Our knowledge base on how to implement selective treatment of cows at drying off (SDCT) and on what impact SDCT has on long-term udder health continues to grow.
At the recent scientific conference of the National Mastitis Council, Dr. Sam Rowe discussed how well the tools that have been proposed to identify which cows to treat or not treat in herds using SDCT end up working. He looked at four different tools that would identify cows that were candidates for not using intramammary antibiotics at dry off:
• Based on research from the United States: cows that had fewer than two cases of clinical mastitis during that lactation and had under 200,000 SCC on all tests during that lactation.
• Based on research from the United Kingdom: cows that had no clinical mastitis over the period of the last three DHI tests and had under 200.000 on each of the last 3 DHI tests.
• Based on research from New Zealand: cows that had no cases of clinical mastitis during the lactation and under 120,000 SCC on all tests during the lactation for heifers or under 150,000 SCC on all tests during the lactation for cows.
• Based on research in the Netherlands, heifers with under 150,000 SCC on the last test or cows with under 50,000 SCC for cows on the last test. The last test had to be within 6 weeks of dry off.
Dr. Rowe took milk culture and SCC data for almost 1,600 cows in 56 herds to figure out how well each of these decision tools would have sorted cows for SDCT. Just under 50 % of the cows had a positive milk culture indicating that they had an intramammary infection at drying off and might have benefited from an intramammary antibiotic at drying off.
Not surprisingly, none of the decision tools was perfect at predicting which animals likely had an intramammary infection and, so, might benefit from an antibiotic treatment. Because each tool used much the same data but in different ways, even though none of the tools was perfect, you could decide to use one or more of the tools to suit your personal dry cow management objectives.
If you wanted to identify as many of your cows as possible that should be treated, you could select a tool that performed better at doing that. It would mean that you would likely end up treating some cows that really didn’t need to be treated but you would still treat fewer cows than if you used blanket dry cow treatment. To do that, you would be better to use the tools developed in the United States or in New Zealand. For both of those tools, you need to have the most data – notably, SCC results from the entire lactation but also you would need to have records of cases of clinical mastitis for the entire lactation on each cow.
If you wanted to avoid unnecessarily treating cows that would not benefit from an antibiotic at drying off — cows that don’t actually have an intramammary infection — you might be better to select one of the other tools. The tools from the United Kingdom or the Netherlands perform like that. Those two tools also use less information to make decisions. You still need SCC data and, if you used the UK tool, you would need information on cases of clinical mastitis too. The trade-off (the downside, actually) is that you’d end up not treating more cows that would actually benefit from being treated – cows that have an intramammary infection but are missed by the tool.
To be able to use any of the tools, you need to have individual cow SCC data. For some tools, you need to keep records of cases of clinical mastitis. Herds that would consider SDCT will likely already have systems on place to collect and organize those data.
Another consideration is that SDCT programs involve the use of an internal teat sealant too. When you use an internal sealant, it is important that it be administered properly so that the sealant has a chance to do its job, so that infection isn’t inadvertently carried into the quarter and so that the sealant can be removed easily from the quarter at freshening.
Next time, I’ll write about some long-term experiences with SDCT.
Dr. Robert Tremblay is a veterinarian for Boehringer-Ingelheim and lives near Guelph.