By Stephen Champion, BSc (Hons) BVM&S MRCVS,
Veterinary Surgeon, CHAMPIONVET.com
Written for Farming Scotland Magazine, November 20

In the second of two articles on parasitic gastro-enteritis (PGE) in cattle, Stephen Champion discusses the options for control of gut worms in calves. You may be employing one or more of these strategies already. It is useful however, to categorise and understand the options available to you, so that logical and cost-effective strategies can be devised for your farm, and the build up of anthelmintic resistance can be avoided.

Methods to prevent outbreaks of PGE can be based on the usage of anthelmintics (wormers) either prophylactically or strategically; or combined grazing and anthelmintic systems; or grazing strategies alone.

I. CONTROL DURING THE FIRST GRAZING SEASON

1. Prophylactic Regimes using Anthelmintics

These are designed to be used in the early part of the grazing season to prevent recycling of the infection acquired from overwintered larvae on the pasture (see Part 1 in last edition). These regimes have largely replaced the traditional approach of treating calves just in July and September.

Boluses have become a popular method of worm control, since cattle are dosed just once at turnout and do not need to be handled again until autumn housing. They protect for the part of the grazing season where animals are most susceptible to PGE. Occasionally they are lost, and in addition there has been some debate about the ability of the animal to build up natural immunity to worms when carrying a bolus, but generally they are an efficient and practical method of control. Most however, carry a long withdrawal period, so beware!

Prophylactic dosing can also be achieved using the other commercially available anthelmintics, although timing is critical and may make these regimes unsuitable for mixed enterprises where other demands on labour may take precedence. However, withdrawal periods are shorter which are more compatible with changing market conditions and unexpected periods of grass growth. The Ivermectin group of anthelmintics have strategies unique to themselves, such as the 3, 8 and 13-week regime of ivermectin (Ivomec) and the 0 and 8-week regime of doramectin (Dectomax). The benzimidazoles and levamisoles are generally repeated at three-weekly intervals until late June/early July when pasture contamination has fallen to low levels. Usually between two to five doses are required depending on when animals are turned out and on the grass growth, stocking density and costs involved. Care should be taken to ensure that calves do not develop disease in late summer because this system may not be totally effective in suppressing pasture larval populations.

Farmers using prophylactic regimes often re-dose before housing if there is likely to be a heavy late season challenge.

2. Strategic Anthelmintic Treatments

By mid-July, infection in calves will have reached sufficient levels to cause clinical disease. Cattle can be set stocked for the duration of the grazing season and are given anthelmintic treatments twice, in mid-July and again in mid-September. This allows cattle to be infected, and hence build up immunity, but is supposed to prevent outbreaks of disease. However, since cattle are carrying worms, sub-optimal growth can occur, and so there is little to recommend this strategy!

3. Combined Grazing and Anthelmintic Systems

These are the so-called evasive or dose and move strategies. All cattle are treated in mid-July (before the major summer rise in pasture larval contamination) and moved onto clean pasture. The main advantage is that only one dose of wormer is required, although, as above, there is sub-optimal growth between turnout and treatment. It also relies upon the availability of clean grazing in July, making the strategy impossible on even the best-run farms! It also limits the use of the first pasture from mid-July onwards to animals other than first year grazing calves (which have a greater-developed immunity).

4. Grazing Strategies

These systems minimise the risk of young cattle coming into contact with larvae and aim to prevent the levels of infection in calves that can cause disease. They rely on minimal anthelmintic input and are therefore attractive to organic systems.

Clean grazing systems may involve turning cattle out in mid-July onto hay or silage aftermath. This is simple and flexible although it is often prohibitively costly keeping cattle housed during the early grazing season!

Rotational grazing systems to obtain clean grazing involve the turning out of cattle in spring onto pasture not grazed the previous year (or not grazed from July onwards). This relies on strict control and record keeping and breakdowns can occur, if for example, contaminated cattle are purchased or fences are poor. Since the most important parasites are not carried in other species, rotation with other species, or arable crops, can be used to provide clean grazing. However, when alternating sheep and cattle, it is worth remembering that Nematodirus battus, a parasite associated with severe disease in lambs, can also cause disease in calves. Veterinary advice should be sought. Minor parasites can also be cross-transmitted and can build up over time.

Mixed grazing within a season with other cattle is employed as a strategy when suckler calves are grazed alongside their dams. The latter do not perpetuate the life-cycle of the parasites. In spring-born calves, the initial intake of grass is usually too low to allow disease to develop. Conversely, autumn and winter-born calves will consume large quantities of grass from turnout and therefore anthelmintic prophylaxis may be advisable.

Mixed grazing within a season with sheep and/or horses is used but is far from ideal. The optimal sward length for horses is shorter than for sheep, which in turn is shorter than that for cattle. Either option must involve the treatment of calves at housing.

Alternate grazing systems have been developed where adults and weaned calves graze the same sward. Older cattle follow the calves through a series of paddocks, “vacuuming up” infection (but producing fewer eggs as they are partially immune). When all paddocks have bee grazed, the young calves are put back on the first paddock. The system can be expensive and time-consuming and the ration of calves to adults must not exceed 3:1.

II. CONTROL IN SECOND AND SUBSEQUENT GRAZING SEASONS

The control of PGE in second and subsequent years depends on the immunity acquired during the first grazing season. In general, the more effective the anthelmintic and the longer the period of protection in the first year, the poorer the immunity will be in the second year! This is because the animal has been exposed to fewer worms to develop its immunity. Some of the strategies described above can be used for second year animals, although beware long withdrawal periods for animals reaching slaughter weight. Bodyweights need to be estimated accurately for dosing and as with many other things, the cost-benefit of treatment should be assessed in the light of likely natural immunity.

There is no single ideal scheme for the control of PGE. It is hoped that before too long, an effective vaccine may be commercially available to help combat this group of diseases. Knowledge of the parasites involved, the strategies currently available to outwit them and the economics of doing so, must always be borne in mind when deciding upon a worm control strategy.

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