Genetic factors affecting milk production in a selected Holstein-Friesian herd
Analyses were made of all normal lactation records (up to the ninth record of each cow) in the Missouri Station Holstein-Friesian herd from its foundation in 1902 to January 1, 1950. There were 314 cows with a total of 933 lactation records. The progeny of 34 herd sires were represented. Within the 34 sire progeny groups are 299 daughters with records and 270 daughter-dam pairs. All lactation records were standardized to a herd test, 305 day, 2x, mature equivalent basis by means of factors derived from the data. A significant upward time trend in production was found. Differences between 5 year periods accounted for 5.5 per cent of the total variance in milk production, 20.4 per cent of the total variance in butterfat production and 38 .8 per cent of the total variance in butterfat percentage. Estimates of the repeatability of contemporary production records were 0.41 for milk production, 0.36 for butterfat production and 0.61 for butterfat percentage. Heritability estimates derived from the intra-sire regression of daughter on dam were found to be: 0.36 for milk production, 0.29 for butterfat production and 0.54 for butterfat percentage. Lifetime average es were used a s the measure of each cow's producing ability but the heritability estimates were expressed on a single record basis. Variation of daughters and dams records was found to be essentially equal. When each cow's lifetime average was expressed as her "most probable producing ability" no increase was observed in the correlation between the records of daughter and dam. The correlation between lifetime average milk and butterfat yield was found to be + 0.89, between milk and butterfat per cent - 0.10 and between butterfat yield and butterfat per cent + 0.35. The corresponding genetic correlations were estimated by two methods: ( a ) by the ratios of appropriate regression coefficients and ( b) by the ratios of genetic covariance to the geometric mean of genetic variance estimates. The estimated genetic correlations by method ( a ) were: + 0.87 between milk and butterfat yield. - 0.52 between milk yield and butterfat per cent, and -0.03 between butterfat yield and butterfat per cent. The three estimates by method ( b) were : + 0.99, - 0.20 and -0.13 respectively. Both the gross and genetic correlations indicate that a slight but real relative decline in butterfat percentage may be expected to accompany increased milk production. The effect of mild inbreeding was analyze d by the intra-sire regression of production on inbreeding. A significant decline of 66 pounds of milk and 2 pounds of butterfat p e r one percent in crease in inbreeding was observed. Th er e was no significant effect on butterfat percentage. The performance of herd sires a s indicated by daughter-dam comparison s is presented for 20 sires having at least 5 daughter-dam pairs. Ten of these sires increased milk production, 15 increased butterfat production, and 13 increased butterfat percentage of their daughters over their corresponding dams. No evidence of nicking was found in a tabulation of sires' daughters according to their maternal grandsires.