Understanding Summer Scour Syndrome

Rischi Robinson Mark McGee David Kenny Bernadette Earley Edward O’Riordan Catherine McAloon

Summer scour syndrome (SSS) is a relatively novel digestive disorder affecting recently-weaned dairy-bred calves during their first grazing season in Ireland and in other countries with pasture-based systems, such as the UK, Australia, and New Zealand. This syndrome typically occurs within two to six weeks following turnout to pasture and often affects groups of calves. Clinical features of this syndrome include persistent diarrhoea, rapid weight loss, ill-thrift and, in some cases, oral ulcerations. Post-mortem examinations of SSS-affected calves consistently identify lesions on the upper alimentary tract, including ulcerations of the oral cavity (e.g. buccal and ventral of the tongue) and oesophagitis (inflammation of the oesophagus). Histopathological examination also identifies epithelial apoptosis, dyskeratosis and necrosis (cell death) in the oesophagus. Research to date has not identified a consistent cause for this syndrome. Therefore, current diagnosis of SSS is primarily based on the exclusion of common causes (differential diagnoses) of calf diarrhoea and ill-thrift at grass including coccidiosis, parasitic gastroenteritis, bovine viral diarhoea (BVD), salmonellosis, ruminal acidosis, and copper and molybdenum toxicities. The present hypothesis in Ireland, which is largely established based on field observation and is not yet experimentally validated, suggests that SSS may be associated with grazing of ‘lush’ grass (i.e. vegetative stage or immature grass containing a crude protein  concentration greater than 200 g/kg dry matter). Such high-crude protein grass is common in intesively managed pasture receiving annual nitrogen (N) fertiliser application rates of 150-250 kg/hectare.

Two Teagasc Grange studies were undertaken to improve understanding on Summer Scour Syndrome. The first was a detailed on-farm clinical investigation describing the occurrence and characteristics of Summer Scour Syndrome in Ireland. The second was a controlled grazing experiment at Teagasc Grange examining the effects of pre-grazing inorganic N fertiliser application rates on rumen and blood variables and its potential to induce SSS in recently-weaned dairy-beef calves within six weeks post-turnout to pasture.

Study 1: On-farm clinical investigation

The investigation was conducted on five commercial farms (three dairy, one dairy-beef, and one mixed dairy/dairy-beef) referred by private veterinary practitioners where calves were suspected of having SSS. Farms were visited two to five days following referrals. Eight to ten calves exhibiting clinical signs of SSS were randomly selected on each farm for clinical examination and thoracic ultrasonography (lung scanning). Additionally, blood, rumen fluid, and faecal samples were collected from each calf. Concentrate samples and herbage samples from paddocks currently and just previously grazed by the calves at the time of visit were also collected. A face-to-face interview was conducted with each farmer to record calf health history and management practices.

On all farms, calves were reported to have developed clinical signs at two to six weeks after turnout. The predominant clinical signs included diarrhoea, weight loss, hypersalivation, poor coat condition and, in some cases, healing oral ulcers. However, the investigation found that only calves sampled on the three dairy farms (defined as ‘case’ farms) met the definition of SSS cases, while those on the other two farms (defined as ‘non-case’ farms) were diagnosed with coccidiosis and/or chronic pneumonia problems. On all farms, BVD and parasitic gastroenteritis were ruled out, and rumen pH values (6.43-7.09), blood copper (14.4-15.7 μmol/L) and molybdenum (6.5-13.3 μg/L) were normal. A novel and consistent finding from this investigation was elevated whole blood ammonia concentrations (129-223 μmol/L; hyperammonemia) in SSS-affected calves, which exceeded reference values (<90 μmol/L) and were substantially higher than for calves in non-case farms (22-25 μmol/L). Importantly, all case farms reported the application of inorganic N fertiliser (protected urea or NPK) to paddocks within one to three weeks prior to grazing by calves. Although the hyperammonaemia finding suggests a potential link with high dietary protein intakes, herbage crude protein concentrations measured at farm visits were found to be low to moderate (106-177 g/kg dry matter), albeit these samples were obtained after the onset of the clinical signs. Overall, this investigation suggests a potential relationship between herbage crude protein intake, hyperammonemia, and the development of SSS.

Study 2: Effect of pre-grazing nitrogen fertiliser rate on calf metabolism and health

To test if high herbage crude protein intake could induce metabolic disturbances consistent with SSS, a controlled grazing experiment was conducted at Teagasc Grange using 54 recently-weaned Aberdeen Angus × Holstein Friesian male calves sourced from a commercial farm. The calves were gradually weaned off milk over 10 days at approximately 10 weeks of age and were transported to Grange at approximately seven days post-weaning. On arrival, calves were accommodated indoors for three weeks and offered concentrates (up to 3 kg/calf/day), with hay and water ad libitum. A clinical health examination was conducted at arrival, and respiratory disease vaccines, mineral supplementation, coccidiostat, and anthelmintics were administered before turnout. The calves (age 100 days, live weight 115 kg) were turned out to perennial ryegrass pasture receiving one of three inorganic N fertiliser application rates (n=18 calves/treatment): 15 (Low-N), 60 (Moderate-N) and 120 (High-N) kg N/hectare/rotation. These fertiliser rates were used to produce herbage with varying crude protein concentrations. Fertiliser was applied as calcium ammonium nitrate 14 days prior to grazing. Post-turnout, concentrate supplementation was offered at 1 kg/calf/day and gradually reduced to zero over 7 days. The experiment comprised a 41-day experimental (28-day treatment exposure and 13-day treatment carryover phases) followed by a 112-day follow-up period. Calves were rotationally grazed in their pasture treatment (24-48 hours/paddock) during the experimental period, and grazed common pasture as one group during the follow-up period. Clinical health examinations were performed, and rectal faecal, rumen fluid, and blood samples were collected during the experimental (19 sampling time-points) and follow-up (two sampling time-points) periods. Additionally, herbage samples were collected from each paddock. Calves were checked twice daily to monitor their clinical health status.

Increasing fertiliser application rates increased herbage crude protein concentrations and resulted in increased rumen ammonia and subsequently, plasma ammonia and urea concentrations (Table 1). Despite these metabolic changes, no calves developed clinical signs of SSS either during the experimental or follow-up periods. Rumen pH was greater in High- compared to Low- and Moderate-N treatments, with no evidence of acidosis (<5.6). There were no effects of fertiliser N treatment on rumen volatile fatty acids, haematological cell counts, and liver enzyme activities, and no adverse respiratory or systemic health effects were detected. However, a higher proportion of loose/runny faeces was observed in calves grazing the High-N treatment, likely reflecting lower herbage dry matter concentration rather than a pathological condition.

Table 1. Effect of inorganic nitrogen fertiliser application rates on herbage chemical composition, rumen and blood variables, and faecal consistency.

^{a, b, c} Values within columns with different superscripts differ statistically; ¹ NS – not significant;  *** P < 0.05

These findings suggest that high herbage crude protein intake alone is insufficient to induce SSS. Instead, SSS likely arises from a multifactorial interaction, potentially involving suboptimal weaning management, inadequate rumen development, and poor nutritional adaptation at turnout.

Transitioning calves to grass: the importance of proper weaning

The transition from housing and milk feeding to pasture grazing presents a major challenge for young calves. Pre-conditioning during the housing period, through implementing proper pre-weaning and weaning strategies, is critical to support adequate digestive tract development and physiological adaptation for turnout to grass. Calves are born as pre-ruminants with an underdeveloped rumen and initially rely on milk feeding to meet their nutritional demands. The development of a healthy and functional rumen is an important factor influencing the transition from the pre-ruminant to the ruminant stage. In the ruminant stage, calves rely on solid feed for producing rumen fermentation products as their main nutrient source. Published research showed that early provision of concentrate and forage (e.g. straw or hay) diets during the pre-weaning period stimulates rumen development (e.g. increases rumen weight, volume, and papillae length) and rumen microbial proliferation, which is essential for nutrient fermentation and absorption. Weaning management also plays an important role in supporting rumen maturation. Gradual weaning is generally recommended over abrupt weaning to support a smooth transition from milk to solid feed, while also mitigating stress that can negatively affect calf physiological and immunological function. Abrupt milk removal, particularly when combined with immediate turnout, increases the risk of reduced feed intake, delayed rumen adaptation, and digestive disturbances at grass. For gradual weaning, milk feeding should be reduced progressively over 7 to 14 days before complete withdrawal, either by lowering the volume fed or reducing feeding frequency, to allow calves time to increase their concentrate and forage intakes. It is recommended that calves should be consistently consuming at least 1 kg of concentrates per day for at least three consecutive days before milk is fully withdrawn. Retaining weaned calves indoors for a short period before turnout is also a recommended practice, allowing the calves to fully adapt to solid feed prior to grazing. Our aforementioned study indicates that retaining calves indoors on concentrate and forage diets for three weeks post-weaning and maintaining concentrate supplementation post-turnout (approximately three weeks is recommended) aids a smooth transition from indoor solid feed to fresh grass diet, potentially preventing Summer Scour Syndrome.

Conclusion

Overall, the similar clinical signs observed in calves with and without SSS emphasises the importance of excluding differential diagnoses of calf diarrhoea and ill-thrift at grass prior to confirming Summer Scour Syndrome. Calves affected by SSS exhibit elevated blood ammonia concentrations. Increasing inorganic N fertiliser application rates increased herbage crude protein concentrations. Under the conditions of our experiment, grazing high crude protein grass increased rumen ammonia, plasma ammonia, and urea concentrations but did not negatively affect health or induce SSS in dairy-beef calves. This finding supports a multifactorial aetiology for SSS, likely involving suboptimal weaning management, inadequate rumen development, and poor nutritional adaptation at turnout to pasture. The successful transition of calves to grass may be achieved through appropriate pre-conditioning strategies, including gradual weaning, optimising solid feed intakes, delaying turnout post-weaning, and providing short-term concentrate supplementation following turnout.

Compiled and edited by Mark McGee and Paul Crosson, Teagasc, Grange Animal & Grassland Research and Innovation Centre, and first published in BEEF2026 – Driving Sustainable Performance, additional reading from BEEF2026 is available here.