Relationships Among Whole-and Part-Lactation Gross Feed Efficiency, Feed Consumption, and Milk Yield

Milk production, feed consumption, and feed efficiency data were collected on 425 first lactation cows by 31 sires to determine if a part-lactation measure of feed efficiency would effectively predict total lactation efficiency. The data were summarized by ten 30 day periods (1 to 30, 31 to 60…, 271 to 300), and a 300-day lactation total basis. Three periods (two, five, and seven) were examined to estimate heritabilities and genetic correlations. Phenotypic correlations among measures of feed efficiency in adjacent periods ranged from .82 to .88. Correlations between the 10 part-lactation measures and total ranged from .64 to .87 with the highest occurring in Periods 4 and 5. The highest phenotypic correlation between part-lactation efficiency and total fat-corrected milk yield occurred during Period 5 (.72). Correlations between feed consumed in each of the 10 periods and total fat-corrected milk and total efficiency ranged from .10 to .69 and −.29 to .32. The highest genetic correlation of the three part-lactation efficiency measures with total occurred during the fifth period (1.00). The genetic correlation between fifth period efficiency and total fat-corrected milk yield was higher (.91) than the other two part-lactation measures but similar to that for total (.93). Heritabilities of the three part-lactation measures of feed efficiency were .44, .44, and .34 as compared to .56 for total lactation efficiency. Selection for efficiency during Period 5 would be 89% as effective as direct selection for total efficiency. Selection for yield during Period 5 would improve efficiency in the same period more rapidly than direct selection