By MARY DREWNOSKI
Beef Systems Specialist
Given the drought conditions in some locations this year, many producers may be asking themselves how to handle the annual forages they have standing in the field that may not have grown as much as would be expected under normal conditions. These drought stressed forages can be high in nitrates and may be potentially toxic to cattle.
So what are the options?
Grazing the forage. Most recommendations for the level of nitrates in forages that cause issues are based on hay and it is suggested that levels above 2,100 ppm N03-N are toxic. Often grazing cattle can tolerate greater concentrations of nitrates than those consuming hay. The amount of nitrates cattle can consume without an issue depends a lot on the situation and management. If the forage to be grazed is high moisture it is often lower risk. Fresh forages release nitrates into the rumen at a slower rate than dry forages. The slower release rate allows rumen microbes that convert toxic nitrite to ammonia, to better keep up with nitrate inflow. If the forage is mature and dried out the rate of release of nitrate and thus the risks would be greater, more like hay.
A few things to understand about risks when grazing higher nitrate forages
1. The capacity of the microbial population in the rumen to detoxify nitrite will increase with exposure to nitrate. Nitrates tend to be greatest in the bottom of the stem and with the least nitrates in the leaf. If given the opportunity, cattle tend to select leaf material first and work their way down the plant, slowly increasing their nitrate exposure over the grazing period. Therefore, the microbial population in the rumen of grazing cattle may have time to adapt to higher nitrate concentrations by the time the animal consumes the stem and lower portions of the plant. Thus, gradual adaptation is a key management strategy. Producers should not strip graze high nitrate forages, but allow cattle to be selective. Losses from nitrate toxicity are more likely in cattle not adapted to nitrate. The bacteria in the rumen capable of converting the toxic nitrite to ammonia will increase in numbers when nitrate is available to them. Adapted animals can be safely fed higher levels than un-adapted animals. To adapt the cattle, start by grazing the lowest-nitrate fields and then work up to the highest. If a lower nitrate field is not available, producers should graze higher nitrate fields lightly to allow animals to selectively graze the plant parts that are lower in nitrate concentration.
2. Grazing cattle often have a slower rate of dry matter intake than when eating harvested forages, again this means that the microbes can better keep pace with the nitrate inflow. Feeding cattle prior to turnout can help slow initial intake of high nitrate feed. Also, if intake becomes restricted at any point (forage runs out or weather impedes grazing) producers should fill them up on lower nitrate hay again, before they go back to grazing the high nitrate forage.
3. Higher dietary energy increases the rate of detoxification. Thus, cattle grazing immature forages can have lower risk than mature forages with the same amount of nitrate. Previous research found that feeding a couple pounds of corn to cattle when feeding mature high nitrate forages can lower risk. So if the forage is not young/immature, producers should consider grain supplementation. This will supply energy for rumen microbes to convert nitrate into bacterial protein and minimizes the accumulation of the intermediate nitrite. Grain feeding may be of limited benefit for high quality annual forages, but is a good idea when grazing more mature forages.
Even though the potential for nitrate toxicity exists, careful grazing and cattle management can be used to eliminate cattle losses. Ultimately, the decision to graze high nitrate fields is a judgement call and a question of how much risk one is willing to take.
Another great option would be to harvest the forage as silage. If done right, ensiling can decrease nitrate content of the forage by 40 to 60%. Ensiling is one of the best ways to decrease the potential nitrate toxicity of a forage so this is an option worth considering. Warm season annuals such as corn, sorghum, sorghum-Sudan and pearl millet can all be harvested as silage as can small cereals such as oats, barley, cereal rye and wheat. The nitrate content of poorly ensiled forages will not be decreased to the same extent as well made silage. Thus harvesting at the proper moisture (65 to 70%) content and getting the forage packed well are key to ensure the best result. Raising the chopper height can also decrease the amount of nitrates in the silage, as the highest concentrations are usually in the lower stem of the plant. Producers need to let the silage ferment for least 21 days for maximal effect. Silage should be sampled and analyzed before feeding to ensure the amount of the silage in the ration provides acceptable nitrate levels in the diet. Producers should target an initial diet content of less than l000 ppm N03-N. Animals can be then be adapted to higher levels by slowing increasing the inclusion of high nitrate silage in the diet. Over time cattle can be adapted to consume diets up to 4,000 ppm N03-N.
Harvesting high nitrate forages for hay, an option of last resort? For some situations, hay may be the only option. The drying process does not decrease nitrates and thus this hay may need to be diluted in the diet with other forages low in nitrates. Grinding and blending of low and high nitrates hay is best to reduce risk. Additionally, feeding a couple pounds of grain per cow each day can also help reduce risk. If blending of hays is not possible, producers should unroll the low nitrate hay first, then after they have consumed most of the low nitrate hay, roll out high nitrate hay. Be sure animals are not hungry when eating the high nitrate hay (slow intake is key to reducing risk). To adapt the cattle, start by feeding the lowest-nitrate hay and then work up to higher levels. Even with adaptation, feeding hay free-choice is risky when the hay has higher than 2,100 ppm NO3-N. This is because there are likely to be “hot spots” in the hay with much high nitrate content that could result in a cow consuming a much higher load of nitrate. Do not feed hay, straw, or fodder suspected of being high in nitrate when it is damp. Damp hay tends to be more toxic because some of the nitrate already has been converted to the more toxic nitrite before being consumed.
Producers with high nitrate forage who need assistance with developing a management plan can contact their local extension educator.