After a cut of silage or hay, rapid regrowth giving lush aftermath which is used for grazing, means the possibility of cases of Fog Fever, a disease generally seen in cattle grazing this type of pasture.
Classically this condition is an acute pneumonia affecting mainly adult beef cattle, which is seen at grass in the late summer and autumn – although cases have been seen in cattle grazing on permanent pasture after warm wet weather has caused a growth spurt in the grass.
Disease is normally seen within two weeks of the animals being introduced to new pasture.
Symptoms range from mild to severe, depending on the degree of ingestion of the regrowth, the animals often presenting with normal temperature, dullness, tranquil behaviour, varying degrees of breathing difficulties and little or no coughing.
The condition is thought to be caused by an increase in the amino acid called L-tryptophan, which is found in large quantities in regrowth.
Rumen metabolism takes up to three weeks to adjust to this rapid change in diet and is unable to cope, leading to the amino acid being converted to a compound called 3-methylindole, which has a toxic effect on the lungs, causing the symptoms of pneumonia.
In some animals, the symptoms can be confused with those of lungworm, but the frequent cough associated with this parasite helps to avoid this.
When this condition is suspected, the animals should be restricted on or removed from the pasture as soon as possible.
This may be difficult with severely affected cases, which may have to be treated where they are and only moved slowly to avoid severe breathing difficulties.
There is no specific treatment but generally anti-inflammatory drugs and antibiotic cover will be given.
Even after treatment mortality among the more severely affected cases can still be high, the speed at which they are identified and restricted from the pasture being a major factor in survival rates.
Once restricted from the pasture the cattle should be offered conserved forage such as hay or silage to dilute the grass in the rumen and so reduce the level of L-tryptophan, although this may take some days to achieve and cases may still be seen for a few days.
Return to grass should only be allowed after full recovery and then in a controlled fashion so as to allow the cows rumen microbes to adapt to the diet.
This disease can be markedly reduced or prevented in most cases by grazing management.
Restricting access to the fresh pasture either by time or by strip grazing will allow a period for the cows to adapt to the change to a more lush diet.
Feeding hay or silage daily before turning cattle on to the pasture may also help.
This type of management is very useful for fresh pasture but if the cattle are grazing permanent pasture extra vigilance is required to watch for a sudden flush of grass, which might precipitate the disease.
It is also worth watching out for of cases of autumn grass staggers, which may also be seen in these conditions.
Iain R Carrington BVM & S MRCVS