by Sally Colby

Dr. Niles Jennett, a veterinarian working as a dairy consultant, said we can’t eliminate stressors in the dairy environment, but should be evaluating approaches to mitigate the effects of stress. “Most important is developing an understanding and attitude with owners and staff on how to best handle animals, minimize restraint and keep cows’ environment clean and safe,” he said. “If we can accomplish this, we can do a lot to eliminate stress.”

Jennett said one important concept regarding the nutrient management system is developing rations not by how they’re put together but how nutrient requirements of the cows are managed. Another factor he considers is the ration support system: anywhere human beings have the ability to impact the environment of the cow is what allows the nutrient management system to work. If the ration fails, it isn’t usually that the ration failed, but more an issue of support of the ration and how staff and ownership handle and care for cows.

One approach is ensuring misters and fans were working. “Make sure everyone is on board with top quality maintenance and function,” said Jennett. “Reemphasize to the milkers to make sure they turn the misters off when cows exit the corral so it improves dryness of bedding and doesn’t allow moisture buildup on beds.”

Another important stress reduction factor involves bedding depth. “There was always the idea that we didn’t want to waste bedding, so the tendency was for bedding to be a little below the level I thought was adequate,” said Jennett. “Increase the frequency of grooming year-round as a way to improve cow comfort.”

It’s important to reduce the sources of secondary fermentation, which is a big stressor on dairy farms. “We adjusted the amount of silage brought to the mixing area to accommodate only one feeding rather than bringing it up for a full day,” Jennett said. “We remained vigilant about inventory turnover and tightened the mangers so there was fresh feed in the mangers. We reemphasized face management on silage piles and bags.”

Dr. Ricardo Chebel, University of Florida, described four general biological stress responses: behavioral, such as avoiding sunlight and seeking shade; autonomic, or fight/flight; cortisone release; and immunological responses, such as those for bacteria and viruses.

“Once the stress response happens, the animal goes from a normal biological function to an altered biological function,” said Chebel. “The altered biological function is the consequence of stress. In most cases, the stress is small and the cow is able to eliminate stress and returns to normal biological function. The problem is when the cow cannot eliminate the stimulus and goes from an altered biological function to a pre-pathological state to a pathological state.”

Reproduction and excess heat top the list of stressors for dairy cattle. Other stressors include environmental stress such as cow comfort, cleanliness, stocking density and heat/cold stress. Periparturient diseases are significant stressors in reproduction, especially metabolic stressors that happen at the end of gestation and beginning of lactation and involve the mammary gland, liver, adipose tissue and rumen. Also important are social stressors that occur when cows establish hierarchy and fight for resources, and personnel stress resulting from the people working with the cows.

Chebel said data show that both uterine and non-uterine diseases such as mastitis affect reproductive performance. “When you look at calving, there are the same problems, with fewer cows that have diseases calving compared to healthy cows,” he said. “If they have both uterine and non-uterine disease, it’s worse. There’s more pregnancy loss in cows with disease, especially in those with multiple diseases.”

Another factor is the consequence of disease to one cow. “If a herd has a lot of sick cows, we tend to believe that herd is more likely to have reproductive problems,” said Chebel, describing a study that showed otherwise. “They classified herds according to prevalence of diseases in the herd including ketosis, purulent vaginal discharge, metritis and endometritis. The proportion of herds with high success at first service (proportion of herds with first service pregnancy with AI above 40%) was much higher when the herd was not above the threshold for disease. That goes hand in hand with the idea that a sick cow is less likely to conceive and a herd with high incidence of disease is more likely to have reproductive inefficiency.”

Diseases are the most important stressors affecting reproductive performance and survival of lactating dairy cows. “Our goal in whatever we do with periparturient cows is to increase the percentage of healthy cows at the end of the first 30 or 60 days postpartum,” Chebel said. “If we don’t manage that, it doesn’t matter if we have high or low stocking density or what kind of grouping we do – if the incidence of disease is high, there’s a problem with fertility.”

A stocking density study involved 324 animals including heifers and cows. The idea was to compare separating heifers from cows to keep 80% stocking density versus 100% stocking density in the prepartum pen to determine how a lower stocking density prepartum would affect health and reproduction. Behavior and immune function were also considered.

Postpartum heifers and cows were housed separately. Chebel said researchers didn’t find any impaired immune function from stocking density even though there were behavioral changes. “When we look at the incidence of disease, there was no difference between treatments,” he said. “Multiparous cows in higher stocking density had lower incidence of metritis than those at 80%.”

The study showed that behavioral changes are important, but cows adapt very quickly to social disruptions and differences in environment to the point those factors don’t have a long-lasting effect on health and reproduction. Despite increased space availability, there were very few effects on hygiene, and the cows housed in lower stocking density were more likely to be diagnosed with metritis. Chebel said the bottom line is that a well-managed herd at 100% stocking density doesn’t have much effect on cow health.

Regrouping can be a stress factor because it disrupts social order. In a grouping study, one group of cows was moved once every five weeks. None of the cows would see a “new” cow until they calved and entered the fresh pen. The other group was managed conventionally, with cows entering the pen weekly, with about five to 15 new cows entering the pen each week. “One of the advantages of all in/all out, beyond not having to do regrouping, is reduced stocking density,” said Chebel. “Not only does a cow not seek a new penmate until she calves, she is also in lower stocking density. Even though we see better behavior in cows because of less competition when there is less changes in grouping, we cannot see any effects on physiological responses, immune responses, health or reproduction.”