An outbreak of frothy bloat in steers grazing forage rape ( Brassica napus )

Case history An outbreak of frothy bloat occurred in April 2020 in a herd of 1,000 recently weaned Aberdeen Angus steers grazing forage rape (Brassica napus) in Buenos Aires Province, Argentina. The affected paddock had been sown on 4 January 2020 with B. napus “Titan” at 4.28 kg/ha and fertilised with diammonium phosphate (78.99 kg/ha). Despite implementation of a gradual adaptation programme, fibre supplementation with corn silage, and restricted grazing periods, cases of acute bloat and sudden death continued to occur over a 4-week period. Twenty-one steers died among the 600 steers grazing forage rape, resulting in a mortality rate of 3.5%.Clinical findings Affected steers developed acute ruminal distension, dyspnoea, tongue protrusion and sudden death after grazing forage rape, particularly during morning grazing sessions. Farmers reported that cattle showed a strong preference for forage rape and largely ignored the corn silage offered before paddock entry.Pathological and laboratory findings Necropsy examinations of three steers revealed marked ruminal distension, mild rectal prolapse and abundant fine foamy ruminal contents with massive gas release on incision. Ruminal pH ranged from 4.5–5. Histopathological examination of multiple tissues revealed no significant microscopic lesions. Nutritional analysis showed that forage rape collected from the affected paddock contained 27.1% crude protein (CP), of which 66.9% was soluble protein (SP). In contrast, kale (B. oleracea “Regal”) collected from an adjacent control paddock where no clinical disease was observed contained 12.9% CP, with SP representing 52.9% of CP. Qualitative nitrate testing was negative for all forage samples.Diagnosis The epidemiological, clinical, pathological and nutritional findings supported a diagnosis of frothy bloat associated with grazing forage rape (B. napus, “Titan”). The high concentration of SP in the affected forage was considered a major predisposing factor for ruminal foam formation.Clinical relevance This report highlights the potential for forage rape to cause severe frothy bloat in cattle despite the implementation of commonly recommended preventive measures. The findings emphasise the importance of nutritional monitoring of Brassica forages, particularly SP concentrations, and suggest that conventional management strategies may not always be sufficient to prevent outbreaks under specific environmental and production conditions.