Effects of sowing rate and grazing management of forage rape (Brassica napus) on grazing behaviour and utilisation by dairy cattle

The increase in total factor productivity in the Australian dairy industry over the last 10 years has been low (1.5%). To help address this issue, 'FutureDairy' is aiming to increase the production of home-grown feed currently achieved from pastures using a complementary forage-rotation (CFR) system. Forage rape (Brassica napus) is a key component of the CFR; however, it is a complex crop to manage and feed, and the interactions between the behaviour and grazing habits of dairy cattle are unknown. The present experiment investigated the effect of the sowing rate and grazing management of forage rape on the grazing behaviour and forage utilisation of lactating dairy cattle. A field experiment was established, with a forage rape crop planted at three different sowing rates of 2, 3.5 and 5 kg/ha. The crop was grazed using either a 'multiple grazing' system, where the forage rape was strip-grazed in a manner to promote regrowth to allow for regrazing, or a 'take-all grazing' system, where the forage rape was grazed once only after reaching maximum biomass. The grazing preferences of cows for the sowing rates during the grazing sessions were visually observed and recorded, and forage utilisation was determined from pre-grazing and post-grazing forage availability. Nitrogen (N) deposition from excreta was estimated using stocking density and time spent. Cattle preferred grazing the forage rape sown at 2 kg/ha, but this preference did not result in higher forage utilisation. Grazing method had no effect on forage utilisation or N deposition. Cows should be removed after ~80 min of grazing in a multiple grazing system to ensure future regrowth. Further work is necessary to fully investigate the effects of grazing method on forage utilisation and N deposition, and more accurate external devices and internal markers should be used in the future to provide better estimates of forage utilisation.