Blood metabolite and hormone concentrations of dairy calves differing in genetic potential for milk fat production : a thesis presented in partial fulfilment of the requirements for the degree of Masterate of Agricultural Science in Animal Science at Massey University

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Massey University
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The present study was conducted at the Massey University Dairy Research Unit to investigate the effect of genetic merit for milk fat production on the physiology and metabolism of Friesian calves. Twenty four Friesian calves divided into four groups namely High Breeding Index (HBI) heifers, HBI bulls, Low Breeding Index (LBI) heifers, and LBI bulls were challenged with four different experimental treatments, ie. fasting, feeding, intravenous arginine infusion, and subcutaneous synthetic corticosteroid injection at ten to eighteen days of age. A total of eighteen blood samples were collected from each calf through an indwelling jugular cannula and the concentrations of plasma glucose, insulin, GH and cortisol were determined. Some statistically significant differences were found in plasma metabolite and hormone concentrations between the HBI and LBI groups. 1. The basal glucose concentration in HBI group was significantly higher than that in LBI group (P<0.05). The basal plasma insulin concentration was also significantly higher in HBI group than in LBI group (P<0.01). The basal GH concentration in HBI calves was higher in HBI calves than in LBI calves, but the difference was not quite significance at 5% level (P=0.059). 2. Following feeding, plasma insulin and GH concentrations in HBI group were significantly higher than those in LBI group (P<0.01, P<0.05 respectively). 3. Acute intravenous arginine infusion induced hyperinsulinemia and hypoglycemia in all calves. LBI calves had significantly higher increments of plasma insulin measured as a percentage of basal levels than HBI calves. The response of GH concentration to arginine challenge differed significantly in terms of level and pattern between HBI and LBI groups, with the HBI calves having more prolonged higher GH concentration than LBI calves (P<0.05). 4. Subcutaneous injection of synthetic corticosteroid resulted in significant increments in plasma glucose and insulin concentrations, and a significant decrease in endogenous cortisol production in all calves. (P<0.01, P=0.05, P<0.01 respectively). But no significant differences were detected between HBI and LBI groups. Effects of sex on plasma metabolite and hormone concentrations were also found in the present study. Plasma insulin concentration was consistantly higher in bulls than in heifers and the differences were significant at the time of fasting, after feeding, and after arginine infusion (P<0.01). Plasma glucose concentrations following feeding were significantly higher in bulls than in heifers (P<0.05). GH concentration was slightly but not significantly higher in bulls than in heifers for most of the experiment. It was concluded that differences exist in some important metabolic and endocrinological characteristics between HBI and LBI calves, and these differences could become significant under certain physiological conditions and experimental treatments such as those applied in the present study. This study also showed the promise of identifying genetically superior Friesian dairy cattle at an early age by using physiological markers. However this possibility has yet to be tested by earring out measurements on calves for which breeding index value for milk fat production will be determined by methods such as progeny test.
Milk fat, Friesian calves, Blood metabolite