Tissue testing your forages
Many have long recognized the importance of testing for hay or pasture for forage quality analysis, but recently, tissue testing for nutrient management has come further into the spotlight.
Traditionally, producers and livestock owners have cored their hay bales or sampled their pasture in order to determine what nutrients their animals will be consuming. Now, we are taking the same approach to evaluate fertilizer management practices, and try to better target what is, or isn't, available to those plants in the soil. While this is not a new practice, we are just now digging more into how important this tool can be.
Soil sampling has been the primary tool for evaluating soil nutrient needs. Fertilizer recommendations are based on how much of that particular nutrient is present in the soil. While extremely useful, and a great start, it does have its limitations. For one, even though the nutrient may be present in the soil, it may not necessarily be available to that plant. We see this a lot here in Montana where the soil may have a lot of phosphorous (P), however that P may be bound to calcium, and not in a usable form.
This is where tissue testing can come in. By taking part of the plant and evaluating how much of a particular nutrient is present, we can determine if it's in adequate quantities or not. The biggest problem is having a "standard" or optimum range for that nutrient, which changes with the maturity of the plant. Because of this, sampling is critically important. We have specific growth stages which are recommended for conducting a tissue test. In alfalfa, we have ranges from early bud to 10 percent bloom. I often recommend somewhere close to 5 percent. For grasses, it is usually at a more immature, vegetative growing stage and can be specific to species.
When to take the sample during the growing season is another important part of the process. For evaluating sulfur levels, first cutting is usually appropriate. This gives a producer time to correct any deficiencies during the growing season. In the case of fresh forage, take samples throughout your field or throughout your hay stack in the case of preserved forage. You don't want to be biased and sample only the good-looking parts of your field — sample the poorer growing parts as well. The number of samples you send will depend on how many operations you are preparing to do. Most producers only want to fertilize an entire field once. If that's the case, then sample randomly throughout the entire field and combine them into one single sample. Make sure it is mixed thoroughly in order to get an accurate representation.
If you are prepared to fertilize parts of your field differently, then taking multiple samples is ideal. For instance, if there is a lower part of the field that isn't producing as well as it is on a hill, taking a sample from each area will help determine if soil nutrients are the cause. Then you can apply more or less fertilizer where needed and save money.
How you analyze your sample is also an important decision. Research has found that taking samples using wet chemistry methods is usually more accurate than using near infrared spectroscopy (NIR) for nutrient analysis. In Montana, we have found that NIR is less accurate at estimating certain nutrients like sulfur when compared to wet chemistry.
Finally, what part of the plant you sample can also be important. In the case of fresh forage, the top six inches of a plant is sampled. Others have made the case for whole plant sampling. Where do hay cores fit in this? Research evaluated different sampling methods and found adequate correlation between all types, meaning you can use any of the methods and come up with similar results. For research purposes, the top six inches is still standard. If you have any further questions, contact myself, your local Extension agent or forage Extension specialist.
Editor's note: Emily Glunk is an Extension Forage Specialist at Montana State University. Her website is animalrangeextension.montana.edu/forage/ and she can be reached at firstname.lastname@example.org or 406-994-5688.