Hay growers can get a better idea of how fertile their alfalfa fields are if they have plants tested rather than soil. That's especially important considering the skyrocketing costs of fertilizer and the high value of hay, says Steve Orloff, Siskiyou County, CA, farm advisor.
“If you soil sample, you can measure how much of that nutrient is in the soil,” he says. “But by testing the plant itself, you're measuring what the plant actually took up.”
Before planting a field of alfalfa, soil test to learn what nutrients may be deficient. “After that I would switch to plant-tissue testing,” Orloff recommends. “Growers should tissue test once a year until they have their fertility programs established; then test every other year.”
It's also good for diagnosing problem areas in a field to determine if the cause is nutrient-related by sampling good and poor growth areas and comparing the results, he says.
The plant-nutrient-measuring method is just catching on in the forage industry — in Orloff's northern California region, at least. Growers of higher-value crops have used the method for years, and Orloff and his University of California-Davis colleagues have been pushing hay growers to use it in recent years.
While soil tests determine the amount of phosphorus and potassium in soil, they do a poor job measuring sulfur levels, he says. Tissue tests, however, can tell how much of all three nutrients — as well as boron and molybdenum — were used by plants.
“The main drawback with plant-tissue testing is that you obviously don't want to test weedy fields because their nutrient concentrations would be different. You also don't want the field to be stressed; if it's stressed from water or insect damage, it could skew the results,” he says.
Other detriments: the time and effort needed to cut and prepare representative plant tissue samples.
“The California method has been to go through the field and collect 40-60 stems. Then you have to divide those stems into thirds and different components are analyzed for different values. It involves walking through a field before it's cut. One of the last things a grower wants to do is run out ahead of a swather and take a sample.”
But Orloff and colleagues believe they have an easier way of gathering tissue samples. “Growers now are taking core samples from bales for forage-quality analysis. A lot of the dairy industry will demand that before they buy a grower's hay, and people are putting a lot of care into collecting good samples for forage-quality analysis. So the thought was, why not use that same sample for nutrient analysis?”
He tested the core-sampling method the past two years on 39 grower fields.
“It looks like it will work,” he says. “I think it's far more convenient — you can just sample a stack and get a more representative sample from the field. If you're collecting 40-60 stems walking a field and bending over each time, your tendency is to cut a plant and get a whole bunch of stems off it because you're tired of bending over.”
Bales should be cored for tissue-test samples at first cutting in cooler-season areas where first cuttings yield well. In longer-season areas, bale cores should be taken from second or third cutting for tissue testing.
The 20 cores currently recommended as an accurate sample for forage analysis are adequate for tissue testing as well, says Orloff.
Accuracy-wise, it's hard to determine whether a bale-core sample is better than a plant sample. “That's what we're keying in on now, but so far the relationship looks pretty good.
“It's been amazing to see the variability in different grower fields; it goes from very deficient all the way up to very high for each nutrient. With one grower who took tissue samples, the lab actually called him because his values were so high. He was able to go several years without applying sulfur.”
Orloff worries that many growers have a fertilizer routine and figure it will continue to work because it has in the past. “But some nutrients could be accumulating or, on the flip side, you may not be putting on enough nutrients,” he says.
In his trials, the farm advisor compared soil samples, whole-plant samples and core samples and sampled at different growth stages.
“One of the concerns is, with ordinary plant-tissue testing in most states, it's recommended to take a sample at one-tenth bloom. A lot of growers now, especially if they're selling to the dairy market, are cutting earlier — usually in bud stage. The nutrient concentration at bud stage is different than it would be at one-tenth bloom, so we've done trials sampling at different time periods to see how values need to be adjusted if alfalfa is cut earlier.”
It costs more to tissue test — possibly double the cost of a soil sample. But with high fertilizer and hay prices expected, and the improved accuracy, tissue testing is worth it, Orloff says.
Growers who want to tissue-test fields should talk with their labs to see if they offer what's needed. Results will then need to be evaluated with the help of the local extension educator or lab personnel, who can help growers find values to compare.
“We plan to have values for cored samples later this year that will be based on our results and values established for whole tops,” he says.