Urinary calculi (commonly known as 'gravel') is not uncommon in young rams or wethers being fed concentrate diets.

The condition arises because tiny mineral crystals are formed in the urine and form a sludge in the urethra (the tube that carries urine away from the bladder).

A blockage forms in the urethra, preventing urine being passed, and the resulting build up of pressure causes either the bladder or the urethra to rupture.

Wethers are particularly susceptible, because their urethra is less developed than in entire males.

Urine contains toxic substances, so its retention in the body eventually causes death due to toxaemia. Consequently, the carcases of affected sheep are not fit for human consumption.

The condition is known as urolithiasis and affected sheep show restlessness, straining and frequent unsuccessful attempts to urinate. Occasionally, blood-stained crystals may be seen attached to the hairs near the tip of the penis.

In early cases, surgery may relieve the condition. Calculi may also form in the urine of female lambs and ewes but, because of the different anatomy of the female urethra, urolithiasis is rarely a problem.

Correct mineral formulation of the ration is key to preventing urolithiasis. If rams or wethers are being fed diets high in concentrates, then a formulation containing the recommended level of minerals should be produced.

Specifically, rations high in phosphorus and magnesium have been shown to predispose to urolithiasis.

Ewe rations frequently contain high levels of magnesium and so should not be fed to rams or wethers. Concentrates should also be introduced gradually and a source of roughage should be provided.

The urinary excretion of phosphorus has been shown to vary between individual sheep. High urinary excretion of phosphorus is associated with urolithiasis. Texel and Scottish blackface sheep were found to have the highest urinary excretion of phosphorus and so these breeds are more susceptible to urolithiasis.

Urinary calculi may also form when a supply of fresh clean water is not available. Water troughs should be cleaned regularly and checks made for frozen pipes in frosty weather.