|dc.description.abstract||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
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