It’s hard to keep nitrogen levels elevated in a pasture or hay field, but a good grazing management plan can help. That’s according to Dr. John Jennings, University of Arkansas Cooperative Extension Service animal science professor and forage specialist. Nitrogen, he explained to Ozarks Farm & Neighbor, is among the most transient nutrients in the field because, “it can leach; it can be used by microbes, weeds, plants and other things. So it is difficult to maintain a high level of nitrogen in pastures at any one time.”
The N level gets a boost by keeping cattle on the pasture; however, most of the nitrogen in their manure and urine is lost back to the atmosphere through ammonia volatilization, and only a small percentage gets recycled back into the soil. “That’s why it needs to be a continual process,” Jennings said, “and a good program helps keep that manure and urine back out on the field.”
Overseeding the field to legumes has a much bigger impact on nitrogen than does animal waste. “The nitrogen fixation from the rhizobium bacteria can be quite high,” Jennings said. “Now, that nitrogen from the bacteria doesn’t directly feed the grass that’s companioned with the legume. It has an indirect effect and again, it’s a nitrogen cycling process, not just a one-time event like fertilization would be. So once we get a good stand of clover established and it’s well inoculated, that nitrogen cycle helps keep more nitrogen in the system.”
Jennings said producers will get the best fertility results if they strive to keep stands at a moderate height. “If you overgraze the plants and keep them continually grazed to where we have short tops, you’ll also have short roots,” he said. “That reduces the nutrient turnover and uptake by the plants. If we keep the plants maintained where it’s nice, leafy and healthy we have a good root system, so as the microbes break down the dead organic material those nutrients will cycle, the plants will pick it up again, and we get good, healthy plant growth.” On the other hand, if plants become overgrown and mature, their vegetative activity slows and they also take up less nutrient.
Dr. Rob Kallenbach, University of Missouri Professor of Plant Sciences and state Extension specialist for forage crops, said about half the nitrogen in the cattle’s urine is lost through volatilization. “When we have animals well distributed on pasture – that is, we use some sort of rotational stocking scheme as the rule – that we have a better distribution over the system of those animals,” he told OFN. “And when we get better distribution of animals over the system, we get better distribution of those mineral nutrients.”
A rotation system that allows pastures to rest can maximize nitrogen use efficiency. Kallenbach said if the pasture has not been utilized after 21-28 days, “it probably won’t accumulate any more nitrogen – in fact, we dilute the nitrogen that’s there, and thus the amount of protein that’s in the feed, which is how we measure the nitrogen content of forage in many cases. When we use a rotational cycle we have the opportunity to have young, growing grass and we have a lot of recycling of those nutrients. A 28-30 day period is kind of an ideal amount of rest period for most of the cool-season grasses, and even many of the warm-season grasses that we use in this part of the world.”
Stands will also use more nitrogen when humid and moist conditions promote rapid plant growth. They’ll use less when it’s droughty, for a couple of reasons. “In drought conditions a great deal of nitrogen doesn’t go anywhere and doesn’t move,” Kallenbach said. “It doesn’t leach out of the system because it’s soluble in water and requires water for the movement out, and in the drought you don’t have that.” In addition, during a drought, “the plants aren’t growing, or are growing very slowly, so nitrogen consumption is pretty low.”
Incidentally, Jennings would like to dispel a couple of myths. “There’s a lot of discussion about grazing increasing fertility in a field,” he said. “That’s really not the case. Grazing is a good method to recycle fertility, and maybe improve uniformity of fertility if we have a good rotational program, but it doesn’t create N, P and K out of nothing.” He also dismissed the belief that deep-rooted weeds in the pasture pick up nutrients from deep in the soil profile, and deposit them on the surface. “That’s really not true,” Jennings said. “If it were, that would mean that the weediest fields have the highest fertility, and we know that’s not the case.”
