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.

Increasing exposure to a prepartum transition diet improved the calving to conception interval, tended to improve the odds of pregnancy and reduced the risk of culling of cattle, but increased the hazard of clinical mastitis.

The prepartum transition diet consisted of ryegrass pasture, cereal hay, grain, grain by-product, protein meals, BioChlor, rumen modifiers, minerals and vitamins and contained 13.9% crude protein, 10.0 MJ metabolisable energy/kg and a dietary cation anion difference of -35 meq/kg dry matter. Forty cows were bled bi-weekly from their introduction to the prepartum transition diet until day 35 of lactation. Blood samples were submitted for estimation of a range of metabolites. Cubic smoothed splines were fitted to scatterplots of metabolite concentration as a function of day relative to calving and the area under the curve (AUC) was calculated. Linear regression modelling determined the effect of days exposed, age, body condition score (BSC) and calving day on AUC. Results The prepartum AUC of blood phosphorus and beta-hydroxybutyrate increased and the AUC of blood calcium and cholesterol decreased with increasing days exposed to the diet. The postpartum AUC of beta-hydroxybutyrate and non-esterified fatty acids decreased with increasing days exposed but increased with days exposed x age and days exposed x BCS, respectively. The AUC of other metabolites did not vary significantly with number of days exposed. Conclusions Increasing exposure to the prepartum diet significantly altered the AUC of blood metabolites associated with mineral, energy and protein metabolism in ways consistent with improvements in production and reproduction previously reported and the result suggests links between energy, protein and skeletal metabolism.