Effect of group size on the consistency of movement direction among cows in a small grazing group

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  • 少頭数の放牧牛群においてグループサイズが移動方向の一致の程度に及ぼす影響

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Abstract

This study investigated the effect of group size on the consistency of movement direction among grazing cows in a small group by analyzing their intra- and inter-patch movement behaviour. Three (GROUP-3) and 6 (GROUP-6) cows grazing in a 1.6-ha pasture were observed. The positions of cows were recorded at 1-s intervals by GPS, and the feeding station (FS) and steps between FSs was observed. A patch was defined as a cluster of FSs. For each intra- and inter-patch movement, actual distance moved (ADM), and distance moved in a consistent direction (DMCD: ADM×COS[α], where α is the angular difference in movement direction between the focal animal and group members' center coordinate) were calculated. The ratio of Σ[DMCD] to Σ[ADM] (DMCD-Ratio) was also calculated. For intra-patch movement, ADM, DMCD, and DMCD-Raio were similar between GROUP-3 and GROUP-6. For inter-patch movement, both ADM and DMCD were greater for GROUP-6 than for GROUP-3 (P<0.01), and DMCD-Ratio was higher for GROUP-6 than for GROUP-3 (P<0.05). The large group size enhanced the consistency of inter-patch movement direction among grazing cows and increased their inter-patch movement distance. The increase of the spatial group cohesion was also observed in the large group.

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