For most cow-calf producers, the routine of selecting replacement heifers from their own calves or purchased animals involves a visual assessment of conformation, structural soundness, anticipated frame size, colour, weight, age and disposition. Occasionally, it may also include a lineage check.
These are all important considerations but are there reasons to also look at lingering health impacts? Should previous bovine respiratory disease (BRD) events play a role in replacement heifer selection?
“The BRD event would need to be bacterial in nature,” says Jeff Caswell, a veterinarian in the Department of Pathobiology at the University of Guelph. “With viral issues, we believe they appear, an immune response is mounted, and if the animal survives it shouldn’t become a problem. A smouldering bacterial infection like a Mycoplasma bovis, however, could cause chronic lung lesions.”
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Three facts to consider
Caswell says there are three possible answers to the question about the impact of one-time BRD on replacement heifers.
First, note if they had pneumonia during their early months of life and they still don’t appear completely sound, or if there are abnormalities or lesions in the lungs at selection time.
A second approach is to consider the idea of “trained immunity” from time in the uterus or early in life. Trained immunity describes the process by which innate immune cells acquire immunological memory. After exposure to certain stimuli, innate immune cells can adjust their response to subsequent insults, resulting in an enhanced response to previously encountered infectious agents.
Lastly, Caswell suggests an examination of genetic background and BRD heritability.
“Regarding chronic or long-term issues, we need to examine the level of lung damage or lesions and to what extent it’s been resolved. Was it clinically apparent, then treated but dragged on for an extended time, resolved itself, flared up again, only to cycle multiple times?”
Feedlot studies have shown when pneumonia is treated and disappears, cattle grow relatively well compared to those who never contracted BRD. This trend should safely translate into replacement choices, but in chronic situations, it is possible animals are predisposed to increased respiratory disease and poor future performance.
“The idea of trained immunity and imprinting, looking at exposure to inflammation in the uterus before birth, or early bouts of pneumonia, is more nebulous,” says Caswell. “Some work has been done on this subject, but with limited data, results could be positive or negative. I’d like to see more research completed in this area.”
Regarding genetics, he says susceptibility to BRD has some heritability aspects. An animal contracting pneumonia might be slightly more predisposed than one never affected.
Testing for susceptibility
Professor Bonnie Mallard of the Ontario Veterinary College has completed work in this field, and is inventor of the High Immune Response (HIR) technology which identifies animals with naturally optimized immune responses.
“The idea of this testing is to predict susceptibility,” says Caswell. “I think it would be an inefficient genetic selection tool to choose a replacement based solely on whether she did or didn’t have BRD. But identifying individuals who mount a higher immune response may indicate a lower prevalence to the disease in the future.”
Caswell suggests it might be plausible to use lung ultrasound as a part of selection criteria. There are ongoing trials aimed at detecting respiratory disease in dairy heifers and tracking their long-term performance as mature cows. Research examining the connection between lung issues and future performance in beef cattle is needed.
Caswell recently took part in a study where lung ultrasound was performed on arriving feedlot cattle. Some findings showed that minor levels of residual lung damage in the front portion of the lung weren’t a concern, but if a major section was affected, it presented as a noticeable problem in performance.
As a starting point, BRD treatment rates could be compared with pregnancy success, along with pre-weaning calf survival to provide an indication of cause and effect.
Caswell says the most concrete conclusion is that heifers experiencing multiple episodes of pneumonia, especially including breathing difficulty, aren’t prime replacement candidates.
“At the other extreme, a female with one previous episode of pneumonia who was treated and recovered is probably fine. The tricky part is determining where in the middle we draw the line. Exactly what measurable effect is related to long-term performance? I don’t think anyone has specifically studied this relationship, but it’s something that should be tackled.”
