Internal parasite in cattle
By: Carl C. Stafford
Senior Extension Agent
Internal parasites better known as “worms” are a production efficiency issue with cattle and other grazing livestock. Cattle producers address this performance problem by using de-wormers known as anthelmintics. There are three chemical classes approved for use in cattle and several non-chemical management methods help reduce the impact of worms on cattle.
An issue with chemical controls is development of pest resistance. You understand that not every pest is killed when treated and those remaining reproduce, concentrating resistance. This can lead to failure of the control used. In the case of anthelmintics, new information is out from a federal study informing us of a possible solution to resistance – the refuge.
As is the case in management recommended for genetically modified corn, using a refuge is a way to avoid exposing the entire pest population to the control. A refuge will allow untreated pests to be available to cross breed with the survivors of the control. This results in a dilution of resistance in a given pest population and slows down the buildup of trait(s) leading to full resistance.
Cattle studies done by Virginia Tech have recommended for at least 20 years that adult cows not be treated for internal parasites. They found adult cows left untreated performed just as well as treated cows. Scientists concluded this was development of age related immunity. In light of this knowledge, it is puzzling to read of long acting de-wormers recommended for the entire herd. If you want this new product to work for years to come, reserve it for the young stock and save money on unnecessary treatments to adult cows. Leaving cows untreated also lets them serve as the refuge. New work at the Virginia Maryland College of Veterinary Medicine shows no benefit to adult cows treated with this long acting product.
De-wormers can also be used to treat cattle for lice as some product labels support this use. Considering the risk of resistance, it seems we could save our de-wormers for the internal parasites and use other lice labeled treatments when needed. Another option is to select for worm tolerance.
Small ruminant breeders have been selecting for worm tolerance in their flocks for years as their
de-wormers have stopped working in most cases. They choose females that tolerate parasites and keep their daughters as replacements. They also check for worm load and only treat those showing signs of needing the treatment. Historically small ruminants have required more frequent de-worming and resistance builds faster because of this.
There are pasture management techniques that help with worm problems. Keeping pasture taller and rotating pastures is another approach to suppressing worm impact on grazing livestock. Nematodes need to stay close to the ground or dehydrate for lack of moisture. Grazing tall grass limits exposure to short grass where the worms live. Making hay from our pastures will also help to remove worms for the short term. A final point would be about tall growing warm season grasses. It is not likely that worms can survive far from the soil; it is just too dry up there where warm season grasses grow. Graze these with confidence and gain access to quality forage when our pastures are least productive.