The influence of pasture crude protein intake on dairy cattle conception efficiency : a thesis presented in partial fulfilment of the requirements for the degree of Master of Philosophy in Veterinary Clinical Sciences at Massey University

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Massey University
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Excessive dietary rumen degradable protein and high blood urea nitrogen (BUN) levels are reported to depress conception efficiency in dairy herds. High protein levels occur in New Zealand dairy pastures. This study explores the association of pasture protein and energy levels, and cow BUN, protein and energy measures with conception efficiency in dairy cows in the Manawatu region of New Zealand. A survey study was conducted on 10 dairy farms on which samples of blood and vaginal mucus were collected from 745 cows within 2 hours before artificial insemination. Blood was analyzed for BUN, serum albumin (ALB), glucose (GLUC), Bhydroxybutyrates (BOHB) and non-esterified fatty acids (NEFA) while vaginal mucus was analyzed for urea nitrogen content (MUN). Pasture samples were collected weekly during the animal sampling period and analyzed for crude protein, metabolizable energy and dry matter content. Reproductive records, including results of pregnancy diagnosis conducted 8 to 15 weeks after services at which samples were collected, were obtained from the farms and entered into the DairyCHAMP computer program. Pasture crude protein levels ranged from 13% to 28%. The associations between dietary crude protein levels classified as high, medium and low and weekly conception rates indicated that as pasture crude protein increased, fertility decreased (P<0.005) in cows in their fourth or greater lactation. There was little variation in pasture metabolizable energy content and no significant association existed with conception rate. The association of the probability of conception with BUN and MUN, and blood levels of ALB, GLUC, BOHB and NEFA when tested by logistic regression analysis, revealed a significantnegative association between BUN and fertility (P<0.005). MUN was positively associated with the probability of conception (P<0.05). Dietary crude protein from pasture and BUN had a similar negative influence on fertility in pastured cattle as was observed in heavily supplemented or fully fed cattle in the northern hemisphere.
Dairy Cattle, Fertility, Reproduction