The growth rates of Merino and Romney lambs were compared at three stocking rates (200, 100 and 50 lambs/hectare) but at the same herbage allowance (2 kg DM/head/day). To achieve this, lambs at lower stocking rates were allocated different areas (0.21, 0.4 and 0.8 ha) and shifted at different frequencies (5.1, 9.8 and 24.5 days, respectively). At each stocking, rate groups of 10 lambs of each breed were grazed both separately and together. Each treatment was replicated. Grazing clocks were used to record grazing times in the mixed grazing groups at both the high and low stocking rates. Romney lambs grew significantly faster than Merinos (P<0.001) at all stocking rates. As stocking rate increased there was an overall decrease in LWG (P<0.001). However LWG of Merinos decreased at a significantly faster rate (P<0.005). The effects of stocking rate on breed differences in LWG were supported by grazing data. At 50 lambs/ha, Merinos and Romneys grazed for 10.5 and 11.1 hrs/d, respectively. At 200 lambs/ha, Merinos grazed for 7.2 hrs/d, compared with 11.0 hrs/d Romneys. The pattern of grazing also changed, with Merinos spending more daytime grazing as stocking rate increased from 50 to 200 lambs/ha (60% versus 82%) whereas Romney continued to spend 80% of total grazing time as daytime grazers.
Proceedings of the New Zealand Society of Animal Production, Volume 52, , 203-206, 1992
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