Cows carrying worm burdens into winter won't automatically eat more to make up for it. In fact, they could eat substantially less, causing problems.

Winter parasite problems in cow herds can go deeper than meets the eye. While signs of lice infestations are usually visible, even moderate internal parasite loads can cause unseen problems that carry into and beyond calving season.

One of these concerns, points out Dr. Rob Rew, Pfizer Animal Health parasitologist, is that worm burdens can depress appetites, potentially affecting the cow's body condition even under normal winter nutrition. The problem could be compounded when cows come off poor summer/fall grazing conditions, such as drought-restricted pastures, and when winter feed supplies are poor quality.

Poor body condition can quickly translate to lost profits. A Texas A&M study superscript 1 that tracked the performance of 422 cows based on body condition score (bcs) at preg check found that, compared to cows in good condition (bcs of 5 or 6), thin cows with a bcs 3 were only 48% as productive, and those with a bcs 4 were only 74% as productive. Lost income due to lower pregnancy rates, lower weaning weights and poorer calf survivability was more than $200 for bcs 3 cows and more than $100 for bcs 4 cows.

Dr. Chip Poland, North Dakota State University Extension Livestock Specialist, notes that body condition score at calving is the single most important factor influencing when a beef cow will cycle and conceive after calving. He adds that while bcs usually is linked to nutritional management, "disease and parasitism can contribute to lower scores even if nutritional requirements are `apparently' being met." superscript 2

The Role of Parasites The primary culprit in parasite-induced appetite suppression is the brown stomach worm, Ostertagia ostertagi, one of the most common and economically significant internal parasites. While the worm's impact on appetite are generally noted in relation to pasture gain, trials documenting the effect are done under drylot conditions with hay-based rations similar to those used to winter cows.

The study technique involves three randomly assigned groups of calves, all fed hay. Consumption is monitored closely. One group is parasitized with Ostertagia, two are uninfected. One of the uninfected groups is restricted-fed to match the amount eaten by the parasitized group, while the other uninfected group receives all the hay they can eat. "Even in studies with moderate parasite burdens, both the infected and the uninfected restricted-fed groups gain about 70% less than the non-restricted group," Rew points out. "This shows that the impact of Ostertagia is intake-related and, more important, that the effect on appetite can be severe."

Sending "False" Signals The appetite loss is linked to changes the parasite has on the digestive tract, essentially causing it to signal the brain to slow the digestive process. The animal simply isn't as hungry. Ironically, this fact is counter to generally accepted logic that says cattle with worm burdens will simply eat more to make up for it.

"Not only will they not eat more," Rew points out, "they will eat considerably less."

How applicable are these studies to cows? Over time, many cows do build some immunity to internal parasites, Rew notes. However, immunity to Ostertagia builds slowly at best. Plus, cows have varying ability to mount an immune response to parasites in general. While first- and second-calf heifers are probably most threatened by the effects of suppressed appetite, due to their need for higher nutrition and a lack of natural immunity to parasites, potential problems can exist throughout the herd.

"The goal is to get your cows through the winter without investing in high-cost supplemental feed, and yet have them in good body condition for spring calving," Rew stresses. "The best chance of doing this on normal winter feed supplies is by making sure appetites are not being limited by Ostertagia."

The bottom line: Just as with spring turnout and summer grazing parasite control, a fall/ winter herd health program that includes treatment of the cow herd for both internal and external parasites is a sound investment. "Lice can be a visible problem, but you don't want to overlook what you can't easily see either," Rew points out. "Using a parasite control product that provides proven protection against both internal and external problems makes sense."

1. Use of performance rations to calculate the economic impact of thin cows in a beef cattle herd, Wikse et. al, Food Animal Economics, JAVMA, Vol. 207, No. 10, Nov. 15, 1995

2. Optimizing reproductive performance starts ahead of calving, Dr. Chip Poland, North Dakota Stockman, Dec. 1999