Agronomic Services — News ReleaseFOR IMMEDIATE RELEASE
WEDNESDAY, FEBRUARY 17, 2010
Contact: J. Kent Messick, field services chief
NCDA&CS Agronomic Division
North Carolina revises nitrogen recommendations
RALEIGH—N.C. State University and the N.C. Department of Agriculture and Consumer Services are advocating a new approach for deciding how much nitrogen to put out on wheat. The revised guidelines take into account crop biomass and often call for significantly less nitrogen than previous recommendations. Growers lucky enough to have a stand, despite the poor planting conditions last fall, may be able to use the new guidelines to reduce input costs without compromising yield.
Split applications of nitrogen are likely to be necessary this year, said Ben Knox, NCDA&CS regional agronomist. The first step in determining nitrogen needs involves a tiller count at green up.
“If the tiller count is low, growers should put out some nitrogen as soon as possible,” Knox said. “If there are fewer than 50 tillers per square foot of row, growers need to apply up to half (about 60 pounds) of the spring nitrogen now. For counts between 50 and 70 per square foot, 30 to 40 pounds of nitrogen should be applied. If the tiller count is high, but the wheat is yellow, an application of 30 pounds of nitrogen is appropriate.
“I have seen fields with as few as 15 to 20 tillers per square foot at this time of year end up making good wheat,” Knox said. “A timely nitrogen application followed by some dry weather and warm temperatures can yield surprising results. However, even if the wheat is thin and has to be abandoned, the nitrogen will not have been wasted. It will have made the wheat a better cover crop.”
After an early nitrogen application, growers will need to wait until Zadok’s growth stage 30 (GS-30) to implement the new guidelines to determine how much additional nitrogen is necessary.
To make that determination, N.C. growers must collect two types of samples: a tissue sample and a matching above-ground biomass sample. Each pair of these samples should be given the same sample ID and submitted to the NCDA&CS Agronomic Division laboratory within 24 hours of collection.
In the past, N.C. growers relied on recommendations developed in Virginia that are most reliable for wheat grown on well-drained soils. By taking into account biomass measurements, the new recommendations detect and account for differences in crop growth, such as those due to planting date, row spacing and moisture levels. Considering the extraordinary amount of rain North Carolina has experienced this fall and winter, the new system should prove to be particularly valuable for determining nitrogen rates this season.
Wheat normally reaches GS-30 any time from mid-February to mid-March, depending on variety, planting date and environmental conditions. When plants begin to stand up straight, growers should try to verify that a crop is at GS-30 by pulling up several plants, splitting the stems from the top to the base, and looking for the growing point. Prior to GS-30, the growing point will be just above the roots; at GS-30, it will have moved about one-half inch up the stem (Figure 1).
|Figure 1. Location of wheat growing point at Zadoks growth stage 30.|
At GS-30, tissue sampling involves cutting 20 to 30 plants about one-half inch above the ground from representative areas throughout a field. In general, two large, fistfuls of leaves make a good sample. Dead leaves and weeds should be removed.
Biomass samples, on the other hand, should contain all the above-ground, wheat-plant tissue from one, representative, 36-inch section of row. In broadcast fields where there are no rows, growers should collect all the plants from one square yard. In either case, soil and weeds should be carefully removed and the sample placed in a paper bag. Write the sample ID from the corresponding tissue sample and the word “biomass” on the bag.
Figure 2. Location of N% and Biomass values on the NCDA&CSPlant Analysis Report. Click to view larger image.
Upon receiving an NCDA&CS Plant Analysis Report, growers should first look for the biomass and N% values (Figure 2). Dr. Randy Weisz of N.C. State University has developed a couple of interpretive tools that use these values to determine an appropriate nitrogen rate (Table 1, Figure 3). Table 1 indicates the nitrogen-recommendation curve to use based on biomass reading (tissue dry weight in grams) and row spacing. Figure 2 provides the nitrogen recommendations based on the N% value from the Plant Analysis Report and the chart color selected from Table 1.
Figure 3. Nitrogen recommendation chart based on crop biomass and percent nitrogen in tissue. Click to view a larger image.
A thorough explanation of Weisz’ recommendations is available online at www.smallgrains.ncsu.edu/. A simple example, however, shows how the approach works.
The sample report shown in Figure 2 gives a biomass value of 36 grams and an N% value of 3.5. Table 1 indicates that wheat with this dry weight planted at a row spacing of 6 inches is a high biomass crop (green square). When you trace the green line in Figure 3 to the N% value of 3.5, you find that the nitrogen recommendation for this crop is 0 pounds per acre. For comparison, look at the amounts of nitrogen that would have been recommended if this crop had been in the medium (45 pounds N /acre) or low (70 pounds N /acre) biomass category.
North Carolina growers who want to know more about this method should contact their NCDA&CS regional agronomist or county Cooperative Extension agent. Regional agronomists, in particular, can offer advice on how to take tissue and biomass samples and how to use report data.