I recently took part in a regular webinar series produced by the Beef Cattle Research Council.
I try to listen to these webinars regularly because they provide a great learning resource and I encourage producers and veterinarians to sign up for the series.
In this webinar, we discussed the long-term implications of drought, which was a relevant topic because significant areas of Western Canada have dealt with multiple years of below average rainfall. In that presentation, I briefly mentioned a herd investigation I was involved with years ago concerning an outbreak of blindness in a group of newborn calves.
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It was a commercial cow-calf herd with Simmental-cross cows. The herd owner had a good vaccination program in place and the cows were in very good body condition, even though there had been a serious drought the previous summer.
The drought had been severe enough that the producer had supplemented the cows with feed in the late summer and early fall while still on pasture. Unfortunately, there had been only some trace mineral salt blocks provided but no other mineral or vitamin supplementation that fall and winter.
That spring, more than 225 animals were due to calve. The herd bulls were usually turned out in late July and calving season typically started at the end of April.
Sometime around mid-May was when the problems were noticed. About 43 cows had calved and the owner had reported that he had seen six calves with neurological signs including blindness, aimless circling and walking backwards. These calves would be very dull and they would occasionally star gaze (arch their neck backwards and look skyward).
Some of these calves were sent for post-mortem examinations at the diagnostic laboratory and although we saw evidence of blindness and retinal detachment in some calves and some congestion in the brain, we lacked a conclusive diagnosis.
Calving continued and more calves had problems. By mid-June, there were about 40 cows left to calve and at least 13 percent of the calf crop was affected at that point with varying degrees of blindness or neurological signs. It seemed to be a little more common in calves from first-calf heifers, but calves from mature cows were affected as well.
One veterinary ophthalmology specialist from the college examined several calves and all had evidence of retinal developmental abnormalities and retinal detachment, which accounted for their blindness.
Our presumptive diagnosis was vitamin A deficiency, which probably occurred at some point the previous fall when the cows were in their first trimester and the fetal retinal development was affected.
Interestingly, the cows that we tested now had adequate levels of vitamin A, probably because they had access to green grass in the spring again and their deficiency had been corrected. Copper levels were still low in the cows as well, reflecting the fact that the mineral supplementation had not been ideal during the winter-feeding period.
Vitamin A is a fat-soluble vitamin. The main source of this vitamin is from the carotenoids in plants, which are sometimes referred to as pro-vitamin A. Beta carotene has the greatest vitamin A activity, and it is found in the orange and yellow pigments in green forage.
Vitamin A is stored in the liver and fatty tissues of the cow. Very little vitamin A passes through the placenta and so the calf relies on the cow’s colostrum to get their vitamin A at birth.
It’s important to remember when you assess vitamin A levels in a young calf that if they are below normal levels, it could be because the dam had low vitamin A levels in their colostrum, or it could be because the calf didn’t receive adequate colostrum at birth.
Some risk factors for vitamin A deficiency would include grazing on bleached pasture, drought-grown hay, weathered forages, harvested crop residues and prolonged feed storage. Many of these issues were experienced on the case-study ranch.
There are a variety of different conditions that can be seen with vitamin A deficiency. There are at least two forms of blindness that we can see. One scenario is where there is damage to the retina when it is being formed in utero. Vitamin A deficiency in growing cattle can also occur where abnormal bone formation pinches the optic nerve and causes blindness.
Neurological signs, stillborn and weak calves, immune deficiencies and poor conception rates are other common effects associated with vitamin A deficiency. One case study described in northern Queensland, Australia, in 2014, was similar to this case. In that herd, they lost more than 40 percent of the calves with similar symptoms, including neurological signs and blindness.
A few research studies in Western Canada have examined vitamin A issues in calves. Dr. Cheryl Waldner and Dr. Fabienne Uehlinger analyzed samples from 887 calves from 150 herds in Alberta and Saskatchewan. These samples were collected from calves less than one month old in the 2002 calving season. Almost 60 percent of those calves had inadequate vitamin A levels.
The studies also demonstrated that vitamin A deficiency was much more likely to occur in herds that experienced drought conditions with less than 200 millimetres of precipitation. They also showed that calves with low serum vitamin A were almost three times more likely to die than calves that had adequate levels of vitamin A.
In another study, Waldner showed that risk of stillbirth is also higher in cows in low precipitation years after controlling for all the other factors such as body condition. Perhaps this is another vitamin A deficiency impact?
Many areas across Western Canada experienced significant drought in 2022-23. In some cases, it has been multiple years of low rainfall. There is no doubt that vitamin A deficiency is potentially a significant issue when drought occurs.
It’s important that producers make sure they are feeding appropriate amounts of vitamin A to their cows throughout the winter-feeding season.
If you aren’t sure, you can get your veterinarian to sample some cows either with blood samples or liver biopsies to measure the status of the herd.
In some situations, you might use injectable vitamin A products if you can’t get the feed levels up to appropriate levels.
Your veterinarian or your nutritionist can provide the best advice for making sure your herd has adequate trace mineral and vitamin levels throughout this winter-feeding period.
John Campbell is a professor in the department of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.
