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Chapter 7 - Plant Development, No. 2, Summer 1987

Development of Ground Cover by a Winter Wheat Crop

Don Wysocki

Plantings of wheat that develop sufficient overwinter ground cover are a management tool that can be used to reduce soil erosion. In Oregon the Soil Conservation Service uses ground cover produced by December 1 to determine estimated soil losses from fields of fall-seeded wheat. Adjustments are made for ground covers of 15 percent or more, or 50 percent or more by this date.

Ron Rickman, Betty Klepper and Paul Rasmussen, STEEP researcher and USDA Agricultural Research Service scientists at Pendleton, OR, are using the relationship between plant leaf development and growing degree days (GDDs) to estimate development of ground cover after planting. This is possible because wheat plants follow a rigid pattern of development according to the number of GDDs they receive. The stage of development of a wheat plant can be predicted by the GDDs received after planting, The size of the plant and ground area protected under its canopy can be inferred from a relationship between leaf number and leaf length (Fig. 1). The cumulative effect of overlapping plants is computed by using the geometry of row spacing, plant spacing and plant overlap (Fig. 2).

This method of predicting development of ground cover allows the selection of seeding rates or planting dates to establish a prescribed ground cover by December 1. For example, using an October 1 planting date at the Pendleton Research Center, let's determine seeding rates that will achieve ground covers of 15 percent by December 1. Using the following data: (1) seeds/pound = 11,300, (2) GDDs for emergence = 150, (3) GDDs per leaf = 100, (4) 90 percent seedling emergence and (5) 10-inch row spacing, the predicted ground cover vs. GDDs for three different seeding rates is shown in Fig. 3. The number of GDDs accumulated between October 1 and December 1 at Pendleton is 410 (Fig. 4). On December 1 the 40, 60 and 80 pounds/acre seedings will have respective ground covers of about 10, 15 and 20 percent. If the ground cover sought for erosion control is 15 percent or more by December 1, a seeding rate of 60 pounds/acre or more should be used.

Fig. 1. Observed Leaf length vs. leaf number for Stephens winter wheat, Pendleton, OR, 1982 (USDA-ARS Pendleton).

Fig. 3 shows that about 550 GDDs must accumulate for a 40 pounds/acre seeding to attain a ground cover of 15 percent. If we interpolated from Fig. 4, this corresponds to a seeding date of about September 20 at Pendleton. Thus this method can be used to match seeding rates and seeding dates to achieve the desired ground cover by a prescribed date.

Fig. 2. Possible overlapping of adjacent wheat plant canopies. (Rickman, USDA-ARS, Pendleton).


Fig. 3. Calculated plant cover development for three different seeding rates.

Row spacing has a strong influence on the development of ground cover. Fig. 5 shows the unprotected area between the rows vs. GDDs for three different row spacings, all at a seeding of 60 pounds/acre. In 14-inch rows, plants are crowded together but do not cover the inter-row areas until they are quite large. In 7-inch rows, the area between the rows closes much faster because plants are more uniformly spaced and cover the inter-row area at an earlier growth stage.

Fig. 4. Expected accumulated GDDs from the first and fifteenth of the month during the fall planting season at the Pendleton Research Center (Rickman, USDAARS, Pendleton).

The procedure for estimating plant development and ground cover has been developed into a microcomputer program, "PLANTEMP." It requires an IBM or IBM compatible computer, Basic language and 256K memory. It is available through Agricultural Communications, Publications Orders, Oregon State University, Corvallis, OR 97331; 503-754-2513. The cost is $25.00 plus 15 percent for shipping.

Fig. 5. Estimated bare area between rows for three different row spacings at a seeding rate of 60 pounds/acre (Rickman, USDA-ARS, Pendleton).


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