Predicting milk yield in sheep used for dairying in Australia.

It is necessary to identify traits that are simple to measure and correlated with milk yield to select ewes for dairying from existing populations of sheep in Australia. We studied 217 primiparous and 113 multiparous (second parity, n = 51; third parity, n = 40; and fourth parity, n = 22) East Friesian crossbred ewes, for 2 consecutive lactations, that were milked by machine following a period of suckling (24 to 28 d). We measured lamb growth, milk production, milk yield, and residual milk during early lactation (<d 60 of lactation) to test whether milk production during the suckling period or the growth rate of the lamb predicts milk yield. Milk production at weaning, or the amount of residual milk, or both, predict milk yield within lactations. These measures also predict milk yield between lactations. Lambs were weighed at birth and weaning and milk production in ewes was measured using a 4-h milk production test at d 5 of lactation and at weaning. Following weaning, ewes were milked twice daily and milk yield was recorded weekly for 8 wk and once a month thereafter. Milk production (using a 16-h milk production test) and residual milk were measured at weaning, and again 1 wk and 4 wk later. Milk yield to 120 d was correlated (r2 = 0.39) between lactations, and 120-d milk yield (primiparous 82.7 +/- 2.0 L; multiparous 107.1 +/- 4.2 L; second lactation 146 +/- 3.7 L) can be predicted after 4 wk of machine milking using a single measurement of either daily milk yield (primiparous 770 +/- 25 mL/d; multiparous 940 +/- 44 mL/d; second lactation 1,372 +/- 46 mL/d, r2 = 0.60 to 0.65) or daily milk production (primiparous 1,197 +/- 27 mL/d; multiparous 1,396 +/- 62 mL/d; second lactation 1,707 +/- 45 mL/d, r2 = 0.50 to 0.53). Residual milk in primiparous ewes (38%) and multiparous ewes (34%) was high (292 +/- 11 and 321 +/- 20 mL, respectively) in the first lactation, but lower (17%) in the second lactation (238 +/- 17 mL). Residual milk and 120-d milk yield were not correlated in either lactation and we suggest that the transfer of milk from the alveoli to the cistern between each milking may be an important mechanism that maintains milk yield in these ewes.