Using curfew weight reduces variation in weight and average daily gain in sheep – implications for grazing studies

Context The weight of sheep and cattle can be measured in a number of ways, including straight off feed (full or liveweight, LWt) or after an overnight fast from feed and water (curfew weight). The use of curfew weight to estimate weight and average daily gain (ADG) has been studied in cattle, however, the effect of different weight measures on ADG in sheep has not previously been reported. Aims To determine whether weight or ADG of sheep grazing different forage treatments was influenced by using LWt or curfew weight for different time intervals between weight measures. Methods Eighty lambs grazed four different forage crops for 42 days: cereal only, cereal + brassica, cereal + legume or cereal + brassica + legume. The weight of lambs was measured straight off pasture (LWt) and after a 16 h overnight fast from feed and water (curfew weight). The ADG of lambs was calculated from LWt and curfew weight for 2, 4- and 6-week time intervals. Key results The correlation between LWt and curfew weight was significant (P < 0.001) at each weight measurement (r2 = 0.92, 0.89, 0.95 and 0.94 for Days 0, 14, 28 and 42 of grazing, respectively). Mean ADG was higher when LWt rather than curfew weight was assessed for Day 0–28 (320.6 vs 294.9 ± 6.48 g day−1) and Day 0–42 (288.8 vs 267.1 ± 5.28 g day−1). The relationship between ADG calculated from LWt and curfew weight was higher between Days 0 and 42 (r2 = 0.87) compared with Days 0–28 (r2 = 0.85) or 0–14 (r2 = 0.57). Conclusions The positive relationship between LWt and curfew weight indicates that LWt can be used confidently for animal husbandry practices. However, when average daily gain data is required, particularly for research studies examining different feeding treatments, curfew weight should be used to minimise the error associated with gut fill and to more accurately reflect the magnitude of gain. Implications The number of sheep required to estimate ADG in grazing studies is less when curfew weight rather than LWt is used. If using LWt, the time interval between weight measures should be at least 28 days.