Month, Year, and Herd Effects on Age Adjustment of First Lactation Milk Yield

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Abstract
First lactation Record of Performance records on 59,778 Canadian Holsteins calving over 4 yr were studied. Within each 6 me year-season, least squares con- stants were derived for combined effects of month of age and calendar month of calving (age-month) on milk yield with herd effects eliminated. Herd by age- month interaction accounted for 2.8% of the total variance compared to 3.6~ for age-months, 41.5~ for herds, and 52.1~ for error by method III of Henderson. Within season, month of calving affected age-month constants, and those effects were similar for all years. A quadratic curve described the relationship between age-month constants and age at calving better than a linear one. Quadratic curves differed across years, months, and month by years; however, differences were small, erratic, and of little practical importance. The quadratic curve pooled within mznth and year of calving was similar to com- parable estimates from New York and to previous results from regression analyses within herd-year-season. This curve al- lows for 213.2 ___ 21.8 kg milk/me and --2.7 +_ .4 kg milk/me 2 for age at first calving and is recommended for age ad- justment of first lactation milk yield.