As a result of the global threat from the emergence of antibiotic-resistant strains of bacteria, the livestock sector is facing mounting pressure to reduce antibiotic usage.  To address the issue, AgriSearch is leading a major research project in collaboration with the Agri-Food and Biosciences Institute (AFBI), Animal Health and Welfare Northern Ireland, LMC and Farm Vet Systems, entitled ‘STrategic AntiMicrobial use in Dairy, Beef and Lamb Production’ (STAMP). 

The project aims to develop both user-friendly systems to benchmark antibiotic usage and decision support tools to help producers improve animal health. It is co-funded through DAERA’s Research Challenge Fund and early results are promising. In particular, within the dairy sector, there appears to be considerable potential to reduce antibiotic usage in relation to dry-cow therapy (DCT).

‘Blanket’ or conventional DCT, whereby all quarters of all cows are treated with an antibiotic at drying-off, has been normal practice on most dairy farms for the last 30 – 40 years.  The use of conventional DCT has been justified by the fact that dairy cows are particularly susceptible to mammary infection during the dry period, and that antibiotic dry-cow therapy provides an opportunity to prevent new infections from arising, as well as treating existing infections. Bulk tank somatic cell counts (SCC) on many farms have, however, fallen in recent years from 250,000 cells/ml in 2009 to 196,000 cells/ml in 2018 (DAERA statistics). Largely a result of improvements in milking hygiene and the more widespread use of sires with the potential to reduce somatic cell counts and mastitis incidence in their offspring.

Taking this into account, and given the concerns about antibiotic resistance, farmers are now being challenged to consider if it is really necessary to treat all cows in their herds with antibiotic dry-cow tubes at drying-off.  The use of ‘selective dry-cow therapy’ (SDCT), whereby only ‘high risk’ cows are treated with antibiotics at drying-off, may provide an ‘easy win’ strategy to reduce antibiotic use within the dairy sector.

Whilst SDCT may provide a clear opportunity to reduce antibiotic usage at the drying-off stage some concerns remain that, by possible increase the incidence of mastitis during the next lactation, any reductions will be off-set by the increased use of antibiotics during the subsequent lactation. To examine the full impact of this practice, a two year study is being undertaken within the AFBI Hillsborough dairy herd. 

Year One Results

The first year of this study (autumn 2018 - spring 2019) involved approximately 220 dairy cows.  These cows were identified as either ‘high risk’ or ‘low risk’ based on their SCC and mastitis history. ‘High risk’ cows were defined as those that had either one or more incidences of clinical mastitis during the three months pre dry-off, or a SCC of greater than 200,000 cells/ml during any of the three milk recordings prior to drying-off. Cows with no incidences of clinical mastitis during the three months before drying-off, and with a SCC of less than 200,000 cells/ml during the three milk recordings prior to drying-off, were considered ‘low risk’. These ‘high risk’ cows received conventional DCT, with all quarters treated with an antibiotic tube and a teat sealant at drying-off. ‘Low risk’ cows where allocated to one of two treatments. Cows on one treatment received conventional DCT while cows on the other treatment were treated with teat sealant only (SDCT). 

Figure 1: Average SCC (‘000/ml) of ‘high risk’ and ‘low risk’ cows during the three milk recordings prior to drying-off

  

 

The ‘high risk’ group had a mean lactation number of 3.0, compared to 2.1 for the ‘low risk’ group.  This highlights that SCC and mastitis incidence generally increases with increasing lactation number.  The ‘high risk’ cows had a mean SCC count of 273,000 cells/ml during the three milk recordings immediately prior to drying off, while ‘low risk’ cows had a mean SCC of 78,000 cells/ml during the three milk recordings prior to drying-off (Figure 1). 

  

 

 

 

Table 1: Effect of drying-off treatment on mean cow performance over the first three months post calving

 

 

Neither milk yield or milk composition during the three months post-calving were affected by drying-off treatment (Table 1).

 

 

  

Figure 2: Somatic cell counts (‘000/ml) of ‘high’ and ‘low risk’ cows during the first three milk recordings post-calving

 

 

During the three milk recordings post-calving, the mean SCC of the ‘high risk’ cows was 214,000 cells/ml.  These ‘high risk’ cows had an average of 0.6 cases of mastitis per cow during that period.  However, SCC of cows in the ‘Low risk’ groups remained low irrespective of whether they were subject to conventional DCT or SDCT (85,000 and 71,000 cells/ml, respectively; Figure 2). ‘Low risk’ cows that had been managed on SDCT had a similar incidence of mastitis during the three months post-calving as those managed on conventional DCT (0.2 cases of mastitis per cow).

 

 

 

 

Outcomes

The results of the first year of this study have demonstrated that SDCT was successfully adopted on ‘low risk’ cows within the AFBI herd without any negative effects on udder health or performance during the subsequent lactation.  However, the success rate of SDCT is likely to be determined by a number of factors:

  • Overall SCC of the herd (the AFBI herd has a low SCC, and this is conducive to the adoption of SDCT)
  • Protocol used to differentiate between the ‘high risk’ cows which will require antibiotic treatment at dry-off and the ‘low risk’ cows for which SDCT is appropriate (participation in milk recording, together with good records of cows treated for mastitis is essential)
  • Hygiene at drying-off (excellent hygiene at drying off is essential to ensure that bacteria are not introduced into the udder at this time). 

If you are considering adopting SDCT on your farm it is recommended that you speak with your vet who can provide you with more information, and help you develop an appropriate protocol for identifying ‘low risk’ cows.

 

Incorporating On-farm Experience 

To complement the results of the current study, AFBI are planning to undertake a survey to gain a better insight into farmer’s experience of the adoption of SDCT.  This questionnaire will seek to identify reasons for adoption, ‘best practice’ protocols, and the impact on cow health and performance.  Consequently, AFBI are seeking to recruit farmers from across Northern Ireland who have already adopted SDCT, and who are willing to participate in this short (30 minute) questionnaire.  If you wish to find out more, or are willing to share your experiences of SDCT with us, please email This email address is being protected from spambots. You need JavaScript enabled to view it. or call 028 92681587.