Q: We are currently suffering through a period of drought, though the horses seem content eating plenty of hay. My question is about what to do when the rains finally do come – I’ve heard lots of horror stories about the dangers of letting horses eat grass that’s grown right after a drought. Should I try and keep them off the grass until it is a certain length, or just limit their time on the grass? One of our ponies has had laminitis in the past.
Tarryn, Te Kauwhata
Vet Dave replies:
As with any change in diet it is worth a gradual introduction rather than a sudden change – this is often easier said than done when it comes to grass, especially when your entire grazing area can be behaving in a similar fashion. The growth spurt you get in grass following a dry period is little different to the growth spurt you get at the start of spring as the wet winter days have disappeared. The grass has come from a period of minimal to no growth and then been given ideal conditions to grow, and so will do with vigour.
With grass, typically the older, longer and more stalkier leaves have a lower sugar and higher cellulose content, so are less of a concern for tipping a horse into a laminitic episode. This does not mean that long grass is safe, as it is equally possible to have younger leaves with a higher sugar content in the mix –more that you should look closely at your grass to determine what is growing and how fast.
I would suggest waiting until the sudden grass spurt that occurs with the first rains has settled, and you have a steadier pace of grass growth before reintroducing your horses – in a similar manner that you would deal with spring grass.
The degree of risk associated with any pasture from a laminitis perspective depends very much on the horse you are putting on the paddock and its own laminitic risk, so there is no one set rule that will keep you safe – rather commonsense and if unsure, a degree of risk aversion to keep yourself on the right side of the laminitic line with a healthy margin for human error.
If you have horses who have not had laminitis, and in your eyes are of a low laminitic risk, or other stock such as sheep or cattle, then I would be introducing these to the paddock first to knock down the new growth before introducing your higher-risk horses to the new grass.
Nutritionist Lucy replies
Yes, this can be a major problem. After a major drought, after the rains finally arrive the young grass comes through, which is lush and full of sugar and low in fibre. This is the time when back-feeding supplemental fibre sources is important to prevent gut problems, such as diarrhoea due to acidosis.
In addition, the drought promotes the production of toxins from fungi due to water stress exerted on the fungi.
Some fungi are visible as spores to the naked eye, such as the ergot soot seen on the heads of paspalum grass.
Others are not visible, such as those in wild-type tall fescues (common on roadside verges) and the endophytes at the base of rye grasses.
In the case of rye grass, those commonly used on dairy pastures in New Zealand have had the endophyte deliberately included to prevent insect damage.
However, the toxins they exude also cause behavioural and physical problems in mammals – commonly manifested as staggers in cattle and horses.
So, when the drought breaks and we get a good soaking of rain, it is wise to ensure the horses are fed an effective and registered mycotoxin binder, if they have to remain at grass.
Supplying hay will help provide more fibre and dilute sugars.
Some horses are more affected by mycotoxins than others, based on their previous history of exposure. If your horse is more sensitive, then removal from the pasture and feeding hay or other forages is important.
However, do be careful as mycotoxins cannot be destroyed by heat, drying or ensiling – so check your hay or baleage is not from high endophyte pastures and check for other potential toxin problems, such as the presence of paspalum or mouldy white clover.
