Dairy industry making big gains in BVD control
The percentage of New Zealand dairy herds that are positive for BVD virus on bulk milk testing has dropped from 15% to 5% over the past decade as annual testing rates continue to rise. However, about 5% of negative herds are experiencing outbreaks every year, which means farmers still need to be careful about managing their BVD risk as long as the disease is still present on other New Zealand cattle farms.
Background
Knowing the current BVD status of your herd is an important first step towards making smarter BVD management decisions. For dairy herds, this has become even easier over the past decade thanks to the availability of tests that can be run on bulk milk samples (1) to look for cows in the milking herd that are actively shedding the virus (PCR) and (2) to measure antibody levels that tell us about the current level of herd immunity to BVD through either natural infection or vaccination (antibody ELISA).
The Livestock Improvement Corporation (LIC) first introduced the BVD Monitor pack in August 2009, which involves testing two bulk milk samples collected in spring approximately two to three weeks apart for both virus and antibodies. These should ideally be done after calving and before mating to catch any PI animals that may have snuck into the milking herd before they have the chance to cause significant production losses during the mating period. A repeat antibody test is then usually performed late lactation to look for signs that the herd may have been exposed to BVD over the period when they are at risk of producing PI calves.
Study Objectives
For this study, LIC generously allowed us to analyse anonymised data from bulk milk test requests from 1 June 2010 to 31 May 2018 to look at trends in testing rates and test results across New Zealand to get a better handle on the current BVD situation for the dairy industry.
Did you know……….
The bulk milk PCR test is sensitive enough to detect a single PI animal contributing milk to the tank even in herds with more than 1,000 milking cows.
Results
There were a total of 156,034 bulk milk BVD diagnostic testing accessions for 10,495 uniquely identified dairy herds over the 8 lactation season period, which covers about 60% of commercial dairy herds in New Zealand. The number of herds performing annual bulk milk testing increased from ~3,000 in the 2010/2011 season to ~6,500 in the 2017/2018 season. The percentage of these herds that had virus positive bulk milk samples dropped from 14.7% to 5.6% while the percentage of herds with negative or low antibody levels increased from 11.6% to 24.6% over this time period.
About 5% of herds that were virus-negative in one season became virus-positive the next season. This risk of having an outbreak was significantly greater for larger dairy herds and herds that had spikes in their bulk milk antibody levels two years prior. This was most likely a BVD introduction that resulted in the creation of a PI replacement heifer that made its way into milking herd when it calved as a 2-year old. For herds that then cleared BVD from their milking cows and remained negative for several seasons afterwards, the S/P ratios tended to drop at a rate of about 0.11 units per year.
Clinical Relevance
The New Zealand dairy industry has already made significant progress towards clearing BVD from its milking herds despite having no formal national BVD control programme in place. These efforts are likely having positive knock-on effects for beef finishers by reducing the number of PI calves that are entering the dairy-beef pipeline each year. As more dairy herds are clearing BVD, we are also seeing their bulk milk antibody levels drop as fewer animals are being infected with BVD. This could make dairy herds more vulnerable to severe outbreaks in the future if BVD gets re-introduced, which reinforces the need for ongoing vigilance until the disease is eradicated from New Zealand.