What Does the Vet Say? - May 2025

The increased incidence of SSS is concerning and with its primary cause still unclear, the need for further research in the area is required, the conference heard. But what we do know is that SSS impacts dairy and dairy-beef calves, causing weight loss, diarrhoea, and ill thrift, and it seems to strike during the calves’ first grazing season. Dr Donlon presented findings – developed by him and a number of other researchers – in a paper titled, Summer scour syndrome in dairy calves: what do we know and what do we need to know?
Discussing the difficulty in diagnosing SSS, Dr Donlon explained that there are plenty clinical signs, but this is not necessarily helpful. These signs can include diarrhoea, ill-thrift, weight loss, ulceration of the mouth and oesophagus in some cases, browning of the coat, and death in severe cases. However, he said, these clinical signs can often be associated with other diseases.
“You are not going to be able to make a diagnosis of summer scour syndrome by just looking at calves,” he said. “When advising vets and farms on how to tackle this problem, the first thing to do is actually rule out all of the other much more common causes such as parasites and respiratory disease,” Dr Donlon added.
Along with this, completing a forage analysis was recommended to rule out molybdenosis – a scour- and ill-thrift-causing disease occurring where excess molybdenum is present within the pasture.

Vet survey

Reporting on a survey that collated the theories of practising vets on the potential causes of SSS, Dr Donlon highlighted the following findings:

1. Insufficient weanling protocols with inadequate quantities of concentrate offered to ensure rumen development;
2. The grazing of very lush grass by recently weaned calves, thus potentially causing sub-acute ruminal acidosis due to its high carbohydrate content; and
3. Nitrogen-rich grass exerting pressure on the microbes in the rumen, resulting in higher rumen and blood ammonia levels.

So, grass quality and weaning management are perceived as the two most important risk factors. In relation to the former, the research paper states: “Grazing an abundant supply of lush grass that is high in crude protein and low in fibre is the common finding in herds affected by SSS.” However, it adds, any association between grazing lush grass and the development of SSS clinical signs is poorly understood, as there is a paucity of studies investigating this apparent association. The research continues: “Possible mechanisms whereby grazing lush grass may cause SSS include the development of sub-acute ruminal acidosis (SARA).” The paper characterises SARA as causing transient depression in ruminal pH, mainly after feeding, and delayed onset of clinical signs.
Elaborating on a possible link between lush grass and inadequate rumen development, the research paper states: “Lush grass typically contains high nitrogen content, e.g. non-protein nitrogen (NPN). When ingested, NPN is converted into ammonia in the rumen. Rumen microbes utilise ammonia to produce microbial protein; however, the utilisation rate is typically lower than the rate of ammonia production from nitrogen metabolism, resulting in excess ammonia in the rumen. The excess ammonia is mainly absorbed into the bloodstream through the rumen wall.
The paper by Dr Donlon and his research colleagues continues: “However, in calves with underdeveloped rumens, this absorption capacity may be impaired, potentially resulting in the accumulation of ammonia in the rumen. An in vitro study (Shen et al. 2023) reported that high ammonia levels reduce the abundance of rumen microbial populations (e.g. bacteria, protozoa, fungi, and methanogens) and subsequently inhibit ruminal fermentation. Currently, it is unknown whether grazing lush grass is an independent factor or, in conjunction with inadequate rumen development, contributes to SSS occurrence.”
While viral infection has also been suggested as a primary cause for SSS, Dr Donlon’s paper points out that analysis of oral and oesophageal tissues from affected calves in Australia detected no common viruses.
Commenting on the above, Dr Donlon said: “It is very possible that on a lot of these farms with summer scour syndrome, calves may be weaned too quickly and are then put out to highly palatable grass that they are not really set up to metabolise. These are all theories currently, one of them may be true but it is also possible that a couple of them are true.”

Clinical investigation

The paper also featured the results of research involving an on-farm clinical investigation by a team from Teagasc and University College Dublin that was conducted in June 2023 to ascertain the characteristics of SSS in dairy-bred calves on commercial farms in Ireland. This involved visiting five commercial farms with suspected SSS cases two to five days following referrals from vets. The visits incorporated: an interview with the herdowner to record the onset of clinical signs; treatments; farm health history; and farm management practices, such as turnout date, chemical or organic fertiliser use, vaccination status, milk feeding, and weaning management. The research paper states that clinical examinations and thoracic ultrasonography (TUS) were performed on eight to 10 randomly selected calves per farm that exhibited clinical signs. Additionally, biological samples, such as blood, rumen fluid, and faeces, were collected from each calf. Concentrates and herbage samples from the paddock where the calves had previously grazed and were currently grazing on the day of visits, were also collected.
Two farms were designated as non-summer scour syndrome case farms after confirmation of coccidiosis or chronic pneumonia. Three farms were classified as having summer scour syndrome. Citing this study, Dr Donlon said: “Probably the most interesting finding of this work was that the blood ammonia was a lot higher in the case farms than the non-case farms and, similarly, the blood urea nitrogen was higher as well.
“That would lend some credence to the presence of excess non-protein nitrogen, but it is putting the cart before the horse to say that is definitely the case because you will get cases where animals that are scouring will have increased ammonia. It may be the case that the ammonia is causing the scour or it may be the case that this is simply a symptom of the animals already scouring. We need to do more work to actually definitively say that is the case.”
An investigation is currently underway at Teagasc Grange into nitrogen fertilisation application rates. It is hoped that this will shed some light on whether or not there is a potential link between artificial nitrogen application strategies and the potential occurrence of SSS.

Final thoughts

Dr Donlon offered advice on a number of on-farm strategies that farmers can apply to help prevent the occurrence of SSS:

Wean calves gradually, making sure that they are consuming sufficient concentrate and forage before moving to grass.

Consider strip grazing calves instead of offering them generous allocations of grass. This will enable calves to consume more fibre as they graze lower into the sward profile, as opposed to being selective and only consuming the leafiest grass.

Dr Donlon is beef herd health research scientist at Teagasc Grange.