Effect of grass crude protein concentration on rumen fermentation, blood variables, and health in recently-weaned dairy-beef calves.

Grass crude protein (CP) concentration is strongly influenced by inorganic nitrogen (N) fertiliser application. Grazing high CP (> 200 g/kg dry matter (DM)) pasture has been anecdotally associated with summer scour syndrome (SSS) in recently-weaned dairy-bred calves. This study investigated the effects of grazing perennial ryegrass-dominant pasture differing in CP concentration, generated through varying calcium ammonium nitrate application rates (15, low (LN); 60, moderate (MN); and 120, high (HN); kg N/ha per rotation applied 14 days pre-grazing), on rumen and blood variables, and the potential to induce SSS in recently-weaned male beef × dairy calves (n = 54; mean live weight at turnout, 115 kg; age, 100 days). An SSS case was defined as persistent diarrhoea and weight loss without evidence of parasitic gastroenteritis or coccidiosis. Calves (n = 18 per treatment) rotationally grazed their pasture treatment (24-48 h/(sub-)paddock) for 28 days, followed by a 13‑day period without pre-grazing fertiliser, after which all calves grazed together in a common pasture for 112 days. Increasing N fertiliser application rates increased grass CP concentrations (196, 227, and 261 g/kg DM for LN, MN, and HN, respectively). Mean rumen ammonia, plasma ammonia and urea concentrations were generally highest in HN, intermediate in MN, and lowest in LN. Effects on rumen pH were inconsistent. Coughing and diarrhoea (loose/runny faeces) proportion was greater in HN than MN or LN. No cases of SSS occurred. Under the conditions of this study, increasing inorganic N fertiliser application rates increased grass CP, rumen ammonia, plasma ammonia and urea concentrations, and the proportion of coughing and diarrhoea, but did not induce SSS.