There are animal, environmental and management techniques that affect the impact of internal parasite infestation on cattle. These include: age, climate, stocking rate, nutritional status of the cattle and grazing behavior.
Age of the Animals. Adult cows and bulls develop a resistence to internal parasites and a positive result from deworming is most likely seen with _thin _ animals grazing short pastures. However, dewormed adults will pass fewer worm eggs in their manure making parasite transmission to younger animals less of a problem. Grazing cattle that range from 3 months to 1½ to 2 years of age will nearly always show a positive response to deworming. Calves under 3 months of age have not been grazing long enough to develop an internal parasite problem.
Climate and Season. Internal parasites are spread by worm eggs which are passed in the manure and hatch into larvae. These larvae climb up blades of grass where they are consumed by cattle. Once inside the animal, they mature to egg-laying adults. This entire life cycle can be completed in as little as 3 weeks. Hatching of the eggs and development of immature worms occur best below 85° F. Freezing temperatures prevent eggs from hatching, but have little effect on immature worms. Parasite transmission is highest in the spring and fall, reduced in the winter in colder areas and very low in dry summer months in the South.
Stocking Rate. The more cattle grazing an area, the more likely cattle are to graze near fecal piles. This grass is more likely to harbor immature worms. Overstocked pasture will have shorter, average grass length concentrating immature worms and increasing the number eaten with each mouthful of grass.
Nutrition and Body Condition Score. Thin, poorly nourished animals cannot afford to share nutrients with worms or have a decreased appetite. Well nourished adult animals in good body condition with adequate nutrient availability may be able to perform well without deworming.
Grazing Behavior. Cattle pick up immature worms near a pile of manure and this is most likely to happen when cattle are grazing or eating feed spread on the ground. Parasite transmission is not likely when cattle are fed hay in feeders or grain in feed bunks.
In summary worms are most likely to be a problem in _thin _ grazing calves on overstocked pasture in the spring and fall. Most producers assume that internal parasites are a problem and no attempt is made at diagnosis. However, microscopic examination of fresh manure from several animals in the herd, can be used to determine which parasites and the approximate numbers present. These tests are inexpensive and can be helpful in selecting the correct dewormer and deciding when to deworm.
Source: James B. Neel, Professor of Animal Science; Gene Burgess, Professor, Plant Pathology and Entomology and Fred Hopkins, Professor, Large Animal Clinical Sciences, University of Tennessee