Survival Curves to Evaluate Age of End of Lactation in Dairy Cows from Different Genotypes

Abstract

Lactancies were analysed in four dairy genotypes: Holstein (H), Canadian Jersey (F1), F1x Brown Swiss (P) and F1x Guernsey (G) between the years 2007 and 2011. The cows were fed on pasture, with the supplementation of concentrated fodder and they were handled in the same milking premises. Survival function was estimated through Kaplan and Meier [10] and the differences between functions were assessed by a Wilcoxon [24] test. Significant differences were found among the survival curves of all the groups in the four lactancies. In all the genotypes cows have a 50 % probability of  ending their first lactancy before they are 1100 days. It is 80% probable that the cows end their second lactancy from their 1500, 1600, 1700 or 1900 days, depending on their genotype: G, P, F1 or H respectively. The probability for F1, G and P  cows to end their third lactancy before their 1900 days is 80 % whereas for H, it is 20 %. Half the H cows are at least 2600 days by the end of their fourth lactancy, while the same proportion of G and P cows end with less than 2250 days and F1 do it with less than 2400 days. In the system under consideration and for the years assessed, the four genotypes showed different results as regards the age when they ended their lactancies. The H cows are more likely to end their lactancies at an older age and with a wider dispersion in time when compared with the F1 cows and –specially the G and P cows. Besides, the F1, G and P cows are more likely to get a third or fourth lactancy than the H cows.