Evaluation of two indirect predictors of current dairy milk yield in Dorset ewes

The aim of the four experiments in this paper was to determine if either a 4-h test or udder volume could be used to select Poll Dorset ewes for high daily dairy milk yields. In Experiment 1, milk yields were estimated using a 4-h test in which ewes were injected into the jugular vein with 1 IU oxytocin, and machine-milked after an interval of 4 h (FHT milk yield). The milk volume from the second milking was multiplied by 6 to give 24-h FHT milk yields. There were no relationships between FHT milk yield measured on ewes suckling lambs and subsequent dairy milk yields. There was a correlation (r = 0.46; P = 0.012) between FHT milk yield of machine-milked ewes and daily dairy milk yields measured the previous day. In Experiments 2, 3, and 4, the udder volumes were calculated from measurements of the lateral and longitudinal semicircumferences of the udder. In Experiment 2, the udder volumes were measured before and after milking on 23 hoggets and 28 mature Poll Dorset ewes, which were 55 and 97 days respectively into their lactation. The correlations between udder volume before milking and dairy milk yield were higher after a log 10 transformation of udder volume and were higher in hoggets (r = 0.64; P < 0.01) than in mature ewes (r = 0.44; P < 0.05). The decrease in udder volume after milking was similar in magnitude to the mean dairy milk yields and there were significant correlations between these two traits (r = 0.59 for hoggets and r = 0.48 for mature ewes). In Experiment 3, the udder volume measured on 90 mature Poll Dorset ewes before lambing was not correlated with dairy milk yields. In Experiment 4, the udder volumes of 69 pregnant Poll Dorset hoggets were measured every 7 days starting 4-6 weeks before lambing. Dairy milk yields were measured every 2 weeks for 10 weeks after lambing. Udder volumes increased exponentially over the last month before lambing. There were moderate but significant (P < 0.05-P < 0.01) correlations (r = 0.37-0.56) between udder volumes measured 8-14 and 15-21 days before lambing and mean dairy milk yields measured over the first 4, 6, and 10 weeks of lactation. Selecting the top 25% of hoggets on udder volume increased mean milk yields by 23-27%. Number of lambs born per ewe had no effect on the dairy milk yield.