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  1. 1. TheSchoolofVeterinaryMedicineandScience The butterfat to protein ratio: A poor predictor of reproductive performance in UK dairy cows A. Madouasse1, W.J. Browne2, J.N. Huxley1, A.J. Bradley1,3 and M.J. Green1 1School of Veterinary Medicine and Science, University of Nottingham 2 School of Veterinary Science, University of Bristol 3 Quality Milk Management Services (QMMS) Ltd., Westbury-sub-Mendip The milk fat to protein ratio is widely used by veterinarians and farmers in the UK as a marker of energy balance. Many studies have found an association between negative energy balance and a lower percentage of milk protein, a higher percentage of butterfat and a higher fat to protein ratio. Most of these studies were performed in a limited number of herds often under similar management. It is therefore hard to assert whether their conclusions are relevant to the UK dairy cow population. Aim of the study: Predict the calving to conception interval from milk recording data collected on the first 2 test-days of lactation Introduction Conclusion Reference A. Madouasse, J. N. Huxley, W. J. Browne, A. J. Bradley, I. L. Dryden, and M. J. Green. Use of individual cow milk recording data at the start of lactation to predict the calving to conception interval. J Dairy Sci, 93(10):4677–4690, Oct 2010. • Monthly collected milk recording data • England and Wales • January 2004 to December 2006 8 million individual cow recordings 1 million lactations 0.5 million cows 2,128 herds Data Discrete time survival model • Multilevel logistic model with time intervals as covariates • Outcome: Conception in an interval (5 intervals between 20 and 145 days in milk) • Herd random effect for the probability of conception in each interval • Covariates corrected for either stage of lactation or stage of lactation and time of the year Predictors of the calving to conception interval • Milk yield on test-day 2 • Protein on test-day 2 • Lactose on test-day 1 • Protein on test-day 1 • SCC on test-day 2 • SCC on test-day 1 • Butterfat on test-day 1 (limited association)  The fat to protein ratio was a poorer predictor than each of its components on its own Why is the fat to protein ratio a poor predictor of conception? The mean percentage of butterfat varies considerably with both stage of lactation (vertical axis) and time of the year (horizontal axis). The mean percentage of protein varies with stage of lactation. Variation with time of the year is only important after 50 days in milk. The variation in the fat to protein ratio with time of the year is large in regard of its variation with stage of lactation, especially at the start of lactation. Study The percentage of butterfat and the fat to protein ratio vary importantly with time of the year indicating a possible effect of feeding on their value. Unless it can be accounted for, this precludes to their utilisation as predictors of negative energy balance.