The consequence for feed dry matter intake of grazing sheep, cattle and goats to the same residual herbage mass

H.A. Collins and A.M. Nicol

Animal Science Group, Lincoln University, Canterbury

Proceedings of the New Zealand Society of Animal Production. 1986, 46: 125-128

Mixed grazing experiments require decisions on the equivalence or interchangeability of grazing species. Often equivalence is based on an assumption of the relative feed intake of the species concerned. Another approach is to equate the species in terms of the effect they have on the pasture e.g. removal of herbage mass. In this experiment the species response to given sward conditions was investigated by controlling the rate of disappearance of herbage mass so that the equivalence of the species in terms of intake was derived rather than assumed.

An autumn/winter ryegrass/white clover sward was grazed separately by growing castrate cattle (320kg), sheep (28kg) and goats (17kg). Plot area and animal numbers were adjusted as necessary (on a daily basis) to achieve the same rate of dry matter (DM) disappearance for each species. The 2 rates of DM disappearance for each species were achieved over 4 and 10 days and were 522 (±23) kgDM/ha/d (Fast) and 215 (±33) kgDM/ha/d (Slow). Initial herbage mass was 3100 (±220) kgDM/ha. Final residual herbage mass was 860 (±310) kgDM/ha.

The relative rate of decline of apparent intake (/kgW 0.75) with decreasing herbage mass was lowest for cattle and highest for goats, meaning the apparent intake of cattle was less affected by lowering the residual herbage than that of sheep or goats. The difference between species was greater when the herbage mass was being slowly depleted than when it was depleted rapidly. This difference between species in the rate of decline of apparent intake at different herbage masses and allowances markedly alters the "stock equivalence" (the number of animals required to remove a given quantity of herbage) of a species. For example at 2 herbage masses, the ratios of "stock equivalence" were calculated as 1:5.5:8 (at 2800 kg DM/ha) and 3.3:44:407 (at 1500 kg DM/ha) for cattle: sheep : goats respectively where unity was the stock equivalent of a 320 kg steer grazing pature of 2800 kg DM/ha.

Keywords: NZSAPAB; Intake; sheep; cattle; goats; relative stock rate; herbage mass; rate of herbage disappearance

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Last Updated 18-03-1997