Crop Nutrient Needs for Alfalfa and Corn Silage

To achieve high-yielding alfalfa and corn silage requires planning. To reach top yields, it’s important to provide crops with the correct nutrients. While alfalfa and corn silage recommendations differ, our agronomy experts both agree that soil testing to determine soil health is the first step.

Dan Putnam, statewide alfalfa and forage Extension specialist at the University of California, Davis, recommends growers take a long-term and short-term view of alfalfa production.

Long-term alfalfa nutrient planning

Before the crop is even established, Putnam recommends conducting a soil test to determine the soil fertility status.

“I’d recommend incorporating several locations within the field for soil variation and to create composite samples,” he says. “Then it’s important to go back to those same spots to sample so you can track changes in soil status over time. Marking spots with permanent markers or some way to remember it would be recommended.”

Alfalfa requires quite a bit of potassium and phosphorus over the year, so be sure to verify your soil’s status. Also, ensure the appropriate soil testing protocols recommended by your state are being followed. Eastern states and western states use a different soil phosphorus test due to differences in pH levels.

“We would not look at nitrogen status because alfalfa is a very effective nitrogen fixer, so you wouldn’t normally consider a nitrogen fertilizer application to alfalfa,” Putnam explains. “A nitrogen application doesn’t impact yields significantly in the studies that I’ve seen conducted across the United States.”

Growers should also make sure their crop is well-inoculated – either through pre-inoculated seed or by inoculating the seed using viable rhizobium bacteria that will then infect the roots and provide free fertilizer from the air.

Short-term alfalfa nutrient planning

If the alfalfa crop is already established, Putnam recommends a combination of soil sampling and plant tissue sampling. Plant sampling can be done following state Extension recommendations on tissue uptake levels.

“Send tissue samples into your laboratory to determine if your alfalfa crop would benefit from an application, particularly of phosphorus,” he notes. “In the western states, we find phosphorus can be exceptionally limiting, and then second would be potassium. In some states, sulfur can be a limiting macronutrient, which is required in large amounts.”

Corn silage nutrient planning

For corn silage, growers should use results from a recent soil test, meaning a sample that’s been collected within the last four years, according to Carrie Laboski, professor at the University of Wisconsin.

“Use a current soil test to look at the fertility levels, starting with pH,” says Laboski. “Make sure your pH is at least 6.0 for good production. In Wisconsin, we’re seeing more areas that have low pH – in the low 5’s and under 5.0 – that are causing poor corn production. Maintaining soil pH is the cornerstone of a good fertility program.”

Next, look at your nitrogen, phosphorus and potassium levels. Given farm economics, producers want to focus on what’s going to be most cost-efficient.

“Based on the research I’ve done in Wisconsin, we know that potassium is a much bigger problem than phosphorous, especially on dairy farms, because there’s often a lot of manure that gets back on the fields, but we may not always get enough potassium on that ground when rotations include alfalfa and corn silage,” she explains. “Both crops remove a lot of potassium. I’d suggest looking at your soil test, and if one nutrient is high and the other is low, then put the low one on. However, if both phosphorus and potassium are low, I’d focus on potassium first and then put money into phosphorous if you have it.”

Nitrogen can be a significant expense in corn production, but it doesn’t necessarily have to be.

“One of the best things growers can do from a nitrogen standpoint is to take credit for on-farm nutrients, so that would be legumes and manure,” Laboski says. “If you run a dairy farm, planting corn immediately following alfalfa is a really great idea because it reduces your nitrogen needs to about 50 pounds of nitrogen per acre or less.”

Your exact nitrogen credit will depend on how good your alfalfa stand was at harvest, but you may end up needing none or only small amounts of nitrogen for first-year corn after alfalfa.

“Manure contains nitrogen, potassium and phosphorus, so it can help reduce your fertilizer bill by adequately taking credit for those nutrients,” notes Laboski. “Also, conserve the nitrogen in the manure as you apply it. Injecting or incorporating it as quickly as possible after broadcast application can also help retain the nitrogen. Both offer options to help reduce your fertilizer bill.”

Laboski cautions that nitrogen is sometimes overapplied, and she encourages growers to follow their land-grant university guidelines for nitrogen application rates.
Carrie Laboski, professor at the University of WisconsinDan Putnam, statewide alfalfa and forage Extension specialist at the University of California, Davis

Headline image courtesy of University of Wisconsin

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Crop Nutrient Needs for Alfalfa and Corn Silage

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