Animal Health Matters: The dose makes the poison

Russ Daly
Special to the Farm Forum
Russ Daly

I was first introduced to this saying during my vet school toxicology course. The instructor held up this ancient phrase to emphasize a couple things. First, anything can be toxic to an animal in a high enough dose. Even things like water and oxygen, consumed in excess, could make an animal sick.

Secondly, and more pertinent to the toxicology course, the phrase reminds us that poisons have to be consumed in a high enough dose to cause adverse effects: minute amounts of even the nastiest poisons don’t cause problems, but higher doses do.

The toxin I wrote about in my previous column is a great example of that principle. Last time, I elaborated upon how excessive levels of nitrates can form in some important animal forages during dry conditions. When toxic doses of nitrates are consumed, ruminants such as cattle and sheep are at risk of death and reproductive losses due to diminished oxygen-carrying capacity in the animal’s bloodstream.

As a South Dakota State University Extension specialist, I feel awkward talking about problems like nitrate poisoning without at least offering some solutions. Nitrate accumulation in forages isn’t necessarily a death sentence for cattle or a reason to trash a whole crop: the dose makes the poison, as they say. It turns out – when faced with potential high nitrate levels in animal feed – there are some things producers can do to reduce the dose.

We can’t do much with high nitrate crops as they stand in the field. But we can work to harvest only the parts of the plant that are lower in nitrates. The lowest parts of the stalk and stems in crops such as corn and sorghum hold the highest concentration of nitrates. Raising the cutter bar during harvest so only the upper parts of the plant are harvested is a good strategy.

Making silage from the risky forage instead of feeding it as hay presents another opportunity to further reduce nitrate levels in the harvested crop. Fermentation has a dampening effect on nitrate levels: silage has roughly a third lower nitrate concentration compared to its non-ensiled version.

Oftentimes, however, we already have put up the potentially toxic forage and we need to manage how much of it we present to the cattle. For this purpose, it really helps to know what nitrate levels we’re dealing with, and testing is of great value.

When feeding forage high in nitrates, it’s important to consider the animals we are feeding. Some are more susceptible to problems than others. Pregnant cows or heifers are prone to abortions and other pregnancy losses when exposed to relatively moderate nitrate levels. Keeping risky feed away from pregnant animals is a very good practice.

There are also some other animal-related considerations to consider. To a certain extent, cattle can adapt to moderate levels of nitrates as their rumen microbes gain the ability to better deal with the compound. But this has to be a gradual process, starting small and making careful, incremental changes over a long period of time.

Beyond that, diluting out high-nitrate feed with feed you know is low in nitrates is another strategy to reduce the overall dose fed to cattle. Of course, this depends on the availability of “good feed” with which to dilute. Frequent, small amounts of high-nitrate feed can be a way to utilize risky forages while minimizing risk. All this, of course, should be approached with caution and careful observation of the cattle.

Grazing potentially high-nitrate forages should be approached with the same attention to “dosage.” Filling up cows with “good” hay and high-energy feeds prior to turning out to graze can help ensure they don’t tank up on the bad stuff once they’re out grazing. It’s important not to overgraze these fields. A high animal stocking density makes it more likely the cattle will want to eat the lower, higher-nitrate parts of the plant. We also should make sure water sources for these grazers aren’t high in nitrates themselves.

Completely avoiding high nitrate feedstuffs can be a challenge in cattle country during dry years. Utilizing these feedstuffs is often possible, but requires attention to detail and careful observation of the cattle being fed to properly manage that risk.

Russ Daly, DVM, is the Extension Veterinarian at South Dakota State University.  He can be reached via e-mail at russell.daly@sdstate.edu or at 605-688-5171.