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Age and Month of Calving Adjustments of Holstein Protein, Milk, and Fat Lactation Yields¹

Department of Dairy and Animal Science, The Pennsylvania State University, University Park 16802

ABSTRACT

Effects of age and month of calving on Holstein protein, fat, and milk yields were estimated from 15,867 lactation records from the Pennsylvania Dairy Herd Improvement Association files. Herds were absorbed, and constants were fit for year of calving before the function of age was derived. A separate quadratic function of age for each calendar month of calving was determined. Herd effects accounted for 28.9% of variation in protein yield, 27.1% in fat yield, and 26.7% in milk yield. Increases in multiple correlations squared were .6% for fat and milk and .8% for protein with inclusion of year in the model and 1.3% for protein, 1.4% for fat, and 2.1% for milk with the inclusion of month of calving. Linear effects of age within month of calving accounted for multiple correlation squared increases of 13.2, 13.4, and 14.2% for protein, fat, and milk whereas quadratic effects increased these percentages additional 5.8, 5.7, and 5.6%, respectively. These models were used to calculate factors for age and month of calving for each trait.

The factors adjust for age and month of calving to maturity and average month of calving.

¹ Journal No. 6319, Pennsylvania Agricultural Experiment Station, The Pennsylvania State University, University Park 16802.