Comparison of cattle and goat grazing behaviour in an Italian ryegrass pasture invaded by Rumex obtusifolius L.

Abstract

This study was conducted to obtain basic information on the development of biological control of Rumex obtusifolius L. by goats. Each of a group of cattle (n = 3) and goats (n = 6) was allocated to one of two paddocks in an Italian ryegrass (Lolium multiflorum Lam.) pasture (approx. 1 ha), which was partly invaded by R. obtusifolius. The stocking rate of the two animals was similar, and their grazing behaviours were observed twice over a 40-day grazing period between April and May in 2009. The percentage of R. obtusifolius harvested by grazing animals to the total number of this plant and the herbage mass per unit area in each paddock were determined. In addition, the relative summed dominance ratio (SDR2′) of R. obtusifolius was measured. On the basis of observations of the grazing behaviour of both animals, we calculated the grazing frequency (GF)—the percentage of each plant species to the total number of plant species grazed in each paddock. Moreover, the selectivity index (SI), based on Ivlev’s electivity index, was estimated from SDR2′ and GF for each of all the plant species. The herbage mass of R. obtusifolius was significantly smaller in the goat paddock than in the cattle paddock (P < 0.05). The percentage of R. obtusifolius defoliated by the animals was higher in the former than in the latter throughout the experimental period (P < 0.05). The GF of R. obtusifolius ranged from 0% to 0.6% for cattle and from 11% to 14% for goats. Although the SI values for R. obtusifolius were negative in both cattle and goats, those for the latter were smaller compared with those for the former throughout the grazing period. In conclusion, both cattle and goats are unlikely to prefer R. obtusifolius to other plant species; however, the extent of food aversion was smaller in goats than in cattle. It is accordingly suggested that goats do not show stronger avoidance of R. obtusifolius than cattle.