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April 2016

Stocking Density Impacts Worm Loads: 
Let Us Count the Ways


By James Morgan, Ph.D.
Round Mountain Consulting
Fayetteville, Arkansas

Most, if not all, aspects of parasitism are impacted by numbers of sheep per acre (stocking density). Similar to the spread of any communicable disease, density of the organism is a key variable impacting worm loads.


Let’s first consider that if all other factors are equal, the more sheep grazing per acre the greater the worm population in the pasture. With 10 sheep per acre, there will on-average be ten times as many worm eggs passing out of the animals each day than with only one sheep per acre.

However, the impact of having ten sheep per acre versus one sheep per acre on worm loads is much greater than ten times. More sheep per acre results in each sheep ingesting more larvae resulting in more adult worms laying eggs. The worm cycle builds and the relationship is exponential.

Two further impacts of sheep density on worm loads include the following: The first one is a decrease in nutrition. In general, lower densities of grazing animals per acre over the course of the year allow more selective grazing and ingestion of higher quality and/or quantity of foodstuffs. Better nutrition improves immune system performance enabling better resilience.

Second increased sheep numbers over the course of the year decrease forage height. Research indicates that 85 to 90 percent of the worm larvae do not crawl above 2 to 2.5 inches. Farm budgets including nutritional predictions and parasite impacts can help determine better stocking densities. Impacts can be nuanced since lower worm loads in ewes and pastures going into the fall/winter affect how fast parasites increase the following spring.

This article will not address rotational grazing programs that have the potential to help manage worms while keeping sheep densities higher. Rotational grazing programs have higher stock densities for shorter bouts of grazing. Rotational grazing programs are effective for parasite management if they control forage height, change paddocks before eggs hatch and larvae become infective (7 days), and/or extend rest periods beyond 45 days so that a higher percentage of the larvae die before grazing resumes. (see Timely Topic; Jan 2015).

Number of sheep grazing days/acre can be decreased by selling lambs and cull ewes shortly after weaning. If worms are killing 15 percent of your lambs, it can pay to sell earlier in the summer. All things being equal, selling those lambs early should greatly decrease yearly worm loads. It may also save the shepherd money by allowing stockpiling of forage for fall. The bottom line at the end year includes amount of purchased hay, dewormer expense, and numbers of dead lambs and not only the selling price per pound of lamb.

In the previous examples, direct effects of total sheep/acre on parasite loads were discussed. Are all sheep equal in terms of loading pastures with worms and creating problems for the year? No. The two most susceptible classes of sheep are lambs and lactating ewes. Does a day of grazing in October have the same impact on numbers of worm larvae ingested or numbers of larvae put back on pasture as a day in late June? No.

A shepherd can change their management so that the classes of sheep that contaminate pastures and are most susceptible to worms are not grazing during peak parasite months. Note that in many flocks the highest density of sheep per year (lactating ewes and growing lambs) is during spring and summer months. These are also the classes of sheep that load the pastures the most. Shifting highest density of sheep and highest density of susceptible sheep to the fall when pasture worm loads are lower should greatly decrease parasite issues.


By selling fall-born lambs by early spring, sheep density is lowest during the humid summer months when increased sheep density greatly increases worm loads. It is not all win-win and requires a farm budget. The gains in parasite management, stockpiling of forage for the fall are partially off set by a 30% lower lambing rate in the fall than in spring lambing for sheep breeds that have multiple lambs.


An additional interaction of sheep density and worm loads is the length of the grazing season. It is not as simple as

calculating that a farm/ranch has 50 ewes/20 acres. In most respects, the same number of sheep grazing for 6 months per year versus 10 months per year will have 60 percent of the days in which feces are being deposited with eggs on pasture. With more worm generations in a ten month grazing season, even higher larval loads on pasture are reached.

Shorter grazing seasons allow most adult worms in the gut to die (enough over winter as dormant larvae in the guts and survive winters on pasture to start cycle again in the spring). Estimates suggest adults barber pole worms survive in the stomach for 60 days. In a grazing season of 6 months, this results in four months of the year in which adult egg laying worms in the abomasum are at a minimum. Sheep in southern Georgia grazing 10 to 11 months of the year always have adult barber pole worms producing eggs year-round and are always replacing larvae that die or adding more larvae to the pasture.

Another way to impact sheep density is to add a grazing species that results in fewer sheep per acre. If enough cattle are added to decrease sheep numbers by 25-50, this can have significant impacts on worm loads. The drop in sheep density can have a huge impact. An added impact includes that cattle and sheep are dead-end hosts for each others worms, so this results in even fewer available larvae for ingestion. Also, in a multi-species rotational grazing system, a shepherd can double the time before bringing sheep back on the pasture by alternating sheep and cattle grazing rotations. One study estimated that worm loads dropped by 50 percent with a 60 day rest between grazing.

Taking a crop of hay off a pasture is another management strategy that affects sheep density. First is an effect on sheep density by decreasing the total number of days that sheep can graze a pasture. Second, it also increases the amount of time before sheep can be rotated back onto hayed pastures resulting in larvae death. Third, the very short grass height following mowing for hay during hot summer months dries out the soil and increases soil temperature which kills the worm larvae.

Removing sheep from pastures and putting them in dry lots, grazing crop fields (wheat, corn), or grazing annual forages on row crop land are effective. These management strategies all decrease the number of grazing days (sheep density) on the permanent pastures so that numbers of larvae ingested and numbers of larvae deposited are less. They allow some of the larvae on the pasture to die before being grazed again. They also allow a proportion of the adult populations of worms in the gut to die before sheep are returned to permanent pastures.

In summary, sheep density affects the severity of worm infestation in multiple ways. If worms are an issue, shepherds have many management tools available to decrease the “effective density of sheep on acre” that do not require decreasing numbers of ewes in the flock. Manipulating season of lambing, adding haying or cattle to the system, and marketing lambs earlier are all effective tools at decreasing the number of sheep grazing days or density.


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