning. Certain cow and calf measurements were obtained to provide information useful in explaining animal variation in cow weight and fatness change during the year.

The Gauss-Newton iterative process was employed to determine the least squares estimates of the Fourier coefficients from data for each cow.

Results indicate that cows with large fall heights, fall weights, fall fat thickness and dry matter intakes had large mean weights during the year. Fall height (P < .13), fall weight (P < .10), milk production (P < .01), digestible dry matter intake (P < .003), dry matter intake (P < .06) and digestibility of the pastures (P < .03) were observed to influence the shape of the cow weight change curve. Fall weight was the only variable noted to influence the periodicity of the cow weight change curve (P < .005).