Nitrogen Applications to Wheat in a Cover Crop Program that Restricts Fertilization Until Mid-March 2008

Richard Taylor, Extension Agronomist; rtaylor@udel.edu

A number of fields were seeded this past fall to winter wheat after being enrolled in cover crop programs that excluded fall applied nitrogen (N). Some fields were seeded quite early in the fall and tillered profusely while other fields were seeded late enough or in dry soil so fall growth, and especially tiller production, was very limited. As late-planted or untillered fields begin to emerge from winter dormancy, N availability will be critical to encourage rapid growth and tiller development in the spring of 2008 for those fields that will be allowed to be harvested for grain. Virginia Tech’s publication entitled “Intensive Soft Red Winter Wheat Production” provides some valuable information to assess the N status of wheat fields in early spring.

To use the information, growers need to determine the average number of tillers (a tiller is a shoot showing at least three leaves visible) per square foot. At normal planting populations, plants often only have two or three tillers present (counting the primary shoot). To assess the tiller count in your field, take a yard stick or dowel rod cut to a 3-foot length and randomly place it next to a typical looking row in at least five (preferably 10 to 20) places well-spaced out around the field. In each section of row, count and record the number of tillers with three or more leaves that are found within the 3-ft section of row. After you’ve counted and recorded the number of tillers in 3-ft of row from at least five locations, add up the number of tillers and divide by the number of locations to get average tiller count (per 3-ft of row). To calculate tiller density multiply the average tiller count by 4 and divide the result by the row width in inches and this will give you tiller density (tillers per square foot). For example in 5 locations you find there are a total of 450 tillers, so 450 divided by 5 equals 90 tillers, on average, per three feet of row. If you multiply 90 by 4 and divide this by the row width (7 inches), your tiller density comes to 51.4 tillers per square foot.

If the tiller density is 60 or less, you need at least 60 lbs N/acre applied as early as the cover crop contract permits. Split applications-one at green-up or around Zadoks stage GS 25 and one at Zadoks GS 30-31 or Feekes GS 4-5 or pseudo stem erection or first node detectable-can increase yields by 5 to 10 bushels per acre but the site characteristics often limit a growers options for N splitting. If you have a tiller density of 100/ft2 or more, no N is needed at this stage (green-up or GS 25); but if the tiller count is 75/ft2, you will need 40 lb N/acre. An N rate of 25 lb/acre will be needed if the tiller count is 85/ft2. You will need to evaluate each field for the likelihood that you will have time and the ability to return and apply a second application of N to the field. If a second split is not possible, you may decide to apply all required N as early as permitted by the cover crop contract. With the high price of N fertilizer, you should consider the likelihood that the stand will produce maximum yields or whether the yield goal should be adjusted down to a realistic level. The lower the tiller count late in the season, the less likely the field will produce maximum yield for the variety and soil type.

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