Pasture, Parasites, and Risk Management

Rory Lewandowski, OSU Extension Educator, Athens County

May through early June is generally a time of good pasture growth and corresponding livestock production. However, if you are grazing sheep and goats this is the time of year that needs careful consideration in regards to internal parasites, in particular Haemonchus contortus, the barber pole worm. One way to approach this grazing season is to think in terms of risk management.

What can be done to reduce or minimize the risk of a heavy parasite infection while sheep and goats graze pastures? To answer this question, the livestock owner must have an understanding of the biology of the parasite and some understanding of the immune response of the animals that are grazing. Let’s take a closer look at each of these factors.

The lifecycle of the Haemonchus contortus is: egg, stage 1 larva (L1), stage 2 larva (L2), stage 3 larvae (L3), known as the infective stage because this is the stage ingested from pasture plants, stage 4 larvae (L4), adult and back to egg. The L4 and adult stage develop inside the animal and are voracious blood suckers. Each fully mature adult female is capable of laying up to 5,000-10,000 eggs per day which are passed in the feces of the animal.

For grazing sheep and goats there are two considerations; the number of larvae that overwintered on pastures that may be grazed during the spring and the development of new larvae from eggs shed by ewes or does during spring grazing. While some of the older scientific literature about Haemonchus contortus says that larvae are unlikely to overwinter here in Ohio, empirical evidence from recent on-farm studies strongly suggests that larvae, if they did not survive the winter years ago, are now in fact surviving our winters and can be ingested on spring grazed pastures. The second consideration is that ewes and does start to shed eggs on spring grazed pastures. These eggs are produced by adult Haemonchus contortus worms that overwintered as L4 larvae in a hypobiotic (inactive) state in the abomasum of the animal. As ewes and does prepare to lamb and kid, the L4 larvae become active, molt into young adults, begin to suck blood and begin producing eggs that are passed on to the pasture in the feces. In spring conditions, with cooler temperatures, it is unlikely that eggs hatch and develop to the L3 infective stage in less than 4-5 days.

The immune response of the animal must be considered when developing a pasture grazing strategy that will reduce the risk of a harmful Haemonchus contortus infection. In general, adult ewes and does have a strong immune response to parasites and can tolerate ingestion of some fairly heavy doses of the infective L3 larvae stage. This is good news. The bad news is that the stress of lactation suppresses this immune response, making lactating ewes and does susceptible to infections and leading to the shedding of extremely high numbers of Haemonchus contortus eggs. Young lambs and kids have essentially no immune protection from Haemonchus contortus infection. Thus, as they learn to graze at their mother’s side, they begin to ingest large numbers of infective L3 larvae developing from the eggs being shed by the ewes/does. Around 6-8 months of age, lamb and kid immune response to parasite infections begins to develop, but there is a nutritional cost to this development.

OK, knowing some of these basic biological facts, what can be done to develop a risk management strategy to reduce Haemonchus contortus infections on pasture? Here are some strategies that are being considered and attempted as a result of 3 years of on farm research. The research did not start out with this focus, but lessons learned over the years have evolved into discussions of these types of risk management strategies.

  • Minimize early spring grazing on pastures that were grazed in September through early October to get away from the overwintered larvae.  Delay using those pasture paddocks until June if possible.
  • As temperatures warm into the mid 70’s and higher, plan pasture rotations so that animals are rotated to a new paddock every 3 days. This strategy attempts to avoid ingestion of L3 larvae which could develop within 4 days of egg hatch under favorable conditions.
  • Plan a pasture use and grazing management so that ewes and lambs (does and kids) do not return together to a previously grazed pasture for a second or third grazing pass. This will require more paddock divisions. The rationale behind this strategy is that the L3 larvae can survive for 60-90 days on a paddock during the growing season. In theory, a second grazing pass might be possible if low numbers of eggs were being shed during the first pass due to low infection levels.  This would require some fecal egg count monitoring.
  • Consider early weaning (60 days) of lambs/kids after going through pasture paddocks with a first grazing pass. Once lambs/kids are weaned the ewes/does recover their immune response and can now go back and do a second or third pass on previously grazed paddocks.
  • Consider moving early weaned lambs/kids into a “safe” pasture grazing situation. This could be a permanent pasture that has not had sheep or goats grazing on it for a year or it could be a paddock of a summer annual such as a sorghum x sudangrass, sudangrass, or even corn.
  • Another option might be to bring the early weaned lambs/kids into a feedlot where Haemonchus contortus is not a concern, and feed them grain and hay for another 30 days and then sell them to a commercial feedlot operator.

All of these types of strategies are being considered because reliance upon chemical de-wormers as the sole component of parasite management is a recipe for disaster at some point. Haemonchus contortus resistance to every class of chemical de-wormer currently on the market is documented. Selective de-worming and limited use of a chemical de-wormer that works on your farm can help to slow down chemical resistance. These risk management strategies are being considered as a way of reducing chemical use, while keeping pasture based sheep and goat production viable.

None of these options are set in concrete. They need to be tweaked to work on your farm and your situation. The important thing is to recognize that understanding the biology of the parasite and the biology of the animal response to the parasite will help to determine a risk management strategy within a pasture production system.

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