Q: Can you safely graze cover crop rye this spring after anhydrous has been applied?
A: Ultimately, the decision to graze is a judgement call and a question of how much risk a producer is willing to take.
We've seen producers graze cereal rye after applying anhydrous ammonia. However, with current cooler growing temperatures, the risk may be greater than in other years with more normal spring growing conditions.
We assume your concern is about increased potential for high nitrates in the plant. The risk depends on growing conditions. We assume the more rapid the plant growth, the less the risk. The N in anhydrous ammonia is immediately available for plant growth. Plants can take up ammonium, which is the immediate form after contacting the soil. Plants can also take up N as the nitrate form.
It is also important to consider how much anhydrous ammonia was applied.
With grazing, the actual risk at any given nitrate level in the plant is unclear. One standard recommendation used for fertilizing cool-season, perennial grass pastures is to wait three weeks after fertilization to reduce the nitrate risk. This recommendation was based on N fertilization of 60 to 120 lbs N per acre.
Current risk levels are based on hay feeding (with levels above 2100 ppm of N03-N considered high risk). Potential for nitrate toxicity is lower in grazed forage than in hay with the same level of nitrate due to several factors:
- Grazing animals eat more gradually than those receiving hay.
- When cattle are grazing they tend to be selective and don't graze the entire field close to the ground (where nitrate is highest). This means that if allowed (not strip grazed) they will likely eat lower nitrate plant parts first, slowly increasing the amount of nitrates consumed during the grazing period.
- Fresh (high moisture) forages release nitrates into the rumen at a slower rate than dry forages such as hay.
- Cattle consuming high-energy diets can handle more nitrates than those on low energy diets. Many of the cover crops grazed are very high energy (brassicas and immature grasses).
Grazing cover crops or cereal rye in this situation is not without risk. The amount of risk can be reduced using certain management methods:
- Make sure cattle are full before putting them on fields. Regardless of the nitrate level, a good management practice is to fill cattle up with hay before turn-out.
- Use lower risk cattle, if possible. The group with the greatest risk of negative consequences due to feeding high-nitrate forages is pregnant cows, as abortion can result. Open cows are the best option followed by growing calves (stockers or developing replacement heifers before breeding).
- Graze lightly to allow animals to selectively graze plant parts that are lower in nitrate concentration. Nitrate level varies with location in the plant. Nitrate tends to accumulate in the lower stem, so overgrazing areas so cattle have to eat the lower stem can cause increased intake of nitrate.
- Consider grain supplementation while adapting cattle. This will supply energy for rumen microbes to convert nitrate into bacterial protein and minimizes the intermediate nitrite production. Grain feeding may be of limited benefit for high quality cover crops.