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Home Tests & Tools for Worm Control Nutrition for Resistance and Resilience to Worms – Sheep

Nutrition for Resistance and Resilience to Worms – Sheep

Nutrition and the condition of sheep can have a major impact on the resistance and resilience of sheep to worms:

  • Sheep that are resistant to worms can prevent some or all worms from establishing and as a result have lower worm egg counts.
  • Sheep that are resilient to worms can grow and produce with less ill-effects from worms.

Good nutrition and managing the condition of ewes and the weight of young sheep is a vital component of WormBoss worm control programs. Nutrition of sheep relates to pastures, forage crops and supplementary feeding and being able to meet the requirements for growth and reproduction.

Protein and energy

The resistance and resilience of sheep to worms is affected by different aspects of nutrition. It is the supply of protein that is most important for regulating the resistance of sheep to worms. This is largely because worm infection results in sheep diverting protein from muscle and wool growth to the immune response and the need to repair the damaged gut. In contrast, both protein and energy are equally important in improving resilience to infection.


Sheep need protein to maintain and grow muscle and wool and sustain pregnancy and lactation. Protein is also needed for the immune response that determines resistance to worm infection. Proteins are made up of building blocks called peptides that are composed of long chains of small compounds called amino acids.

The protein supply in grazing animals comes from two sources:

  • Rumen microorganisms convert plant proteins consumed by the sheep to microbial protein. This is the major source of protein to grazing sheep. A pasture with higher crude protein will generally lead to more microbial protein available for digestion by sheep.
  • Some proteins cannot be broken down by rumen microorganisms. These proteins are called bypass or protected proteins and are usually given to sheep as supplementary feeds. These bypass proteins are usually insignificant in the pasture and are most valuable when given to animals with a high protein requirement such as pregnant ewes, twin-bearing ewes and growing sheep.

Extra protein can be given to sheep in one of several ways:

  • Improving the crude protein content of the pasture (ideal crude protein levels are 10-15% of pasture dry matter; lower for dry sheep and higher for young and reproductive sheep) and ensuring animals have sufficient pasture to graze (at least 1,000–1,500 kg green pasture dry matter per hectare; more for higher production). Pasture quality can be improved by inclusion of legumes, improving soil fertility and grazing management.
  • Feeding protein-rich supplements such as cottonseed meal, canola meal or copra meal or (less effectively) by protein-rich grains such as lupins, pulses and oilseeds. Protein meals (which can be pelleted) contain high levels (i.e. 30–60% or more) of bypass proteins.
Image: Rumen and sites of digestion (Source: Professor Nick Sangster, University of Sydney)


Sheep need energy to maintain themselves, grow, produce wool and sustain pregnancy and lactation. In contrast to protein, energy is less important for immunity and resistance to worm infection. Energy in pasture is mainly provided to animals by carbohydrates, such as cellulose and sugars, and to a very small extent, by fats.

Carbohydrates are fermented by rumen micro-organisms to provide metabolisable energy (ME) to the animal. ME is the energy used by the animal for maintenance, production and reproduction and the amount is expressed in units of megajoules (MJ) per kilogram of dry matter (MJ/kg DM).

The supply of ME to grazing animals comes from pasture or supplements. Rumen microorganisms convert the carbohydrates in pasture or supplements (e.g. grain, hay, silage) consumed by the sheep to provide ME. A pasture or forage with higher digestibility (i.e. young, green and leafy) will lead to more ME available for the sheep.

Extra metabolisable energy can be provided to sheep in one of several ways:

  • Improving the digestibility of the pasture or forage (ideal levels are 70–85% of pasture dry matter; higher for young and reproductive sheep) and ensuring animals have sufficient pasture or forage to graze (at least 1,000–1,500 kg green pasture dry matter per hectare; more for higher production). Pasture quality can be improved by inclusion of legumes, improving soil fertility and grazing management. Sheep are able to eat more from a high-quality pasture and in that way increase both energy and protein nutrition.
  • Feeding energy-rich supplements such as cereal grains, lupins or oilseeds or (less effectively) by hays and silages. Cereals, lupins or oilseeds can provide 11–13 MJ ME/kg, but considerable variation exists. Hays and silages can provide 8–11 MJ ME/kg, with even greater variation.

Major and trace minerals

Major and trace minerals are generally less important than protein and energy in regulating resistance and resilience to worm infection. Mineral supplementation may only be useful in regions or situations where minerals are normally deficient.

  • Diets with low phosphorus levels (less than 0.2% of dry matter) can reduce immunity to scour worms. In addition, worms impair the absorption of phosphorous from the gut.
  • Cobalt deficiency (less than 0.1 mg/kg dry matter) can impair the immune function of sheep and make them more susceptible to worm infection.
  • Supplementation with copper (trials conducted with copper oxide wire particles) can substantially reduce the establishment of barber’s pole worm and small brown stomach worm. These worms themselves also cause reduced absorption of copper. Supplementation with copper should be discussed with an advisor as high levels can accumulate and be toxic.
  • Dietary supplementation of penned sheep with molybdenum has been shown to increase rejection of established barber’s pole worm and black scour worm under experimental conditions.

Trace minerals have complex interactions and supplementation should be discussed with an advisor.

Classes of sheep with the highest nutritional requirements

The classes of sheep most susceptible to worms are young growing animals in their first year and ewes during late pregnancy and early lactation. These animals have increased requirements for protein and energy.

Young sheep

Susceptibility of young sheep to worm infection is a major constraint to sheep production. Young sheep, especially Merinos, are most susceptible to worms between weaning and one year of age.
Young animals often can’t get enough protein from pasture to meet their requirements, especially low-quality pasture. It is even more difficult for them when they are infected with worms because this normally reduces the amount of pasture consumed.

The worm resistance and resilience of young sheep will benefit from providing them access to high-quality pastures and from supplementary feeding with a protein-rich meal or grain. Contact an advisor to discuss your feeding plan and to establish growth targets for your young sheep.

Various experiments have demonstrated:

  • Feeding extra protein allows young sheep to more rapidly develop resistance to new infections (i.e. from infective larvae) and more quickly reject established worm infections.
  • Feeding extra protein or energy (or access to higher quality pastures and forages) will allow young sheep to greatly reduce the impact of worms on production.

An example that illustrates these effects was an experiment conducted with young Merino sheep grazing a high-quality pasture (11 MJ ME/kg DM; 20% crude protein). They were given either an effective anthelmintic capsule or 15,000 black scour worm larvae. Over the 10 weeks of the experiment the weight gains of the animals given either the capsule or the worm larvae were the same despite worm egg counts reaching 800 epg for the worm-infected animals and remaining at zero for the treated sheep.

Reproductive ewes

During late pregnancy and early lactation, the resistance of ewes to worm infection is usually lost or greatly reduced, resulting in a rise in worm egg count around the time of lambing. This is due to the peri-parturient relaxation of resistance (PPRR). As a result, worm eggs from ewes can be a major source of worm infection for young lambs.

The loss of resistance occurs at the same time as nutrient requirements are increasing. The requirement for protein and energy increases 2.5–3.0 fold during late pregnancy and early lactation and ewes often cannot eat enough pasture to meet these needs. As a result, ewes will start to mobilise fat and muscle and this highlights the importance of having ewes in the optimum body condition (CS 3.0, or CS 3.3 for twin-bearing ewes) at lambing.

Various experiments have demonstrated:

  • Increasing protein supply to ewes in late pregnancy and early lactation has led to greater resistance where ewes have been able to reduce worm burdens by 30–80%. In contrast, increasing energy supply typically has little effect on resistance to worms.
  • A higher energy diet is generally as effective as extra protein at increasing maternal performance of worm-infected ewes.

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