Parasite Control in Heifers

Posted: January 22, 2008

Dr. Arlen Mills, Extension Veterinarian, Capital Region, Pa.
April 19, 2002

Research has shown that Holstein heifers should calve between 22.5 and 23.5 months of age to maximize lifetime performance. But it has also been shown that weight at calving has a greater impact on first lactation performance than did calving age. This suggests that heifers be bred by weight and not just by age. This dictates that Holstein heifers weigh 750 to 850 pounds when bred to produce a fresh heifer that weighs in at 1250 pounds. One hindrance to raising heifers that meet these standards can be intestinal and stomach worms. We occasionally see the heifer that has been devastated by parasites but much more common is the chronic parasitism that results in reduced growth.

Roundworms are the most devastating internal parasite of cattle. These include the brown stomach worm, Ostertagia. All of these worms share a common life cycle. In an infected animal, adult worms produce eggs that are expelled from the host in the feces, contaminating the pasture. A pasture can remain contaminated for up to a year.

Once the eggs are passed unto the pasture, they hatch and become immature worms called larvae. Infection of a new host occurs when these larvae are consumed with blades of grass. These swallowed larvae mature in the new host, starting the life cycle all over again.

The level of parasite infestation in livestock and pastures varies with season and management practices. Parasite activity peaks in the spring, varies during the summer depending on conditions, and declines during a normal winter. Therefore, steps to control worm problems should focus on the times of higher worm activity.

Age of livestock should also influence decisions concerning parasites. Younger cattle are more susceptible to worm infections. A mature cow under a good nutrition program will develop some acquired immunity to parasites so is less bothered by their presence than young cattle. Thus parasite control should focus on the young livestock.

Therefore, focus parasite control on young stock during the time of high worm activity and
combine management with proper use of deworming medications.

Deworming medications should be used in a timely manner so as to reduce infection before symptoms of disease occur. Treatment should be aimed at interrupting the life cycle of the parasite in an effort to minimize pasture contamination.

There are a number of dewormers on the market. Most of these products are either avermectins/milbemycins (ivermectin, doramectin, eprinomectin, and moxidectin) or benzimidazoles (oxfendazole, albendazole, fendbendazole). Avermectins/milbemycins have the advantages of providing control of some external parasites and persistent protection for days to weeks after treatment.

Putting this all together, we need to concentrate our efforts on controlling parasites in the young, growing stock by timely use of dewormers to limit contamination of pastures or lots. This will limit parasite burdens in the most susceptible stock. To do this, all young stock should be treated with an appropriate dewormer prior to being tuned out onto a lot or pasture. This should be done each spring in March or April to counter the spring rise in worm activity. If possible, place less susceptible, mature cattle on contaminated pastures and attempt to prevent overgrazing. Animals on overgrazed pastures graze closer to the ground and pick up more worm larvae.

Animals on pasture may need to be dewormed again in midsummer as worm egg counts tend to rise in lots or pastures where livestock are maintained. A final treatment needs to be done in the fall to remove any remaining worms. At this time be sure to use a product that removes external parasites as well before animals are grouped for winter housing.

Having your veterinarian do fecal egg counts can monitor levels of parasite infection and treatment success. Realistically, a herd should be sampled on multiple occasions to determine worm prevalence.

Most losses due to worm infestations go unnoticed due to the chronic nature of the problem. But this does not diminish the magnitude of the end result. Parasite control in young stock will provide a significant return on your investment.