Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 2, 2010
Publication Date: March 1, 2011
Citation: Cole, J.B., Null, D.J., De Vries, A. 2011. Short communication: Best prediction of 305-day lactation yields with regional and seasonal effects. Journal of Dairy Science. 94(3):1601-1604. Interpretive Summary: In the United States, best prediction methodology is used to calculate lactation yields from individual test day yields, and uses lactation curves that do not account for differences among regions of the country or seasons of calving, which may result in biased estimates of 305-d lactation yields. Multiplicative factors were used to pre-adjust test day records for regional and seasonal effects and compared to lactation curves that differed by region and season. Mature equivalent yields were similar in all cases. Actual yields were slightly more accurate after seasonal adjustment. Multiplicative factors are preferred over seasonal and regional lactation curves because they are easier to calculate.
Technical Abstract: In the United States, lactation yields are calculated using best prediction (BP), a method in which test day (TD) data are compared to breed- and parity-specific herd lactation curves that do not account for differences among regions of the country or seasons of calving. This may result in biased estimates of lactation yields. Data from 538,090 lactations of 348,123 Holstein cows with lactation lengths between 250 d and 500 d, records made in a single herd, at least 've reported TD, and twice-daily milking were extracted from the national dairy database (NDDB). Herds were assigned to one of 7 regions of the country, individual lactations were assigned to 3-month seasons of calving, and lactation curves for milk, fat, and protein yield were estimated by parity group for regions, seasons, and seasons within regions. Multiplicative preadjustment factors (MF) also were computed. The resulting lactation curves and MF were tested on a validation dataset of 538,090 lactation records of 348,123 Holstein cows sampled at random from the NDDB. Mature equivalent (ME) milk, fat, and protein yields were calculated using the standard and adjusted curves and MF, and differences between 305-d ME yields tested for significance. Yields calculated using 50-d intervals from 50 to 250 DIM and using all TD to 500 DIM allowed comparisons of predictions for records in progress (RIP). Differences in ME milk ranged from 0 to 51 kg and were slightly larger for first- than later-parity cows. Milk and components yields did not differ significantly (P > 0.05) in any case. Correlations of yields for 50-d intervals with those using all TD were similar across analyses. Yields for RIP were slightly more accurate when adjusted for regional and seasonal differences.