Lack of hard freeze snarls farmers' grazing plans
November 11, 2021
We’ve had some nice harvesting weather this fall but the lack of a hard frost has thrown a wrench into grazing covers. For those with cattle on cover crops, Jenny Rees, extension educatior, warns, “Please be aware of the potential for nitrate and prussic acid poisoning with the light frosts. Nitrogen moves from the roots up the plant. When a frost occurs, nitrates accumulate in the plant, and, we had lighter frosts for several days in a row. For sorghum species where prussic acid poisoning is also of concern, we say to wait at least 5 days before returning animals back to the field after frost. And, for every light frost, the 5 day window resets until a hard freeze occurs (at 26 F or lower).”
Rees advised “It’s been hard to find any recommendations regarding nitrate accumulations in brassicas after frost…and what happens to the nitrates after a frost. We know weathering in general reduces nitrate levels in plants by spring. Just advising to watch cattle with these light frosts-especially those in seed corn fields that had milo in corners.”
A study conducted by Mary Lenz, a graduate student, found brassicas accumulate more nitrate than small grains, millet, sorghum and sudan grasses, or cover crop mixes. Forty-eight percent of the brassica samples submitted to Ward Labs were considered “highly toxic” for nitrate levels compared to 20 to 28 percent of other cover crop species. Yet what’s interesting is how often fields in the “highly toxic” level (or with no testing) are grazed with no impacts.
Mary Drewnoski said brassicas and immature grasses are also high in energy and that cattle consuming diets high in energy can handle more nitrates. So, Rees thought this may be why we thankfully don’t see more issues grazing turnips and radishes high in nitrate. Other factors for consideration are that cattle are selective and will graze the upper-most parts of plants first which are lower in nitrates, grazing animals eat more gradually than those receiving hay, and the high moisture forages that are grazed release nitrates at a slower rate than with dry forages like hay.
Ways to reduce nitrate concerns when grazing include: turning out cattle full before grazing the covers, using lower risk cattle such as open cows and stockers (as pregnant cows have risk of abortion when fed forages high in nitrate), graze lowest nitrate fields first for adaptation, graze highest nitrate concentration fields lighter so not as much forage is removed, or there’s also the option of not grazing fields that are very high in nitrates.