Jerk the slack on internal parasites in a cow herd, and you could pocket another $20/head each year, easy. Take the see-no-evil, do-no-evil management path, though, and those same parasites can cost you more weight and reproductive efficiency than genetics are gaining.

"I think the actual losses from parasites are underestimated by most producers. Even some well-managed operations suffer more losses due to parasites than they need to," says Lou Gasbarre, an immunologist with USDA's Agricultural Research Service facility in Beltsville, MD.

"There is no way to put an accurate number on an individual, but if you look at an average figure across the American cattle industry, realizing in some herds it will be overestimated and underestimated in others, I think $20-40 per head per year is a reasonable number."

According to Gasbarre, Ostertagia ostertagi (round stomach worm) - the most common internal cattle parasite in the U.S. - is the primary culprit. The lion's share of economic loss stems from appetite suppression, which hinders gain and cost of gain, as well as potential suppression of the animal's immune system, which makes them more susceptible to other health challenges.

Even though controlling the parasite comes at a price, Gasbarre explains the benefit usually runs well ahead of cost.

"If you look at research trials, you'll find everything on the gain spectrum from huge weight gains in cattle treated with one product or another, to cattle treated for parasites weighing less at the end of the trial than cattle that received no treatment," says Gasbarre. "But, I think a good average figure to use for return is 10-20 lbs. of weight gain by weaning time."

Alvin Loyacano is a Louisiana State University (LSU) animal science professor at the Dean Lee Research Station in Alexandria. He says even when weight gain points to apparent control of internal parasites, its presence can disrupt metabolism enough to alter the reproductive performance of young breeding stock.

Increasing Reproductive Potential Until now, rule-of-thumb logic reckoned that parasite-based reproductive challenges were the result of the lower weights and body condition score (BCS) caused by parasites. But, an LSU research project that Loyacano began in 1993 indicates parasites may be robbing reproductive potential in other ways.

In a nutshell, Loyacano began the project with 84 females and has added another 96 heifers to it every year since. Each set of heifers is divided into four groups of equal numbers: a control group left untreated for internal parasites; one treated for Ostertagia ostertagi only; one treated for liver flukes only; and the last group is treated for both stomach worms and liver flukes.

As most producers would suspect, the heifers treated for worms gained significantly more weight and had significantly higher BCS than the control group. Those treated for flukes alone outpaced the control group, but not by as much. And the group treated for both worms and flukes posted the highest performance of any (Table 1).

What is surprising folks, however, is that significant weight and BCS gains yielded by the control of stomach worms is not significantly improving subsequent pregnancy rates, at least from a statistical standpoint. Controlling liver flukes has not served up significant gains in weight and BCS, but it is improving pregnancy rates substantially.

"That tells me that liver flukes directly have an impact on pregnancy rates," says Loyacano.

So far, 67% of the heifers treated for liver flukes only remain in the breeding herd after weaning their first calf, compared to 53% of the control group and 63% of the females treated only for worms. Although 78% of the females treated for both internal parasites remain in the herd after weaning the first calf, controlling liver flukes seems to offer the single most powerful tool to increase reproductive potential.

Loyacano explains females treated for both parasites have returned an average of $555 through first calf weaning, compared to $507 for the control group.

No one is sure exactly how liver flukes hinder reproduction, but Loyacano suspects it has to do with a disruption of hormonal balance and normal metabolism. A recent Spanish study underscores that theory.

In the study, heifers infected with liver flukes reached puberty 39 days later than heifers not infected with flukes. Researchers believe the delay is the result of infected heifers being unable to metabolize estrogen normally. Keep in mind that liver flukes are cropping up in places folks don't normally expect to find them. Although they have long been endemic in the Southeast, South and Northwest, the species of mud snail - necessary for the liver fluke's life cycle - and flukes are showing up in places like Nebraska, too.

Loyacano explains liver flukes can flourish where snails are present and soil temperatures are 50-90 degrees F.

The Potential Of Parasite Control For all of the losses associated with internal parasites, however, Gasbarre says if most producers think of parasite management at all, they think in terms of products and tradition rather than a specific control strategy.

As an example, according to the National Animal Health Monitoring Service (NAHMS) 1997 beef study, almost 75% of beef producers reported deworming their cattle. But, well over half did so based on tradition rather than laboratory testing or animal health assessment.

Plus, Loyacano points out, "If producers have treated for parasites, they have usually done so on a least-cost basis." That means producers are using what's cheap, not necessarily what is the right product for the right parasite at the right time for the right length of time.

"The best parasite control is not waiting to treat animals until they are sick, but keeping parasite numbers from building up in the pasture," says Gasbarre. "I believe the best programs are the ones that have some sort of early-season treatment."

What's more, Gasbarre explains, the most effective programs are built around knowing which specific parasites exist in the environment.

Moreover, effective control demands understanding the environment. Loyacano points out in his neck of the woods parasite season lasts all year long, typically making mid-summer the best time to attack worms and September the best shot at flukes.

In other words, since parasite loads vary from herd to herd and from pasture to pasture, it's impossible for any animal health professional to offer producers a one-size-fits-all solution. But, Gasbarre says, producers can make some rational assumptions that can help them establish a control strategy.

* First, Gasbarre explains, "Parasites succeed by taking advantage of the conditions most optimum for forage growth." In other words, the more forage you have, the greater the risk of internal parasites.

Likewise, the heavier the stocking rate, the greater the risk. Consequently, operations maximizing range resources with intense management likely face more parasite challenges than the one that expects 20 head to make a living on a section of cactus and rocks.

* Next, parasite control is just that. "Eradication of parasites is not only impractical and probably impossible, it's not desirable, either," says Gasbarre. He explains exposure to parasites allows parasite-naive calves to develop immunity as they mature.

So, the trick is to keep parasite loads low enough to avoid economic losses, while allowing cattle enough exposure to develop immunity.

* Finally, the more a producer puts into parasite control up front, the greater the anticipated returns. In this case, more exacting control of internal parasites requires understanding the specifics of a given situation.

"The only way I can tell for sure if my cattle are infected is to do a fecal egg count," says Loyacano. However, he cautions producers that, while fecal egg counts will identify specific parasites, the counts say nothing about the infection rate of individuals or herds.

Some Control Tips Really, Gasbarre believes producers can step up to more efficient parasite control fairly simply. "It takes producers setting a schedule where they will treat early in the season first, then keep a careful eye on the environmental conditions," says Gasbarre. The more forage-friendly the weather is, the more parasite-friendly it is.

Along the way, he encourages producers to take advantage of the scientific expertise available to them through local and state Extension veterinarians and government animal health professionals such as himself.

Most important, Gasbarre explains, "No one knows their own animals like a producer does. As a producer, if you look at an animal you know is on a good nutritional plane but they aren't doing as good as you know they should be, the first thing that should go through your mind is parasites."

With that in mind, he says just comparing the performance of calves under a specific parasite control strategy this year to performance last year should offer producers a guide to how much their parasite control is helping or hurting their herds' efficiency.

Bottom line, increasing gain and reproductive potential through parasite control requires understanding potential parasite impact even when performance seems to be on track.

"The best parasite control is the producer working with someone who understands the biology of the parasite and can point to potential control schemes," says Gasbarre. "The biggest thing is developing a plan, staying with it, then modifying it to meet the individual needs of the operation."