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Production in a Large Jersey Herd as Affected by Sires, Dams and Yearly Variations^1,2,

Iowa State College

Abstract

Milk and fat records of dairy cattle are affected by many uncontrollable factors which make comparisons difficult, especially when the records are made at different times and in different herds. The records made in the Sibley herd from 1912–1928 were made under as uniform conditions as is likely to have prevailed in any other herd of comparable size or duration. Nevertheless changes have occurred in the herd from year to year. Presumably part of these are due to differences in the merit of the sires used, part to changes in the average merit of the dams and part to changes in management or other influences which varied from year to year. A little more than one fourth of the variation in production is determined by causes which varied from year to year or with the different sires used. For the part which can be separated roughly into sire effect and year effect, it appears that the influence of the sire is only a little greater than the influence of the other factors which change from year to year, but no simple way has been found for a rigorous separation of these effects. Any a priori assumption of uniformity in environment will therefore give the sire credit for more influence than the data actually justify.

The variance of the dams was significantly smaller than the variance of the daughters. This may have been caused by closer culling of the dams. There is also evidence of a greater uniformity among the mates of a single sire, but it appears that at least part of the heterogeneity between groups of dams is due to the fact that each group of dams was tested within a comparatively few consecutive years in a herd where the production was changing during the period studied.

¹ This investigation was started at Michigan State College at the instigation of Dr. Harrison R. Hunt of the Department of Zoology.

² Journal Paper No. 131 of the Iowa Agricultural Experiment Station, Ames, Iowa. Project 317.

³ Interested Prince was born on the Island of Jersey, Dec. 9, 1899, and was kept in service until his 15th year. Spermfield Owl was born April 9, 1899, and died in 1909.