‘Why is my horse urinating more than usual?’

Marshall obligingly having a pee to illustrate the point

This is a common question we get asked –

it is because of excess nitrogen in the grass which gets converted into urea in the liver so it can be excreted in the urine.

The removal of excess nitrogen takes water – the horse drinks more and pees more – this process was not necessary when they evolved in semi-arid low nitrogen environments where water is scarce!

Horses need nitrogen in order to construct protein and they obtain this by consuming plants. Protein in a forage analysis is known as Crude Protein which comprises some ‘True Protein’ (strings of amino acids with a nitrogen molecule as part of the structure) but also has a Non-Protein Nitrogen (NPN) component – mainly in the form of nitrates

‘Nitrate’ is the form of nitrogen most used by plants for growth and development. Grass absorbs nitrates through their roots and stores them in the bases of stems and shoots – interestingly lucerne/alfalfa also stores them in their leaves.

However horses have no requirement for the nitrate form and it needs to be excreted immediately so it doesn’t get converted to the highly toxic ammonia in the blood.

Nitrate levels accumulate in the grass any time the weather conditions inhibit growth:- growing in the shade, cool night-time temperatures, frosts, droughts or consecutive cloudy days when there is no sunlight for photosynthesis.

One way nitrates are excreted is by latching on to important cations – preferably sodium but if not available (sodium is always scarce in grass) they attach to calcium and/or magnesium, thereby robbing the horse of these important minerals. Just one reason why adding salt to feeds helps so much.

Excess nitrogen is also converted to urea in the liver so that it can be excreted with the urine. When the liver can’t keep up, nitrogen converts to ammonia

Have you ever noticed burnt patches where your horse has urinated? This is because of the high urea content in his urine. Dog urine has the same effect.

What can you do about it?

• Where possible leave the grass to mature before grazing. Think standing hay.

• You also need to take into account the metabolic state of the horses grazing it. For many access will still need to be carefully managed.

• Avoid adding to the horse’s nitrate load by feeding lucerne/alfalfa, fresh herbs, green/wet haylage. Lucerne stores nitrates in the leaves whereas in grasses it tends to accumulate in the bases of stem

• Best not to harrow too often. A couple of times a year is good, eg: early spring. Whilst some nitrogen application may be warranted avoid heavy urea/nitrogen fertilisers

• Add salt to feeds rather than relying on salt licks (approx. 10gms per 100kgs)

• If you cannot control access to green grass make sure they eat as much plain grass hay as possible every day. This reduces nitrogen intake and ensures fibre requirements are met.

Burn patches on the grass are caused by the high urea content of urine and bright green manure indicates a high nitrogen diet.