Effect of ewe prolificacy potential and stocking rate on primiparous flock performance

• Ewe prolificacy potential had no effect on lamb ADG after six weeks of age.
• Increasing ewe prolificacy potential and stocking rate reduced animal performance.
• Flock output increased as prolificacy potential and stocking rate increased.

The objective of this study was to investigate the effect of ewe prolificacy potential (PP; predicted number of lambs born/ewe/year) as dictated by sire breed type, stocking rate (SR; ewes/ha), and their interaction on animal performance in a primiparous flock. The study had a 2 × 3 factorial design, consisting of two PP levels and three SR incorporating 360 primiparous ewes, comprising of two ewe genotypes (180 medium prolificacy potential (MP – Suffolk crossbred ewes) and 180 high prolificacy potential (HP – Belclare crossbred ewes)). Stocking rates were low (LSR; 10 ewes/ha), medium (MSR; 12 ewes/ha), and high (HSR: 14 ewes/ha). There were 60 ewes per treatment and each treatment was assigned to an individual farmlet of five paddocks for the duration of the study. There was no PP by SR interactions observed for ewe body weight (BW) and body condition score (BCS; P > 0.05). Medium prolificacy potential ewes had a higher BW and BCS at initial mating, pregnancy scanning, 6 weeks post-lambing and weaning (P < 0.001). At lambing and second mating ewe BCS did not differ by PP (P > 0.05). Lambs born to HP ewes achieved a lower average daily gain (ADG) up to 6 weeks of age, but a higher ADG from 10 to 14 weeks of age compared to MP lambs (P < 0.01). Post-weaning, and lifetime lamb ADG, carcass traits (weight, conformation, fat score), and days to slaughter (DTS) did not differ by PP. Low SR ewes had a higher BW than ewes at the MSR and HSR, which did not differ from each other at second mating (P < 0.001). Lower lamb ADG was recorded at the MSR and HSR pre- and post-weaning (P < 0.01) compared to LSR, with MSR and HSR lambs subsequently requiring longer to reach slaughter (P < 0.001). Total herbage utilisation was highest at the HSR and lowest at the LSR (with the MSR not differing from either; P > 0.05), above the post-grazing sward height of 4.0 cm pre-weaning and 3.5 cm post-weaning (P < 0.05). Carcass output per hectare increased as both PP and SR increased (P < 0.001). In summary, Primiparous flock performance was significantly reduced as SR increased from 10 ewes/ha to 12 ewes/ha and 14 ewes/ha, with no further decrease in performance from 12 to 14 ewes/ha. Increased carcass output per hectare is achievable through the use of greater PP and SR levels within a temperate grass-based lamb production system.