Nitrogen is typically the most limiting nutrient for grass growth. In fact, many Extension recommendations for pasture N management include the phrase “use nitrogen strategically to increase grass growth during certain seasons” since nitrogen is such a driver of production. But what does strategic N use really entail?
I think it’s easier to start with understanding what is not a strategic use of N, and this can be summarized as two main types of applications: (1) applying N at a time or place where the plant can’t utilize the nitrogen, and (2) applying N when we already have more growth/ pasture productivity than we can utilize on our farm.
When not to apply
Avoiding N applications at times that plants can’t utilize N is fairly straightforward, since this usually has to do with weather. In winter, applying plant-available forms of N isn’t a strategic practice, since plants can’t take up nitrogen when they’re dormant.
During the dry part of summer, applying N isn’t strategic since water availability is more limiting to growth than nitrogen availability — plants can’t turn more nutrients into more growth if they don’t have enough water to keep growing. Plus, most pastures get urea as their main N source, and urea applications in dry weather are less effective due to increased volatilization.
Many recommendations for strategically increasing pasture growth with nitrogen include advice about green-up applications to maximize early-season growth. Applying N in spring can be useful for many pastures since spring rains help the fertilizer enter the soil and become plant-available. But, spring rains can also limit whether pastures are fit for grazing or haymaking.
If the weather reduces your ability to utilize increased pasture growth, then applying N at this time point would not be strategic. This also requires planning ahead a bit, since N application takes around 5 days to start impacting pasture growth, and the full effects will accumulate over another 15 to 45 days (depending on the weather).
When to apply
So, when are some of the times where plants can utilize N applications and we might want to increase plant growth using N? Applying some N at green up can still be a helpful practice on some acres. But choose to place your green-up N on pastures that you plan to get into early in the season.
Later in the season, applying N after a hay cutting or heavy grazing can be useful since increased N availability improves plant regrowth. Make applications when there’s rain in the 24 to 48-hour forecast, and split fertilizer into two to three applications across the season.
Right rate
Another important aspect of the N strategy is picking the right rate. Most Ohio forage grasses on loamy soils see economic returns of up to 125 to 150 pounds. N per acre per year. Even when forages are growing quickly, they can take up around 50 to 80 pounds of N per acre at a time; applying higher rates in a single application increases losses.
Optimal N rates for pastures with 20 to 35% legumes are about half of what we see for all-grass stands (around 75 pounds N per acre per year, with each application including up to 40 pounds N per acre). Pastures with more than 30 to 35% legumes rarely see increased productivity when N is applied.
In summary, applying nitrogen on forages is a great way to boost productivity as long as the N is applied at a time the plants can utilize the nutrients and you’ll be able to utilize the increased pasture biomass. Returns on nitrogen improve when (a) N is split into two to three applications, (b) each application is below 80 pounds/ acre, and (c) applications are made shortly before rainfall.
