Sentence:  Male cattle ('riders') in intensive housing are prone to mount herdmates willing to stand to be mounted (<WSD>'bullers</WSD>'). The <WSD>buller</WSD>-steer syndrome has been estimated to cost $70 per head, which can represent significant losses in large feedlots (Blackshaw et al., 1997). Incidence of <WSD>bullers</WSD> in a feedlot was about 2% (409 <WSD>bullers</WSD> in 20,428 mixed breed steers, Irwin et al., 1979). 
Meaning of the word marked with <WSD> tags:  A steer that allows itself to be mounted by bulls when in the crowded conditions of a feedlot.