DRY COW THERAPY

By removing any reservoir of infection from the udder, dry cow therapy plays an extremely important part in the control of contagious mastitis. Treatment of mastitis during the dry period has several advantages over treatment during lactation.

Although clots and clinical signs may disappear, probably half the cows treated for mastitis during their lactation remain carriers. This is especially true for staphylococcal infections. In addition there will be a second group of animals which pick up infection but never show any clinical signs - they go straight into the subclinical phase. Both groups provide an important reservoir of infection for the other cows in the herd, and both are best treated during the dry period, when the udder tissue regresses. This is known as dry cow therapy.

Special long-acting antibiotic preparations can be used because there are no problems with milk­withholding periods. Treatment during the dry period is also far more effective than when the cow is milking. It is an especially important opportunity to eliminate chronic Staph. aureus infections and the dry cow antibiotic used should therefore be chosen with this organism in mind.

Table 7.1 shows that for clinical cases of Staph. aureus treated during lactation, viz when clots are seen, the response to intramammary antibiotics can be as low as 25%. This is a particularly low figure, however, and if infections could be treated early, if possible at the high cell count subclinical stage, response rates would be higher. With dry cow therapy, response rates are much higher, and even with Staph. aureus reach 65%. This figure applies to younger animals.

Table 7.2 shows that if the staphylococci become well and truly established in the udder, as they would in an older cow, then even in the dry period response rates may be as low as 33%. In such instances, culling remains an important option for control.

Many new infections are picked up soon after drying off and these can cause mastitis either in the dry period or in the next lactation. For example, in one trial, 25% of quarters were infected in cows being dried off. Although 5% of these quarters elimi­nated their infection naturally, another 10% became infected during the dry period, so that at calving 30% of quar-

Table 7.1. Response to treatment (% bacteriological cure rates).

Adapted from Dodd, 1978.

Table 7.2. Response of Staph. aureus infections to treatment during the dry period. First and second lactation animals respond much better than older cows.

ters were infected. Dry cow therapy increases the number of quarters which lose their infection during the dry period and it also reduces the rate at which new infections become established. Various trials have shown that cows are fifteen to twenty times more likely to contract infection during the first two weeks of the dry period and for the two weeks prior to calving. This is discussed further on page 206. Management of cows before calving and maintaining a clean environment at this stage is therefore extremely important. Pre calving teat dipping may also help.

There has been a suggestion by some that continued use of dry cow therapy reduces udder infections to such a low level that it lowers the cow’s resistance to E. coli mastitis. This theory has not been conclu­sively proved or disproved. My advice to the reader would be to continue with dry cow treatment for all cows. I suspect that the advantages far outweigh the disadvantages. It is doubly important during July to September, when there is a risk of summer mastitis (see page 216).

At drying off, milking should be discontinued abruptly, even at yields of 20-25 litres per day. This is because once a day milking or, even worse, alternate day milking leads to an increased risk of mastitis (see page 178) and to a massive increase in cell count. Probably the worst procedure is to leave the cows for four or five days and then give ‘one last milking’ before inserting the dry cow tubes.

Dry cows should be removed from the milking herd. This avoids the risk of milking a cow which has been given antibiotics and eliminates the stimulation of a let-down which might otherwise encourage further milk production.

Points to note regarding dry cow therapy are:

• It is an important means of removing reservoirs of infection from the herd.

• It helps prevent new infections during the dry period, including summer mastitis.

• Its main effect is against contagious mastitis.

• Tubes should be administered hygienically and gently.

• Dry off abruptly and remove dry cows from the milking herd.

• It is much more effective than treatment during lactation.

• There is no cost of discarded milk.

• The risk of bulk milk antibiotic contamination is reduced.