Ingestive behaviour of heifers grazing monocultures of ryegrass or white clover

Abstract

The ingestive behaviour of dairy heifers grazing monocultures of either perennial ryegrass or white clover was characterised. Two different types of jaw movements were characterised: bites (i.e. when herbage was gathered in to the mouth and then severed) and chews (i.e. when herbage severed by a bite was processed before swallowing). The heifers had a total jaw movement rate (i.e. bites+chews) between 70–80 min −1. Of these, 80–90% were bites, with bite mass varying between 0.15–0.30 g DM per bite. Heifers grazing grass spent longer eating (536 min per day versus 436 min per day) compared to those on clover. As average instantaneous dry matter intake rates min −1 were identical on grass and clover swards (both 12.9 g DM min −1), this gave rise to greater daily intakes of grass compared with clover (6.93 kg DM per day versus 5.61 kg DM per day). However, clover had a higher digestibility than grass (59.9 DOMD versus 77.2 DOMD, % DOM in DM) and so animals on the two different swards had similar intakes of digestible organic matter (4.17 kg per day versus 4.27 kg per day). This gave rise to similar daily liveweight gains (0.97 kg per day versus 0.99 kg per day) for heifers kept on grass and clover, respectively. The masticatory processing (chewing) of the herbage appeared to be delayed from grazing to ruminating, with the heifers kept on grass spending longer ruminating (526 min per day versus 267 min per day) and tending to have more ruminating chews per kg DM ingested (5906 per day versus 3117 per day) than those on clover. These findings suggest that the heifers were regulating their intake by adopting a foraging strategy aimed at minimising grazing time.