The Effect of Selective Dry Cow Therapy on Scottish Dairy Farms

Selective Dry Cow Therapy (SDCT) is becoming commonplace on many dairy farms in response to increasing pressure to reduce prophylactic antibiotic use.  Cows with no evidence of existing infection can be successfully dried off with only a teat sealant, and the use of antibiotic dry cow therapy can then be targeted to only those cows with evidence of infection at dry off (usually indicated by a high somatic cell count or SCC in late lactation).

Over the last three years, 17 dairy farmers in Scotland have been taking part in a project to investigate the effect of SDCT.  The aim was to determine whether there was any detrimental effect on udder health, increase farmer engagement and build confidence in SDCT.  The project was funded by the Scottish Government’s Knowledge Transfer and Innovation Fund (KTIF) and managed by SAC Consulting with additional support from Zoetis, the University of Edinburgh and Müller.

Dry Period Performance

The dry period performance was assessed using two main criteria:

• Dry Period Protection Rate (target 90%): cows who had a low SCC (under 200,000 cells/ml) for the three months prior to drying off, and whose SCC remained low at the start of the next lactation.
• Dry Period Cure Rate (target 80%): cows who had a high SCC at some point in the three months prior to drying off, and whose SCC then went low at the start of the next lactation (i.e. they had got rid of any udder infection during the dry period).

There was dry period performance data on 3342 cows, of which 57% were given teat sealant only at drying off (SDCT), and 43% were given both antibiotic dry cow therapy and teat sealant (ADCT).  The effect on dry period protection rates and cure rates are shown in Figure 1.

There was no difference in dry period protection rates between the treatment groups (SDCT 82% versus ADCT 84%).  Interestingly, 373 cows in the SDCT group were wrongly classified at drying off (they had at least 1 high SCC prior to drying off) and should have received antibiotics.  As expected, their cure rates were slightly lower (74% versus 79% in the ADCT group).  This shows the benefit of the responsible use of ADCT to target those cows that had evidence of udder infections at drying off.

Clinical Mastitis Rates

The effect of drying off treatment on the incidence of clinical mastitis within the first 30 days of calving was investigated.  Cases occurring during this period are referred to as mastitis of dry period (DP) origin, as there is a high probability that mastitis in the first month of lactation is due to the original infection being picked up during the dry period.

Across the 17 farms, a total of 2059 mastitis cases were recorded, of which 353 were of DP origin.  There were fewer mastitis cases of DP origin in cows on SDCT compared to those on ADCT (see Table 1), and the total number of mastitis cases was significantly higher for cows on ADCT.  This is to be expected as these are the problem (or high SCC) cows more likely to have chronic recurring udder infections, which would sporadically result in clinical mastitis.  When expressed as a percentage, mastitis cases of DP origin out of the total number of cases was slightly higher for cows on SDCT, but the biggest risk was for first calving heifers.

First lactation heifers are a good indicator of dry period infection when they calve in, as they have no previous mastitis history, and have not received any prior dry cow therapy that could influence mastitis or SCC rate after calving.  A high incidence of heifers calving in with mastitis or a high SCC at first milk recording after calving may point towards cleanliness of the dry cow environment, as well as how heifers are managed and transitioned into the milking herd.  Heifers have different challenges compared to cows over the transition period and they likely experience more stress with moving from the in-calf heifer group to the dry cow group and their introduction to the milking parlour.  Stress will suppress the immune system and increase the risk of infection.

Conclusions

In conclusion, SDCT can work very well and it is possible to achieve industry targets for dry period performance with high rates of SDCT.  Although there was a slight increase in the rate of clinical mastitis cases immediately after calving for those cows on SDCT, heifers contributed the most to mastitis cases in the first month after calving on many farms, and so their management and introduction to the herd must be carefully managed to minimise stress and reduce the risk.

Recommendations for Implementing SDCT

• Work closely with your vet to choose which dry cow therapy is most appropriate to your farm and regularly review performance and selection criteria based on bulk tank SCC, individual cow SCC, clinical mastitis data and bacteriology to identify the main mastitis causing organisms on your farm.
• Hygiene at drying off is crucial to the success of SDCT. The gold standard is to disinfect clean teats with cotton wool soaked in surgical spirit.  There are a number of other recommendations to follow during the drying off procedure (i.e. order of teat disinfection and order of teat sealant insertion) but most successful farmers take their time to dry off cows and do not carry out this procedure during normal milking time. They recognised it is an important task for care and attention to detail and treated it as such.
• The dry cow environment can have a significant effect on the infection risk during the dry period. Avoid overstocking dry cow facilities and straw courts should have plenty of space (ideally 1.25 square metres per 1000 litres herd average production) and calving pens cleaned out daily.  Cubicles should be scraped and bedded every day and if dry cows are at grass ideally they should be on pasture which is rested for four weeks and grazed for two.
• Look at the management of heifers into the dry cow group and milking herd. Results showed that heifers contributed significantly to mastitis in the first 30 days of lactation and are under more stress in the pre-calving period than cows.  Look at ways to minimise stress on heifers at this critical time.

Lorna MacPherson, Dairy Consultant

lorna.macpherson@sac.co.uk; 07760 990901