Milk production and body composition of single-bearing East Friesian × Romney and Border Leicester × Merino ewes

• Lactational performance of EF × Romney (EFR) and Border Leicester × Merino (BLM) ewes were measured.
• Potential milk production and component yield was higher in EFR than BLM ewes and declined as lactation advanced.
• Changes in tissue energy estimated using dual energy X-ray absorptiometry scans were highly correlated to average estimated energy balance.
• EFR ewes have greater potential milk yield than BLM ewes but this was not realized as increased daily gain of single lambs possibly because of insufficient difference in milking pressure.

The introduction of East Friesian (EF) genetics into commercial sheep milking herds has the potential to improve milk yield and offer a robust dam for meat lamb production systems. This study was conducted to measure the milk production, feed intake and longitudinal changes in body composition of EF × Romney (EFR) and Border Leicester × Merino (BLM) ewes over the first nine weeks of lactation. Sixteen seconds parity single-bearing EFR (n = 8) and BLM (n = 8) ewes that had been mated to a purebred EF ram were housed in individual pens from 2 weeks prior to, and until 9 weeks after, lambing and fed a commercial pelleted diet with additional chaff. Milk yield was measured twice a week using the four-hour milking interval technique after injection of oxytocin and body composition was determined by dual energy X-ray absorptiometry (DXA) at 1, 3, 5 and 9 weeks of lactation. Potential milk production was higher in EFR than BLM ewes (2.57 vs. 1.92 kg/d, P = 0.028) and declined as lactation advanced. Similarly, the yields of milk fat (P = 0.015), protein (P = 0.018) and lactose (P = 0.062) were all higher in EFR than BLM ewes and declined as lactation advanced. Energy intake increased over the first 4 weeks of the study, before reaching a plateau for the remaining 5 weeks of the study. However, there was no significant difference in energy intake or energy balance between EFR and BLM ewes. Changes in tissue energy between DXA scans were highly correlated (R2 = 0.51, P < 0.001) to average estimated energy balance over the same periods. There was no effect of breed on birth weight of lambs nor was there any effect of dam breed on average daily gain of lambs. In conclusion, EFR ewes have greater potential milk yield than BLM ewes but this was not realized as an increased average daily gain in the single lambs possibly due to insufficient milking pressure failing to produce a difference in actual milk yield. It is likely that this difference would be expressed in response to a greater milking demand (i.e. from a milking machine or twin/triplet lambs). The EFR ewes ate more than BLM ewes during lactation and mobilized less fat than BLM ewes to maintain similar nursing lamb growth. However, a greater milking demand of twins or triplets may allow the greater potential milk yield of EFR to be expressed. Alternatively, machine milking EFR would most likely allow the expression of their full milking potential. Finally, DXA was able to predict body tissue mobilization and accretion in lactating ewes and these changes in body composition were related to estimated energy balance indicating that DXA can be used to serially determine body composition in lactating sheep.