Why are some cows skinny?
An evaluation of stress and immune factors
By Amanda K. Lindholm-Perry, Larry A. Kuehn, Colby A. Redifer, Carol G. Chitko-McKown, Kacie L. McCarthy, Matthew S. Crouse and Brittney N. Keel
Have you ever walked out in the field to look at your cows and wondered why body condition might vary amongst cows that are utilizing the same feed resources? That is exactly what our colleague, Dr. Harvey Freetly, thought as he noticed a few thin cows in a large group of cows with adequate body condition all grazing the same pasture. This observation generated a discussion within our group about whether immune or stress issues could be contributing factors to animals that are unable to maintain appropriate body condition.
This was the start of a research study that became known as the “Skinny cow project” at the USDA, ARS, U.S. Meat Animal Research Center (U.S. MARC) located in Clay Center, Nebraska.
Mature cow body condition is a critical component of production because of its close relationships with feed consumed, forage availability, nutrient requirements and productivity. While there have been research studies to evaluate the effects of dietary restriction or low-quality forage on the body condition of cows at different productions stages, there is limited research studying the potential underlying causes of mature cows that naturally have low body condition scores (BCS).
We hypothesized that low body condition may be caused by higher levels of stress or subclinical illness and could have effects on animal well-being and overall health rather than on production issues like lower birth weights. The 2016 edition of the Nutrient Requirements of Cattle (NRC) showed that calf birth weights decreased as BCS decreased below a BCS of 3.5, otherwise birth weights are relatively constant between BCS 3.5 to 7. The NRC also showed that birth weights decreased as BCS increased above 7 (NRC, 2016).
Body condition scores of ≤ 4 account for only 1% of the U.S. MARC cow herds. The vast majority of these cows have a BCS of 4, with only a couple of animals identified with a BCS of 3 in any given year. The small number of cows fitting our low BCS criteria, and the possibility that there may be more than one contributing factor, prompted this multi-year “skinny cow project” to evaluate whether stress and/or immune factors were responsible for cows with low BCS (≤ 4).
As standard operating procedure, cattle operations staff at U.S. MARC body condition score over 6,000 cows each year at pregnancy diagnosis in the fall. In fall 2020-2022, blood and hair samples were collected from each cow that received a body condition score of 4 or less. To serve as a control animal, we also sampled an animal with a body condition score of 5-6 that was of the same age, management group and breed/breed-type. Each skinny cow had a “matched” control cow that was ‘moderate’ for BCS under the same management conditions and genetic influence. The average age of the cows on study was 5 years.
The blood samples were used to assess hematology parameters that included both white blood cell and red blood cell data and can serve as a measurement of both immune and health status. Blood was also used to evaluate concentrations of circulating immune system factors called cytokines. Cytokines are protein messengers that help to signal immune cells to fight bacterial or viral infections, and they also help to control inflammation.
The hair samples were used to evaluate long-term cortisol concentrations, which serves as an indicator of long-term or chronic stress. Potential sources of stress could include disease exposure, behavioral (social stressors), or environmental (weather conditions).
Several red blood cell hematological parameters were associated with BCS. These included hemoglobin, hematocrit and red blood cell count. These parameters were lower in cows with low BCS than moderate BCS cows. Hemoglobin is responsible for carrying oxygen in red blood cells to the organs and tissues of the body. Tissues like muscle use the oxygen to generate energy to function properly. Lower red blood cell counts and hemoglobin concentration may indicate that low BCS cows have lower oxygen carrying capacity, and may affect the animals’ ability to meet metabolic demands.
Three white blood cell parameters were also associated with BCS. These included neutrophils and monocytes which were greater in low BCS cows. Elevated neutrophil and monocyte counts can indicate infection, inflammation or stress. Increased lymphocytes, another type of white blood cell that helps to fight infection, would indicate an animal experiencing illness; however, decreased lymphocyte concentrations were detected in the low BCS cows.
The ratio of white blood cells is important and a higher ratio of neutrophils and monocytes with lower concentrations of lymphocytes is indicative of a stress leukogram. A stress leukogram is a change in neutrophil cell counts in response to the release of cortisol triggered by pain, stress or excitement.
In this study, higher cortisol concentrations were identified in the low BCS cows. Along with elevated neutrophils and monocytes, and lower lymphocytes, the high cortisol suggests that these animals may be more stressed than those with a BCS of 5 to 6. None of the 17 cytokines that we tested were associated with BCS. This lack of association also supports the lower lymphocyte counts that indicate that these animals are not experiencing acute or sub-clinical illness.
The combination of the hematology results and cortisol concentrations identified in this study suggests that low BCS cows may be experiencing some type of chronic physiologic stressor. The effects of this stress may not reduce fertility or cause clinical health issues, but they are likely sub-optimal for the animal’s performance and well-being.
The body condition score of these cows may be improved by management strategies that minimize stress and promote general immune health. While skinny cows may not directly cause management issues or financial loss, it stands to reason that cows with less chronic stress are likely to be more productive and efficient over their lifetime.
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CitationsNational Academies of Sciences, Engineering, and Medicine. 2016. Nutrient Requirements of Beef Cattle: Eighth Revised Edition. Washington, DC: The National Academies Press. https://doi.org/10.17226/19014.
Lindholm-Perry, Kuehn and Keel are research geneticists, Redifer and Crouse are research physiologists, and Chitko-McKown is a veterinary immunologist, all with the USDA Agricultural Research Service. McCarthy is an assistant professor and Extension beef cow-calf specialist with the University of Nebraska-Lincoln. Photos courtesy of U.S. MARC.
