North American and Dutch Holstein-Friesian (HF) cattle have undergone genetic selection for increased milk production, however, at the same time, reproductive efficiency has decreased. Overseas HF also have greater propensity to mobilise body reserves during early lactation than New Zealand HF cows. It is possible that reproductive differences are a consequence of metabolic and endocrine differences between the strains. This trial was designed to investigate effects of genotype and diet on endocrine response to a challenge of intravenous glucose in HF cows. Twenty-five HF cows of New Zealand (NZ; n=13) and Overseas (OS; n=12) origin, were fed either pasture (Grass; n=12) or a total mixed ration (TMR; n=13) in a 2x2 factorial design. At two weeks postpartum, they were challenged with 300mg of glucose/kg live weight. Data were analysed for effects of genotype and diet. Following glucose challenge, the NZ TMR cows had higher plasma insulin concentrations than the NZ Grass, resulting in a significant interaction between diet and genotype in the area under the response curve; but the OS groups had similar patterns. Mean plasma leptin concentration was higher in NZ TMR than in NZ Grass, OS TMR or OS Grass. Plasma concentrations of glucose and IGF-I were not affected by genotype or feed regime. These results demonstrated metabolic differences between the strains and also diets. A better understanding of the genetic and nutritional basis of hormonal regulation of nutrient partitioning will be important to develop nutritional strategies to meet the requirements of lactating cows with different genotypes, and to find ways to improve reproductive efficiency.
Proceedings of the New Zealand Society of Animal Production, Volume 63, Queenstown, 31-34, 2003
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