title: 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.
authors.affiliation: Hannah Research Institute, Ayr KA6 5HL, UK. knightc@hri.sari.ac.uk
authors.forename: Christopher H
authors.initials: CH
authors.lastname: Knight
authors.forename: Mohammed A
authors.initials: MA
authors.lastname: Alamer
authors.forename: Annette
authors.initials: A
authors.lastname: Sorensen
authors.forename: lan M
authors.initials: lM
authors.lastname: Nevison
authors.forename: David J
authors.initials: DJ
authors.lastname: Flint
authors.forename: Richard G
authors.initials: RG
authors.lastname: Vernon
mesh_terms: D000818:Animals; D001823:Body Composition; D001835:Body Weight; D002417:Cattle / Q000235:genetics* / Q000502:physiology; D005260:Female; D013006:Growth Hormone / Q000008:administration & dosage / Q000009:adverse effects / Q000097:blood; D006339:Heart Rate; D007328:Insulin / Q000097:blood; D007334:Insulin-Like Growth Factor I / Q000032:analysis; D007774:Lactation / Q000235:genetics*; D012044:Regression Analysis; D013974:Thyroxine / Q000008:administration & dosage / Q000009:adverse effects
chemical_list: D007328:Insulin; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone; D013974:Thyroxine