Forage 2021: Give and Take: When Should Farmers and Ranchers Consider Fertilizing Pasture or Hay Ground?

Nature has provided the perfect patterns for renewal of the soil; as animals graze, they return “recycled” plant material rich in nitrogen to the ground in the form of manure. They trample some grasses as they graze as well, and this also adds to soil health. Rain and snow also nitrogen to the ground, and roots of legumes such as alfalfa fix nitrogen in the soil.  

What happens when we interrupt this process by removing a crop mechanically, year after year, without putting anything back? Should hay producers fertilize? If so, when and how much?  

It depends… 

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Jerry Volesky, University of Nebraska-Lincoln professor of Agronomy and Horticulture, says that soil testing is a good first step in determining just what each field’s soils may need.  

“Nitrogen is probably the most important factor in grass production, but keeping everything balanced, including phosphorus and potassium, is key,” he said. “We always encourage doing a soil test to find out the needs of each field. With a hay crop, it’s all taken off, and you are removing nutrients year after year. It’s a slow process, but you can see it over time. With phosphorus, for example, we did some testing on hay plots and we could see the phosphorus levels slowly decline each year to the point where yield was affected. Everything from the soil goes into the hay and each year you haul it away. Rains and snowmelt replace a small amount of nitrogen, but it’s a losing battle if you don’t ever put anything back.” 

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The next step is to push a pencil to determine whether investing in added fertilizer will pay off. Volesky said that varied soil fertility levels in Nebraska can indicate a need for anywhere from 40 to 70 pounds of nitrogen per acre.  

“Once you know how much the ground needs you can figure the cost of fertilizer and get a pretty good idea of what kind of a return you’d be looking at based on the value of the hay. In many cases, applying fertilizer will pay very well, but not in every instance.” 

Sara Bauder, South Dakota State University Extension agronomy field specialist, said that a general rule of thumb is roughly twenty-five pounds of nitrogen per ton of expected forage production per acre. Hay land soils should be tested annually and pastures every two to three years. Soil nutrient levels change more quickly when a hay crop is removed year after year in comparison with pastures, that benefit from the natural nutrient cycle. Legumes such as alfalfa and clover fix atmospheric nitrogen in the soil, but phosphorus and potassium are also key macronutrients vital to soil and plant health. 

“It is not recommended to add nitrogen on alfalfa as it is a legume and should not need any additional nitrogen,” Bauder said. “As long as the stand is at least one third alfalfa, you should not need any additional nitrogen. There will likely not be any need to add nitrogen during the lifespan of the alfalfa/grass stand unless it’s very old and the alfalfa has become very thin, leaving mainly grasses. However, soil tests should be periodically taken as phosphorus and potassium will likely be needed.” 

Proper fertilization helps with plant vigor, increasing longevity and production of alfalfa.  

“It’s imperative to make sure that you’re feeding your alfalfa stands so they stay healthy and productive as long as good soil management profitability allows,” Bauder said. “Potash can help improve winter survival as well.” 

Variables to consider while planning fertilizer applications include the amount of expected moisture, the anticipated value of the hay crop per ton, historical tonnage produced and increased tonnage expected after fertilizing. Wetter regions and irrigated fields are more likely to give a much greater return for investment, while drier, arid areas will see a smaller increase in tonnage. Available moisture is key to getting the benefit out of a fertilizer application. 

“You’ve got to have rain to get the value out of it,” Volesky said. “For dryland applications in western South Dakota and Nebraska with lower annual rainfall, it’s a good idea to make sure you have the fertilizer available, then watch the forecast for a significant rain coming up and make a quick application right before a rain. On good irrigated pastures or hayfields you can put down a lot more fertilizer because you have the moisture available.” 

A lack of moisture can also increase risk of nitrate toxicity in plants. It’s critical to test feedstuffs that are drought stressed especially when nitrogen fertilizer has been applied. 

“Nitrate toxicity is very dependent on the situation,” Bauder said. “Essentially, when your area has received less than normal precipitation and you applied nitrogen to your forage crops you need to monitor the situation. If you feel plants are drought stressed or if you applied more nitrogen fertilizer than recommended, it is worth the money to get the feed tested to be safe. Nitrates typically accumulate at the bottom of the stalk of most forages, so it’s best to make sure you are taking whole plant samples (from the soil surface and up).” 

Timing of fertilizer application is everything. Most hayfields in the upper Midwest are seeded with cool season grasses such as smooth brome, crested wheat grass or intermediate wheat grass. Getting the fertilizer in place prior to plant growth in the spring is ideal. For warm season grasses such as switchgrass, big bluestem or foxtail millet, a June application is best. 

“April to early May is great for getting fertilizer down before those cool season grasses are starting their growth,” Volesky said. “Here in the Nebraska Sandhills, hay meadows can be pretty wet in April so some guys get their fertilizer down even earlier while the ground is still frozen, but April is the most common timeframe.” 

Pastures and native rangelands in western South Dakota, the Nebraska Sandhills, and similar arid regions are not typically fertilized, but Volesky said that producers in areas with more rainfall, such as eastern South Dakota and Nebraska might find an application of fertilizer would boost growth, especially in pastures that are predominately smooth brome or other tame grasses. Data from the University of Wyoming, the University of Minnesota, as well as studies done in Oklahoma show that applying nitrogen to tame grass pastures can increase tonnage yield per acre and improve carrying capacity. But producers need to first make sure that adequate moisture is present, and secondly, take care not to overgraze.  

Pastures tend to benefit significantly more than hayfields from the natural fertilization cycle. 

“When pastures are grazed, nutrient cycling happens as manure and urine are deposited and trampling of some of the grasses occurs,” Volesky said. “These natural benefits can be applied to hayfields by grazing them in the fall or early winter. In some cases, producers feed on a hay field and then drag it in the spring to break up the manure. These practices can greatly improve soil health and nutrient levels, but they are not always possible due to location and availability of water and winter protection.”