Extension Update by Jenny Rees

Nitrates and Grazing Forages: the fact that we haven’t had a hard frost is throwing a wrench into grazing covers. For those with cattle on cover crops, please be aware of the potential for nitrate and/or 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 26F or lower). 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, grad student for Dr. Mary Drewnoski, found brassicas accumulate more nitrate than small grains, millet, sorghum/sudan grasses, or cover crop mixes, and that 48% of the brassica samples submitted to Ward Labs were considered “highly toxic” for nitrate levels compared to 20-28% 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. Dr. Mary Drewnoski has shared that brassicas and immature grasses are also high in energy and that cattle consuming diets high in energy can handle more nitrates. So, this may be why we thankfully don’t see more issues grazing turnips and radishes high in nitrate. She shared 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.

Corn Nitrogen Calculator: For those desirous to calculate N rates utilizing different sources and prices, the UNL nitrogen calculator has been updated. Before, it didn’t allow for the prices we currently are experiencing and there’s been some updates to the manure credits. The UNL nitrogen equation itself has remained the same, and honestly, I still say it’s conservative. For example, the nitrogen credit from soybeans is 45 lbs/ac in the equation. However, on-farm research conducted in the mid-2000’s in this part of the State found a credit of 1 lb for every bushel up to 60 bu/ac was achievable in irrigated soybean.

To utilize the N calculator, go to: https://cropwatch.unl.edu/soils, scroll down to “Corn Nitrogen Recommendations Calculator” and download the excel spreadsheet. You will want to open the “October 2021” version. They’re currently working on an online calculator for the future.

A few questions I received this week: “How much yield loss should I expect if I reduce N rates?” and “With higher corn prices, how much profit is one giving up with lower yields due to lower N application?”. These are valid questions and ones every operation will have to determine for oneself. That’s where on-farm research is really helpful too.

We have lots of data through the years of comparing a grower base rate +/- 30 lbs. of nitrogen mostly showing no yield differences. Most of this data is from the 1990’s for this part of the State, with data from the past 10 years from other parts of the State. In 2020 and 2021, a grower conducted nitrogen rate studies in York and Hamilton counties using his rate +/- 50 lb N. As I’ve shared the results with growers with the above-mentioned questions, they’ve been surprised at the yields and profits. I’ve summarized the grower’s results at jenreesources.com and also showed the profit based on today’s fertilizer prices. There’s a few ways to approach this data. It shows minimal benefit to yield and profit for increasing nitrogen application above the grower’s rate. Thus, there may be opportunity this year for those who have been over-fertilizing for high yield goals to try cutting back. It also shows that yield is reduced to some extent by reducing N rate 50 lb/ac below what the grower’s rate typically would be; however, with today’s prices, profit may be comparable. We also have several farmers who are already doing a great job with fertilizer rate for their realistic yield goals, so there may not be room to cut back. Appreciate all the farmers conducting on-farm research so we have data to share to help in answering questions!