Forage Fertilization Considerations

As pastures and hayfields begin to green-up, we need to start thinking about our fertilization program. Forages, just like row crops, need fertilizer to be productive.

We will quickly discuss fertilizer applications for your alfalfa and grass hay, and for your pastures.

In each case, determining application rates by soil testing would be the gold standard. Soil testing will give you the most complete assessment of the needed application rate. You can use PM 1688 article “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa” to determine fertilization rates.  Although there is not always time, or the resources to complete soil testing as spring is filled with many tasks.

Fertilizing your alfalfa and grass hay field                        

A good fertilization program for your alfalfa hayfield is a key component for persistence and high-producing stands. As spring is here, fertilization of your alfalfa stands should be on your mind.  The key aspects of your program should include maintaining a neutral pH and applications of potassium and phosphorus. Additionally, throughout parts of Iowa there have been some sulfur deficiencies noted. Sulfur deficiency symptoms include light green to yellow leaves and stunting (Figure 1). Tissue samples should be submitted to test for sulfur deficiency. 

Outside of soil testing you can determine how much to apply by estimates of how much you removed. It is estimated that with every ton of alfalfa harvested 15 to 20 pounds of phosphorus is removed and 50 to 60 pounds of potassium. So, if you removed 5 tons/acre of alfalfa hay off your field last year, you should replace at least 75 lbs/acre of phosphorus and 250 lbs/acre of potassium. Potassium is particularly important to help prevent winterkill.

Similar to alfalfa, grass can remove a large amount of K and P from your system. The potassium removal rate is estimated to be at 50 lbs per ton and 15 lbs per ton for P. Outside of P and K, grasses need nitrogen for growth, while alfalfa and other legumes can fixate their own. Typically, a split N fertilization application rate is recommended at 40 lbs at spring green-up and directly following each hay cutting. This will help boost the regrowth and yields. Although a single application can be applied early in the spring at green-up of approximately 100-120 lbs of N/acre, but you will see a better response across the season with split applications.

Fertilizer for pastures

Fertilization rates recommended for pasture are very similar to the recommended rates for hay. A few slight differences include the removal rates and N fertilization timing. In a pasture much of our nutrients is recycled through the cattle to manure and urine. This helps maintain P and K. It is estimated that about 80% of these nutrients are recycled back into the pasture, so soil testing will give us a better idea of fertilization rates.
Grazing management strategies vary based on when paddocks are grazed, rested, or left with a rest period. For those employing rotational grazing after each cycle, applying 25-50 lbs of nitrogen per acre can enhance forage productivity throughout the season. While this approach may complicate split applications post-grazing, it's advisable to administer at least two split applications of nitrogen (60-120 lbs per acre) in the spring, with 30 – 60 lbs of N/acre early spring, followed by an additional 40-60 lbs per acre in late spring. A final application of 40-60 lbs of N/acre in late summer.

Application of 30-60 lbs of N /acre can intensify the spring flush, likely leading to more forage than you can graze. When applying N in early spring, be prepared to have more growth than you may need, but you could harvest for stored feed or stockpile for grazing later in the summer when our cool seasons will have limited growth (this will be low-quality feed and not recommended with tall fescue). If unable to utilize excess forage in early spring, skipping the early spring nitrogen application may be feasible, but applying 40-60 lbs of nitrogen per acre in late spring can aid in stimulating regrowth. Moreover, the late summer application is important to help stimulate growth as we are exiting the summer slump and especially if planning to stockpile, the additional N can lead to greater yields for later use.

One last note: many of our pastures have been through a multi-year drought, making many of them slow to green up. An additional boost may be beneficial this year, so the application of early spring N may be a good consideration, but it would need to be completed sooner rather than later.  

Remember the timing of application is very important as well to get the desired response. We want to apply N when the plants are actively growing. If our plants are starting to reach reproductive growth, N application will not help yields as much as application in early vegetative growth.

Lastly, if you have a mixed stand that has greater the 30% legumes on a dry basis, this should supplement enough N for the grasses within the mixture. Also, among our grass species there is variation on the amount N needed for each species, knowing your plant species is beneficial in determining your N rate.