It’s been a rough summer for growing many crops in Western Canada, despite some official reports of a bountiful harvest. Some parts of the western Prairies had a cool-wet growing season, while eastern areas were baked by drought. Such polarized weather is ideal for toxic accumulation of nitrates in forages. Fortunately, nitrate testing of forage samples is inexpensive. If a winter feed inventory is found to contain toxic levels of nitrates, measures can be taken to reduce potential nitrate threats in rations.
Mature cows and replacement heifers are the most vulnerable group to the threat of nitrate poisoning. Their typical diets are made up mostly of forages, which could contain damaging nitrates (grain is very low in nitrates). A small cross-section of plants harvested for cattle forage and known to accumulate nitrates under the “right” adverse growing conditions are: barley, corn, oats, canola, soybeans and millet. Field weeds such as pigweed, kochia and Canadian thistle have also been sighted as high nitrate accumulators.
It’s a bit of a myth that most nitrate-poisoned cattle become lethargic and then die under our nose. That type of acute cattle loss infrequently occurs in beef cattle that have not had time to adjust to toxic nitrate forage. A high-risk situation for example, might involve hungry cattle that go without feed during a storm only to engorge themselves on high-nitrate feed put in front of them.
Such beef cattle might adapt to high- (not toxic) nitrate forage over time, yet even most non-lethal nitrate poisonings are subclinical and very subtle in nature. Producers might subconsciously notice there are a number of unexplainable abortions in the mid- and late-term cows, while calves born of other nitrate-afflicted cows are premature and have a higher newborn mortality. Poor growth and body condition, a higher incidence of disease with slower recovery and poor milk production amongst all cow members are other common symptoms of nitrate poisoning.
Oddly enough, nitrates are not particularly poisonous to cattle. Rumen microbes are naturally able to break down low levels of nitrates from forage into ammonia, which is safely incorporated back into bacterial protein. In contrast, excessive forage nitrates overwhelm this microorganism capacity to process nitrates, which then causes an abnormal nitrate/nitrate pool to form. The latter nitrites are then absorbed across the rumen wall and into the bloodstream where they diminish the oxygen-carrying capacity of hemoglobin in red blood cells, which causes the animals’ body tissues literary to suffocate.
As much as excessive nitrates in forages can be a blood-poisoning hazard in cattle feeding, it should be noted nitrates present in the soil and taken up by plants are all part of plants’ own natural life cycle. These same nitrate compounds are ultimately responsible for helping cattle meet their own daily protein requirements when they consume forages in their rations.
It is under these normal and less-stressful circumstances when nitrates are taken up by the plants’ root system, transported through the stems and finally to the leaves, where it is converted into plant protein by photosynthesis. If this natural process is interrupted or slowed, plants begin to accumulate toxic levels of nitrates in their roots and lower stems to the extent that high nitrate concentrations are stored in the lower portion of the plant, rather than made into plant protein.
Any conditions that reduce their photosynthesis capacity of most crops, such as a cool and cloudy spring, promotes nitrate accumulation during the growing season. It’s also easy to see why so many hailed-out areas becomes high-nitrate cattle feed; not only do hail-shredded leaves shut down photosynthesis, but any surviving leafless plants still have an active root system which keeps on pumping nitrates into the plant until the crop is cut down. Likewise, in drought-stricken areas, not only are plant leaves wilted, but a lack of moisture also impedes any uptake of nitrates by the remaining and functioning leaves.
Test suspect feed
All such nitrate-suspected forage (re: a hailed-out barley crop or drought-stricken corn field) should be tested before feeding as the best assurance for safety. Producers should collect samples in the field and then collect another set of samples once the crop is harvested. Send in all samples into a reputable laboratory and request a common nitrate test, which should cost no more than $20 per forage sample. It is also recommended that water samples be collected and tested for nitrates, too.
A routine laboratory printout will show forages and other feeds analyzed for nitrate content are commonly reported as nitrate (NO3) or nitrate nitrogen (NO3N). Research has proven mature cattle and replacement heifers can safely consume a total diet containing nitrates that are below 0.5 per cent Nitrate (NO3) or expressed another way; below 0.12 per cent nitrate nitrogen (NO3N) on a dry matter basis.
There are three recommendations for reducing the threat of nitrate poisoning in beef cattle if the forage analysis proves high levels of nitrates:
- Don’t feed high-nitrate forage. For example, allowing pregnant animals to graze drought-stricken corn stover is unwise, because most nitrate accumulation occurs in the lower 20 cm of cornstalks.
- Ensile high-nitrate forage. Making barley and corn silage reduces nitrate levels as much as 60 per cent, yet high-nitrate silage may result in lower but still toxic nitrate silage. In addition, making dry hay from high-nitrate forage has no effect on nitrate levels.
- Dilute high-nitrate forages. Grind contaminated forage such as hay and mix it with low-nitrate feeds such as clean hay, silage or grains to acceptable safe limits in a TMR mixer. It is not suggested to feed high nitrate forage without processing. Feeding whole high-nitrate bales alternated with whole low-nitrate bales in bale feeders doesn’t solve the problem. †