Net energy efficiencies of Holstein, Jersey and Holstein-Jersey F1-crosses

Net energy efficiencies were calculated from data of an experimental herd w ith respect to type of cow, lactation number, stage of lactation and diet. The trial consisted of 71 Holstein-Friesians, 71 Jerseys and 71 Holstein-Je rsey F1-crosses in 1st, 2nd and >2nd lactation. Data were collected during 210 days of lactation, from calving to week 30 and included total dry matte r intake, energy content of foods, milk yield, milk solids, body weight, bo dy condition scores and several body measurements. The cows were divided in to four feeding groups : high and low energy content of roughage as well as high and low proportion of concentrates. Net energy efficiency was calcula ted as the ratio of milk energy to total net energy intake after subtractin g energy utilized for maintenance and body tissue change. Body tissue chang e was calculated either via body-weight changes or body condition-score cha nges. Due to the distribution of the efficiencies being skewed, efficiencies were transformed applying a Box-Cox transformation. Transformed net energy effi ciencies were analysed using a repeated measurements design considering the sequential nature of the observations. Mixed models with a compound symmet ry structure for the variance components were applied. Final models contain ed the fixed effects of type, lactation number, feeding group and the covar iates week of lactation and its square. Holstein-Jersey crosses were more e fficient than purebreds, second lactation cows were least efficient, cows g iven low energy roughage and a lower proportion of concentrates were more e fficient than cows on the other diets. Least efficient were the cows belong ing to the high energy roughage and higher proportion of concentrates group . The coefficients of determination of the final models were between 0.357 and 0.492.