VitiFeed made from steam distilled grape marc can be included at 10% of the dairy cow diet with no reduction of milk production Brendan Cullen 1 , Brenton Mengersen 2 , Jeremy Desfreres 3 , Frank Dunshea 1 , Kenny Oluboyede 1 , Nick Boyd 1 and Long Cheng 1 1 Faculty of Veterinary & Agricultural Sciences, The University of Melbourne, Victoria 3010, Australia. 2 Tarac Technologies Pty Ltd, 20 Samuel Road, Nuriootpa, South Australia 5355, Australia. 3 AgroParisTech, Paris Institute of Technology for Life, Food and Environmental Sciences, France. * Corresponding author. Email: bcullen@unimelb.edu.au Short title: Grape marc feeding to dairy cows ABSTRACT An experiment to test the hypothesis that inclusion of steam distilled grape marc (‘VitiFeed’) at 10% of the dairy cows’ diet would not have a negative impact on milk production was conducted in a pasture-based dairy production system in northern Victoria, Australia. At the commencement of the experiment the cows were on average 70 days in milk and had a liveweight of 675 kg. Two groups of 18 cows were fed a diet consisting of daily offerings of 8.1 kg DM/cow concentrate feed (16% crude protein (CP) and 12.5 MJ ME/kg DM), approximately 5 kg DM/cow pasture and 8 kg DM/cow of silage. The control group received 8 kg DM/cow of silage made from balansia clover pasture (20.2% CP, 10.5 MJ ME/kg DM), while the VitiFeed group received 6 kg DM/cow of balansia silage and 2 kg DM/cow steam distilled grape marc silage (14.7% CP, 8.5 MJ ME/kg DM). Silage was fed on a feed pad, and refusal was estimated as <5% of silage offered. The experiment included a 7- day adaptation period followed by 35-days of data collection. Daily milk production, fat and protein percentages and rumination time were recorded using the Lely Astronaut milking machine. Treatment differences were analysed by ANOVA. There was no significant difference in milk production (37.8 vs. 38.8 kg milk/day, 2.73 vs. 2.74 kg milk solids/day) and rumination time (414 vs. 398 minutes/d) between the treatments (Control vs. VitiFeed). These results indicate that feeding VitiFeed to dairy cows at 10% of the diet can sustain milk production but further research is required to confirm this finding. Keywords: Grape marc; dairy cows; silage; rumination; milk composition INTRODUCTION Grape marc (the skins, seeds, stalk, and stems remaining after grapes have been pressed to make wine) is a waste-product that may be used as a feed supplement in dairy production, but its nutritive characteristics can vary widely (Russo et al. 2017). Previous work showed that grape marc contains high levels of condensed tannins and crude fat (Moate et al. 2014). Feeding tannins to dairy cows has been demonstrated to decrease enteric methane, reduce urinary nitrogen loss and improved nitrogen use efficiency in the cows (Grainger et al. 2009) while including dietary oils in the diet of dairy cows can reduce their methane emissions and increased milk production where energy was added to the diet (Moate et al. 2011). However, while grape marc either fed as dried, pelleted grape marc or ensiled to dairy cows can reduce their enteric methane emissions by around 20%