It’s believed transition cows have suppressed immune systems, but Dr. Stephen LeBlanc of the University of Guelph says it’s not as simple as just blaming compromised immune systems for all transition dairy cattle problems.
“Metabolically, yes cows have reduced aspects of immune function through the transition period and part of some of the reasons for that is there is a lot of other changes happening in terms of management and metabolism,” said LeBlanc during the virtual webinar for Animal Nutrition Conference of Canada’s annual conference.
Why it matters: Cows are most vulnerable to disease in the period after calving. High immune function to kill off bacteria and disease is important so the cow remains healthy.
The immune system focuses on bacterial physical barriers and neutrophils play a big role in this.
“They are the rapid first responders. They are not the only ones, but the most important.”
Neutrophils, which are types of white blood cells, play a role in the immune system to kill off bacteria by engulfing them, trapping them or by excreting oxygen reactive species and killing the bacteria extracellularly.
“A number of these functions of neutrophils are compromised, through the transition period,” says LeBlanc.
Immune system compromise is known to start before calving, “bottoms out” within two weeks after and normal function will not return for three to four weeks.
“The killing ability (of neutrophils) is one that seems to be particularly compromised in transition cows. It has some practical implications because things we do in terms of management, and in terms of diet can have differential effects.”
How well the neutrophil system works is influenced by glucose, calcium, and other compounds that will directly or indirectly help or harm their function.
“Then on top of that we can layer on social stressors. If cows are crowded, if feed bunks are empty, if they’re experiencing heat stress or competition for feeding space, these can directly or indirectly feed into how well the neutrophil system works,” says LeBlanc.
Uterine disease levels are about how much bacterial challenge the cow faces, type of bacteria, bacterial load and how well the cow responds to the challenge. The immune response of the cow to the bacteria in the uterus, or in the udder, has more to do with how the cow gets sick.
When cows have a retained placenta, their immune function, specifically neutrophils, fail to break down the placenta attachment to the uterine wall.
It’s “not just because they have a tough calving,” LeBlanc said about studies on specific animals. “There were measurable differences in cows’ immune function going back as far as two weeks where you could see differences in neutrophil function in terms of their killing capacity.”
Cows that are going to have retained placenta are already eating less than healthy cows at least one week before calving.
Metritis tells a similar story.
“They were eating a lot less when they when sick, but in fact, they were already eating substantially less as close up dry cows, weeks before calving and weeks before they ever got sick,” says LeBlanc.
Work from the University of British Columbia shows that some of the lowered consumption has to do with competition at the feed bunk. At the same time work from Doug Herrman at the University of Utah shows cows that had metritis or endometritis had lower neutrophil killing capacity.
“Two weeks before metritis, five weeks before endometritis, those cows already had compromised immune function, but specifically the killing ability of neutrophils,” says LeBlanc.
The same study from the University of Utah showed that the cows who had the greatest dry matter intake as close up dry cows for three weeks before calving, had stable neutrophil killing function throughout the transition period.
The cows that were in the lowest quartile of intake as close up dry cows showed substantially less neutrophil killing capacity.
Although feed bunk space is important to avoid these issues, numerous studies showed that when cows were crowded in a controlled way at the feed bunk immune functions did not diminish.
“The take home here is not that space and competition don’t matter, but they’re not in amongst themselves the one key feature that is going to guarantee immune function in transition cows,” says LeBlanc.
Not crowding cows at the feed bunk, especially transition cows, gives producers a buffer for other variables such as changes in feed quality, heat stress — factors that are not controllable.
“If you don’t have (the buffer) other bad things are in fact likely going to happen when other variables aren’t controlled the way they were in controlled studies,” says LeBlanc.
As well, a study in Quebec, by Ster et al. in 2012, shows that as the concentration of non-esterified fatty acids (NEFA) within the blood goes up, the killing ability of neutrophils goes down.
“This is at the concentrations of NEFAs we would very typically see in transition cows, they aren’t crazy high.”