title: Effects of increasing days of exposure to prepartum transition diets on milk production and milk composition in dairy cows.
abstract: OBJECTIVE
  To evaluate the effects of length of exposure to prepartum transition diets on milk yield, fat and protein production.
  
  
  METHODS
  Prospective cohort study. The number of days that the cows were fed the prepartum transition diets was the exposure of interest.
  
  
  METHODS
  Holstein and Holstein x Jersey cows (n = 1008) were enrolled. Diets given in the far-off dry period (from end of lactation until approximately 3 weeks before expected parturition) consisted of ad libitum access to perennial ryegrass pastures. Prepartum transition diets included perennial ryegrass pasture, ryegrass silage, cereal hay, grain, grain by-product, protein meals, BioChlor, sodium monensin, virginiamycin or tylosin, MgSO(4), trace elements and vitamins. On a dry matter basis, these contained 16.0% crude protein, 4.2% rumen undegradable protein, and 9.9 mJ metabolisable energy/kg. Diets provided an estimated metabolisable protein balance of 286 g/day and dietary cation anion difference of -150 meq/kg dry matter. Statistical models controlled for effects of herd, calving day, breed, age and gestation period.
  
  
  RESULTS
  Increasing length of exposure to the prepartum transition diets significantly increased the 4.0% fat- and 3.2% protein-corrected milk yield and milk-protein yield as a linear and quadratic effect. The optimal duration of exposure to the prepartum transition diets was 25 days for fat- and protein-corrected milk production and 22 days for milk protein production. Milk-fat percentage decreased significantly and linearly with increasing exposure to the prepartum transition diets; however, milk-fat yield or milk-protein percentage did not vary significantly with duration of exposure to the diets.
  
  
  CONCLUSIONS
  Increasing exposure to prepartum transition diets increased milk and milk-protein yields and decreased the milk fat-percentage, but not the milk-protein percentage or milk-fat yield.
authors.affiliation: Veterinary Clinical Studies, University of Sydney, NSW, Australia.
authors.forename: P J
authors.initials: PJ
authors.lastname: Degaris
authors.forename: I J
authors.initials: IJ
authors.lastname: Lean
authors.forename: A R
authors.initials: AR
authors.lastname: Rabiee
authors.forename: C
authors.initials: C
authors.lastname: Heuer
mesh_terms: D000821:Animal Feed; D000824:Animal Nutritional Physiological Phenomena / Q000502:physiology*; D000818:Animals; D002417:Cattle / Q000502:physiology*; D015331:Cohort Studies; D003612:Dairying / Q000379:methods; D005223:Fats / Q000032:analysis; D005260:Female; D007774:Lactation / Q000378:metabolism*; D008892:Milk / Q000737:chemistry* / Q000378:metabolism*; D008894:Milk Proteins / Q000032:analysis; D011446:Prospective Studies; D013997:Time Factors
chemical_list: D005223:Fats; D008894:Milk Proteins