Discipline: selection; Keywords: lactation length, spring-calving, pasture-based, management factors, genetic parameters.
Lactation yield estimates standardized to common lactation lengths of 270 or 305 days are commonly used in management decisions and genetic evaluations. The use of such measurements to quantify the merit of individual cows fails to penalize cows that do not reach the standardized lactation length, or vice versa to reward cows that lactate for more than the standardized lactation length. The objective of a study by Dr M. Williams and colleagues was therefore to quantify the genetic and non-genetic factors associated with lactation length in seasonal-calving, pasture-based dairy cows. They published their results in the Journal of Dairy Science, Volume 104 of 2021, page 561 to 574 using the title: Genetic and non-genetic factors associated with lactation length in seasonal-calving, pasture-based dairy cows.
A total of 616 350 lactation length records from 285 598 Irish cows were used. Linear mixed models were used to estimate the associations between lactation length and calving month, parity, age at calving, previous dry period length, calving difficulty score, heterosis, recombination loss, breed and herd size, as well as to estimate the genetic and residual variance components of lactation length.
The median lactation length in the data set was 288 days, with 27% of cows achieving lactations of at least 305 days. Relative to cows calving in January, the lactations of cow calving in February, March, or April was, on average, 4.2, 12.7, and 21.9 days shorter, respectively. This will correspond with spring in South Africa. The lactation length of a first lactation cow was, on average, 7.8, 8.6, and 8.4 day shorter than that of second, third and fourth lactation cows, respectively. Norwegian Red and Montbeliarde cows had, on average, a 4.7- and 1.6-day shorter lactation than Holstein-Friesian cows, respectively. The heritability estimate, coefficient of genetic variation and repeatability estimate of lactation length were 0.02, 1.2%, and 0.04, respectively. Based on the genetic standard deviation for lactation length of 3.3 days estimated, cows ranked in the top 20% for genetic merit for lactation length would be expected to have lactations 9.2 days longer than cows in the bottom 20%, demonstrating exploitable genetic variability.
It was concluded that with only 27% of the cows in the study achieving lactations of 305 days or greater, the suitability of yields standardized to a 305-day lactation, without the consideration of lactation length or persistency for genetic evaluations and decision support tools, is questionable. Whereas the heritability estimates for lactation length were lower than previously reported for cows in confinement production systems, genetic variation exists for lactation length in pasture-based dairy cows. Given the array of genetic and non-genetic factors associated with lactation length, an approach combining selective breeding and improvements in management practices may prove an efficient and effective strategy to lengthen lactations.