Metabolic safety-margins do not differ between cows of high and low genetic merit for milk production

Abstract

Three galactopoietic stimuli, frequent milking (4X), bovine somatotrophin (bST) and thyroxine (T4) were used in an additive stair-step design to achieve maximum output (metabolic capacity) in six peak-lactation cows of high genetic merit (HT) and six of low genetic merit (LT). A further six of each merit were untreated controls (HC, LC). Milk yield was increased significantly by 4X, increased further by the combination of 4X and bST and increased further still and significantly by the full combination of 4X, bST and T4. The magnitude of the yield response to the sequence of treatments did not differ significantly between HT and LT. The yield response to 4X and bST was sustainable without significant loss of body weight or body condition score for the 6 weeks during which these stimuli were administered. The response to the full combination, which included T4, was accompanied by significantly elevated heart rate and significant loss of body weight and condition compared with the combination of 4X and bST. As a result, treatments were discontinued, on an individual cow basis, before completion of this 6-week phase. Time on experiment did not differ between HT and LT. The results do not support the commonly held belief that selective breeding of dairy cows for high milk production has rendered them markedly more susceptible to metabolic disturbances.

Changes in milk composition in obese rats consuming a high-fat diet

“…Further maternal over-nutrition has been demonstrated to result in offspring metabolic programming (2,3) , in multiple organ systems, for example, pancreas and liver (2) , as well as in central and peripheral nervous systems involved in energy homoeostasis (4)(5)(6)(7) . Human epidemiological (2,8) and experimental animal studies (9)(10)(11)(12) have shown a correlation between maternal diet and milk composition. Few studies address the mechanisms by which maternal obesity induced by a high-fat diet regulates liver and mammary gland (MG) differentiation and function during lactation and the implications for the observed changes in milk synthesis and composition (13) .…”

Increased plasma leptin attenuates adaptive metabolism in early lactating dairy cows

“…Accordingly, the reduction in circulating thyroid hormones in early lactating dairy cows has been viewed as an energy sparing mechanism (Vernon et al 2002). This view is supported by increased energy expenditure and excessive weight loss in cows treated with exogenous thyroid hormones (Thomas 1953, Knight et al 2004. The hLeptin infusion partially corrected the thyroid hormone deficit and exacerbated the loss of body weight without impacting milk production, suggesting an increase in a nonmammary component of energy expenditure.…”

Effects of genetic capacity on milk production and on plasma metabolites in dairy cows during post-ruminal or intravenous carbohydrates or amino acid infusions

“…Plasma glucose and NEFA concentrations were not affected (P N 0.10) by genetic group. Lower plasma glucose and higher NEFA concentrations would have been expected in cows with higher genetic capacity due to their lower energy balance (Veerkamp et al, 2003) but responses have varied (Westwood et al, 2000;Åkerlind et al, 1999b;Knight et al, 2004). Westwood et al (2000) found reduced plasma glucose but similar NEFA concentrations in high merit cows, and explained the similar NEFA by a higher utilization for milk fat synthesis.…”

“…In late lactation, a numerical, but not significantly, higher (P = 0.18) IGF-1 concentration was observed in H cows than in L cows. In the studies of Michel et al (1991) and Åkerlind et al (1999b), IGF-1 was not affected by genetic merit, but higher IGF-1 concentrations in high genetic merit cows have been reported (Snijders et al, 2001;Knight et al, 2004).…”