A major issue facing dairy producers of the '90s and beyond is not whether deworming is the right thing to do, but whether producers are administering the right product at the right time to the right category of animal. Effective deworming results in maximum economic benefits, as well as long-term parasite control.
For progressive operations, deworming cattle after parasite build-up has occurred is a practice of the past. Deworming strategies are now designed to control parasites by correctly timing treatments to interrupt parasites’ life cycles and prevent reinfection before significant economic loss has occurred.
Dairy farmers answer that question themselves by spending millions of dollars each year to deworm their animals. They have been convinced of the benefits of parasite control by the increased milk production gained with deworming. As the efficiency of the dairy industry improves and as more potent dewormers are approved for use in dairy cows, it is increasingly important for each producer to have a deworming strategy in place.
Recently, attention has focused on deworming practices. This is due, in part, to the improved efficacy of current dewormers compared to those available just a few years ago. Also, the increased availability of convenient, non-handling dewormer formulations--such as medicated blocks, pellets and free-choice minerals--has made deworming possible for more producers. A better knowledge about timing treatments to provide the greatest long-term benefits of parasite control, however, is the single biggest factor for increased dewormer use.
Young dairy cattle are very sensitive to parasites and their ill effects. Because of the easily recognizable symptoms of parasitism that often develop in dairy replacement heifers--including diarrhea, weight loss, slowed growth, delay of puberty, breeding and calving--the economic significance of an effective parasite control strategy has been clearly proven by investigators.
Unlike young animals, adult dairy cows develop a fairly strong natural resistance to parasites--except during a three-to-four-month period following calving. Additionally, the milking cow is typically on a high plane of nutrition that often masks the most detrimental effects of gastrointestinal parasites. During the post-calving period of parasite susceptibility, however, losses in milk production can be significant. Although difficult to measure accurately even under carefully controlled conditions, these losses can be immediately quantified by producers in terms of decreased income and cash flow.
Milk production in dairy cattle is a highly elastic trait that is easily influenced by many factors. The first 100 days of lactation is the period of greatest elasticity. And it is during this time that high-producing cows are most susceptible to negative external stimuli, such as parasite infections.
Soon after calving, high producing cows are under considerable nutritional stress--a state that continues for several weeks, depending on the level of milk production. During the early post-calving period, high producing cows seldom meet their nutritional needs through feed consumption alone and, thus, metabolize body fat. In fact, high-producing cows can often lose 100 to 200 pounds of body weight during that time. The lost weight is usually regained later in the lactation when production needs are lower.
The fluctuations in body weight, dry matter intake and milk production for a moderately high-producing cow during a 305-day lactation are conceptualized in Figure 1. The peak in milk production occurs well before the time a cow can consume enough dry matter to meet her energy needs. Consequently, until feed intake exceeds the amount needed for milk production, a cow uses body reserves as is indicated by weight loss. During this period of negative energy balance and physiological stress, even a small number of worms can significantly reduce milk production.
There is a definite relationship between the economic losses due to decreased milk production and the time of worm exposure, length and degree of exposure, as well as the cow’s stage of lactation. Furthermore, three separate phenomena appear to be involved:
During the post-calving, early lactation period, dairy cows experience varying degrees of “periparturient relaxation of resistance” to parasites. In other words, this state allows the parasites more freedom to cause harm. Research in cattle has demonstrated that non-lactating animals acquire fewer parasites compared to lactating animals when grazed together on infected pastures. This suggests that the endocrine changes associated with early lactation temporarily decrease the immune response to ingested worm larvae resulting in greater susceptibility to new worm infections, maturation of inhibited larvae and increased fertility of existing parasites.
The effect of gastrointestinal parasites on production parameters in lactating cows was first identified at the University of Wisconsin. Dairy cows in various stages of lactation from one of the University research dairy herds were given infective larvae and responded with marked reduction in milk production. (Figure 2). Cows that were less than 90 days fresh at the time of parasite infection began producing markedly less milk within 24 hours compared to similar cows that were not infected.
Compared to non-infected animals, milk production of infected cows was reduced by 6.47 lbs. per day; however, milk production in cows more than 90 days fresh at the time of parasite infection was only reduced 0.38 lbs. per day compared to similar non-infected cows. It is reasonable to conclude that the decreased milk production was a result of the induced parasite infections.
Investigators from major dairy regions throughout the world have examined the economic importance of gastrointestinal parasitism in lactating cows. Most of the studies were field trials on commercial dairy farms and involved determining the effects of a wide range of dewormer regimens on milk production. Although the results of the studies widely vary, treatment timed during the first 100 days of lactation was key for maximum parasite control.
Treatment of cows after 100 days of lactation or during the dry period appears to be a waste of time, money and resources. Cows often become reinfected during the dry period and, if not treated at freshening, remain infected during lactation. Furthermore, the dairy cow is under a minimal amount of physiological stress during late lactation, so deworming at this time has little or no effect on milk production.
Research data indicate that a range of 100 lbs. To 1,000 lbs. more milk per lactation can be generated from strategically deworming high-producing cows and by maintaining minimal parasite exposure during the first 100 days of lactation. The degree of exposure dairy cows have to parasites will determine the deworming program that should be used. For cows exposed to parasites, for example, a single treatment at calving will add approximately 300 to 500 lbs. more milk per lactation, while a second treatment 6 weeks later can significantly increase this benefit.
Dairy cows can be successfully dewormed two different ways. The best method of treatment appears to be on an individual basis according to each animal’s lactation cycle. Seasonal treatment of an entire herd once or twice a year depending on the level of parasite infection, however, can also be successful.
After the treatment at freshening, the strategic choice for the second treatment is 6-8 weeks into lactation. Treatment at freshening and again after 6 weeks--the "0-6" program--is based on the 6- to 7-week life cycle of gastrointestinal parasites in the adult cow. Deworming 6 weeks after freshening is timely because of the following:
The first step in designing a deworming program for lactating dairy cows is to determine the approximate level of parasite contamination animals are exposed to throughout the year.
The general guidelines for determining parasite exposure under different types of herd management are:
It is not recommended to deworm at dry-off. During the dry period, reinfection can occur and place milk production at risk during the critical post-calving period.
Based on approximate parasite exposure level, the following individual deworming programs are recommended for dairy cows:
In dairy operations where individual cow treatments are impossible or inconvenient, parasite control can be achieved with seasonal herd deworming. Dairy cows that are exposed to moderate or high levels of parasites--grazing pastures or running on contaminated exercise lots--should be dewormed as a complete herd in late fall after the first hard frost and again approximately 6 weeks after spring green-up or turnout.
Dairy cows exposed to low levels of parasites need to be treated as a herd only once a year, preferably in late fall.
Strategic deworming first began in the United Kingdom during the late 1960s. At that time, investigators found they could prevent severe parasite infections from occurring in late summer by treating young cattle just prior to the period of highest challenge followed by moving them to “parasite-safe-pastures”--pastures where cattle had not grazed during the same calendar year. This system was called the “dose and move” deworming program.
The success of dose and move deworming programs was very limited, however, since most producers did not have access to parasite-safe pastures during mid-grazing season. Other investigators found that deworming repeated in succession--3 to 4 weeks apart--at the beginning of the spring/summer grazing season also produced clean pastures and provided significant weight gain benefit over conventionally treated cattle.
The economic importance of a strategic deworming program for replacement heifers has been studied in a number of states. One study conducted in Ohio, for example, showed that dairy heifers strategically dewormed during their first year of grazing had weight gain increases of 88.0 lbs. more than control heifers.
Parasite control of pastures is accomplished by preventing pasture recontamination. Several years of strategic treatment of cattle are usually required to substantially reduce pasture infectivity. Before putting young cattle on pastures, they should be treated to prevent introducing new parasite infections. Retreating the cattle after grazing has begun is the second step in developing parasite-safe pastures. The second treatment prevents the parasites acquired from the pasture early in the grazing season from developing into a second-generation build-up on the pastures. This can be accomplished best by carefully timing treatments with an efficient dewormer around the life cycle of invading parasites.
For replacement heifers, preventing parasite build-up on pastures will improve rate of gain and decrease time to breeding size.
The first treatment given prior to spring grazing or before turnout onto pasture is very important. This treatment is necessary for all animals that have been exposed to parasites to prevent infected animals from contaminating the pasture at the start of grazing. Calves weighing less than 300-400 pounds at springtime are not considered to have been exposed to parasites.
The following summarizes strategic replacement heifers, bulls and steers of various weights.
The two treatments are recommended at 3 to 4 weeks apart after the initial deworming because it takes that length of time for parasites to complete a life cycle in young stock, depending on the animal’s size and age. It takes longer for parasites to develop to maturity in adult cattle. For example, a young calf may develop a mature infection as early as 3 weeks following exposure, while a yearling calf may take 4 weeks and a mature cow 6 weeks.
Since parasites survive on all types of winter pasture conditions throughout North America, most spring pastures that have been grazed by cattle during the previous summer or fall are assumed to be contaminated. The larval stages of parasites that survive the winter, however, demonstrate a limited life span during the following grazing season. If not consumed by grazing cattle, the over-wintered larvae die within 2 to 3 months after the spring/summer grazing season begins.
A strategic deworming program for the whole herd achieves these objectives:
Dairy producers in the ‘90s should consider deworming as they would any management tool with an eye toward maximum return on investment. By carefully following a strategic deworming program with an effective and convenient treatment regimen, today’s dairy producer can realize economic benefits from lactating cows, steers and replacement heifers that far outweigh the costs.
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