Variables Affecting Ratio Factors for Estimating 305-Day Production from Part Lactations

Complete lactations for 12,561 Holsteins, 2,262 Guernseys, 990 Jerseys, and 459 Brown Swiss recorded in Michigan DHIA-IBM from June 1954 through July 1957 were analyzed to ascertain the relative effects of breed, herd, age, parity, and season of freshening on the relationship of total to part milk production, and then were used to derive ratio factors for extending partial records to 305 days.The ratio of total milk produced on ten nmnthly test days to milk produced on each test day was used to measure the relationship between total and part production.Components of variance of ratios indicated that lactation number had a larger influence on the total to part relationship than did age at freshening.Season of freshening also exerted an influence, but to a slightly less degree than either lactation number or age, while the effect of herd was snmll and unimportant.Differences between breeds existed both in the components of variance and in the total to part relationship.Ratio factors for extending records from each of ten monthly test days and from cumulative test-day production were computed for different ages, lactation numbers, and seasons of freshening for each of the four breeds.Only small differences exist between the factors which adjust for ages and those which adjust for lactation number, indicating that either set of factors should adequately extend incomplete records.However, factors based on age are more useful in extending those records in which age at freshening and lactation number do not coincide.In practice breed, age, and season of freshening should be considered in extending partial records to 305 days.More than one-fifth of the lactations started in DHIA are not completed because the cows die or are sold (1, 7).Seldom are the records terminated early in lactation used in programs to improve dairy cattle, and what information they contain is lost or ignored.One method of salvaging the information which is lost when a cow leaves the herd prior to completing a lactation is to extend the incomplete record to estimate from its production before leaving the herd what the cow would have produced had it remained.The most important use for extended incomplete records is to make possible including in a sire's progeny test a record for every daughter that comes into production.This would help to standardize sire proofs by compensating for selective removal of poor producers from the herd prior to completion of the first lactation.Extended incomplete records can also be used to project what a cow will produce in a lactation while that lactation is still in progress.This early information would facilitate selection prior to the completion of a lactation.At present, selection of a bull on the performance of his offspring is delayed until several of his daughters have completed at least one lactation.Projection of records still in progress could reduce the time needed to prove a sire by up to 9 me.Several workers (2-5, 8-11) have reported multiplicative and regression factors for estimating complete lactation yield from various portions of the lac-