Relationships between some estimates of growth hormone and prolactin secretion and rates of accretion of constituents of body gain in rams

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Abstract
Relationships between estimates of some factors affecting growth hormone (GH) and prolactin (PRL) secretion and rates of deposition of fat, protein and ash and rate of gain were investigated in Suffolk ram lambs. Factors affecting secretion were determined from measurements made by radioimmunoassay on blood samples collected at 15-min intervals during a 6-h period at a body weight of 43 to 58 kg. Depositional traits were estimated over the period of growth from 32 to 73 kg. The endocrine secretory measurements were adjusted statistically for weight at blood sampling. The only single endocrine secretory measurement that significantly correlated with a growth or depositional trait was frequency of GH peaks which was negatively related to rate of ash accretion. Multiple regression analysis was used to estimate the association between endocrine secretory measurements and rates of gain and accretion. R2 values were 0·43, 0·29, 0·42 and 0·70 for average daily gain and accretion rate of protein, fat and ash, respectively. Models with minimal error mean squares contained linear, quadratic and/or crossproduct terms for the independent variables, thus suggesting nonlinear relationships of the hormones in the mediation of rate and pattern of growth. These results emphasize the value of assay of multiple samples from each animal, the subsequent estimation of secretory characteristics and the use of expanded statistical models to determine the relationship present in the system under investigation.