Pasture Management for Worm Control

It’s a good idea to revise your worm control regime once a year, preferably prior to spring. As the temperatures increase, the worm eggs that have remained dormant on the pasture over winter will hatch into infective larvae, exposing your horse to rapidly increasing parasite burdens. Rather than simply relying on drenching your animal, there are several other ways you can reduce the numbers of infective larvae, thus reducing the levels of infectivity your horse is exposed to.

It is important to remember that it is not the worm eggs on the pasture that are the concern, but rather the infective larvae that hatch from these eggs. While the temperatures are very cold (under 6°C), the eggs lie inactive in the faeces or on the pasture and present no danger to the horse if eaten. However, increasing temperature will stimulate these harmless eggs to hatch into the next stage of the worm life cycle – the infective larvae. This can take 12-14 days at temperatures of 10°C. Once temperatures are 25-33°C , it takes only 3-4 days for eggs to hatch into infective larvae.

When swallowed by the grazing horse, these larvae will go on to complete their life cycle and develop into adult, disease-causing worms. This is what we need to avoid! Below are some strategies to help reduce the amount of infective larvae on the pasture.

1. Dung Removal

By far the most effective way of reducing larval numbers on your paddocks is via removal of the dung. If you remove the manure (and the worm eggs each pile contains) then there will be far less eggs left on the pasture to hatch into the infective larvae. As temperatures increase so does the hatching and so dung removal should be performed every three days. Obviously this may not be practical for large farms, so other methods will need to be used to minimise exposure.

2. Resting Paddocks

On bigger farms, mares are often moved from paddocks to the working area to be under lights or to foal down. This means that certain paddocks will be partially rested when the mares are moved, and therefore the shedding of eggs onto these pastures (and the number of infective larvae that could develop) has been reduced.

3. Cross-Grazing

It would be a wise idea to take some manure samples from these mare paddocks to carry out Faecal Egg Counts. This will help to gauge which are the “dirtier” paddocks with the higher egg counts. As the old saying goes: “Drench the paddock, not the animal.” If the egg counts from a particular paddock are high, it would be better to allow older, less susceptible animals to graze these pastures compared with younger stock, or even rest the paddock completely. If cattle (steers) are available, then they should be moved into the contaminated paddock to help “mop up” the worm eggs and larvae. Yearlings could also be moved onto these paddocks if the contamination isn’t too high to grow these young animals out. Make sure if you are putting your yearlings onto these pastures that they have been drenched with an effective treatment at least three days prior to moving them.

4. Harrowing

As the temperature and moisture levels should be ideal for eggs to hatch into infective larvae, it could be a good idea to harrow the paddock before putting cattle or other animals onto these pastures. Harrowing, followed by one to two months of hard grazing will effectively reduce the infectivity of a paddock by up to half. Imagine your animals being exposed to 1 million infective larvae in a paddock versus the exposure to only 100,000!

5. Using Older Animals to “Clean” Paddocks

Generally older animals are less susceptible to worm pressure than younger animals as their level of natural immunity is higher. If cattle are not available to cross graze, you can use older animals to do the same. This is known as a “leader-follower system”. Using this system, younger animals (that are more susceptible to worms and will shed more eggs) are grazed first in a paddock, and then the older animals are used to graze afterwards to “mop up” all the eggs and infective larvae. This also allows the younger, growing animals to graze the new spring protein-rich pasture. If cattle are available it will allow you to turn the steers off into the fat market more readily.

Thinking ahead, it might also be time to think about what paddock(s) you will want this year’s foals to graze in over the autumn/winter of next year when they are weaned. At this time, a weanling will be under the greatest worm pressure as its immune system is still trying to develop. Also, the number of infective worm larvae on the pasture the foals are exposed to during this period is at its greatest.

Preparing the paddock can be done by grazing it with cattle from November through to the end of January the following year. At the end of January, the paddock should be rested to let the pasture recover and then make use of the first autumn rain and growth in March. In March, the weanlings can be put back onto this paddock.

If cattle aren’t available, the same effect can be achieved by drenching older, less susceptible horses and putting them in the paddock in November at double the normal stocking density. Do this for no more than 30 days. The paddock is then rested until January when you would repeat this process: drench the same horses at the start of the month, and then put double the normal stocking density on the paddock during January for no more than 30 days. From the start of February, rest the paddock again until the first autumn growth in late March early April. It is then ready for the weanlings. After these two short hard grazing periods, the number of infective larvae on this paddock will be greatly reduced, therefore minimising the exposure to these very susceptible young animals.

By implementing some of these simple pasture management strategies, you can effectively decrease the numbers of infective worm larvae in your paddock, which in turn will reduce your horse’s parasite burden. This is turn should mean you can extend the interval between worming treatments and therefore minimise the number of drenches administered to your horse.