Lactation Curves for Milk Yield and Composition of Moroccan Holstein Dairy Cows

The objectives of this study were to describe lactation curves for milk yield and composition using the Wood’s model and to estimate lactation curve characteristics. Data including 202 283 test-day records be-longing to 27 108 first three lactations of Holstein cows recorded from 2012 to 2016 in a total of 193 herds were analyzed for milk yield, fat, protein, lactose, total solids (TS) and solids not fat (SNF) contents, as well as milk urea nitrogen (MUN) concentration and somatic cell score (SCS). The lactation curve and lac-tation parameters of interest were intercept (a) and curve shape (b and c) of the lactation curves, as well as the peak production (ym), the day at peak (tm) and persistency. The test-day averages milk yield, fat, protein, lactose, TS and SNF contents, as well as MUN and SCS were 24.4 ± 7.87 kg/day, 3.57 ± 0.78%, 3.04 ± 0.35%, 4.88 ± 0.26%, 12.2 ± 1.02%, 8.73 ± 0.39%, 15.8 ± 7.12 mg/dL and 4.37 ± 1.84 units, respectively. Standard lactation curve was the most frequent shape for milk yield (63.9%), lactose content (46.7%) and MUN (42.4%), while reversed standard was the most common shape for fat, protein, TS and SNF contents (59.4, 57.3, 57.8 and 54.0%, respectively) and for SCS (43.2%). The milk yield increased from calving to peak production of 28.2 kg/day that occurred 61 days from parturition and then decreased regularly to dry-off with a persistency of 6.87. The peak for lactose content and MUN concentration of 4.95% and 17.3 mg/dL, respectively was reached at lactation days 105 and 115 post-calving, respectively. Estimated mini-mum for fat, protein, TS, SNF percentages and SCS was attained at lactation days 100, 69, 83, 68 and 95, respectively with values 3.26%, 2.78%, 11.7%, 8.16% and 3.92, respectively. It was concluded that the knowledge of lactation curves for studied traits will help to improve herd management and to increase pro-duction.