The main aim of worm management in spring is to limit larval load on pastures. There are two ways of doing this:
1) Manage manure: pick up where possible, where not possible cross-graze with other livestock
2) Worm horses that are shedding high numbers of eggs (check via FEC)
The theory behind this is that in spring, as the weather warms and stays damp, worm eggs will hatch faster (in as little as 4 days) and build up a large population of infective larvae on the pasture. Remember that of all the worms on your property, 90% of those worms are free-living larvae on the grass. These larvae will survive for up to 6 months in the environment. Horses are then grazing more contaminated pasture, possibly leading to a higher worm burden, or higher burden of encysted larvae, which then can cause trouble later in the following autumn/winter.
1: Picking up and removing manure is the most straight forward method of limiting larval contamination. It directly breaks the lifecycle of the worms, preventing the step from egg to larvae. As it isn’t always possible to collect all manure, cross-grazing pasture with ruminants (cows/sheep) between rotating/resting paddocks can also help by removing some of the infective larvae (ruminants and horses do not share worms).
2: Most horses do not need to be wormed in spring for their own health. Over spring, the fact that worming a horse will, for a short time (6-10 weeks), stop eggs being shed onto the pasture, and therefore reducing larval load is the main reason why a horse will be treated. Egg shedding can be reduced by 80% by only worming 20% of horses, as some horses are naturally high egg shedders, while others are not. Monitoring all horses in spring via FEC will allow you to selectively worm those that are shedding the most eggs, and monitor the health of others, in case some do need treatment.
One could say “why not worm all horses in spring? It’ll help even more with the pastures!” And while this is true, this approach will lead to drug resistance developing faster. Refugia is an important concept in parasite management, where there are always some worms left untreated, and unexposed to the drugs, to help slow drug resistance development.
Another point to make is that there is no set requirement for what wormer type to use over spring. There is now no longer any need to “rotate” wormers (there’s a blog post on the website about why rotating is a myth), so chose a wormer based on FEC results, and how long you’d like egg shedding to be suppressed for. E.g. Ivermectin has an egg reappearance period of about 8-9 weeks, while oxfendazole only 5-6 weeks.
Overall, in spring, make sure you don’t have a build up of manure, and get your horses checked via FEC before any decisions are made about using deworming medications.
If you’d like to organise FECs for your horses this spring, head over to the website to see drop off locations, or how to send samples via post.
Happy poo-pick up!
