Breeding a better cover crop: a screen of rye germplasm for weed suppression

Cover crops are an essential tool for farmers interested in sustainable agriculture, but have been ignored by plant breeding programs. The potential for breeding a better rye cover crop was explored by an interdisciplinary group of researchers at North Carolina State University. Two hundred and sixty-eight accessions of rye were chosen from the National Rye Collection and grown under field conditions. Accessions were screened for allelopathic potential using bioassays. The most allelopathic genotypes were used to start a breeding program that hopes to optimize allelopathy and other key cover crop characteristics.

Introduction

Concern over non-point source pollution emanating from agricultural areas has led to interest in reduced pesticide use and reduced tillage. Cover crops are an important tool in reducing non-point source pollution. They protect the soil from erosion, trap nutrients, enhance soil organic matter, protect the soil from erosion and suppress weeds (Echtenkamp and Moomaw 1989, Hartwig and Hoffman 1975). When cover crops are killed and left as mulches on the soil surface for the following crop, their environmental benefits are even greater. The residues continue to inhibit soil erosion, while increasing infiltration of rainwater (Creamer et al. 1996). Residues also inhibit weed growth by shading the soil, physically inhibiting the growth of seedlings and releasing weed-inhibiting chemicals, also known as allelochemicals, into the soil. The weed control afforded by cover crop mulches can significantly reduce herbicide inputs (Teasdale and Mohler 1993).
While all cover crops provide these environmental services, they are not all equally effective. Rye is known as one of the best winter cover crops and has been widely used by farmers. With vigorous root growth, rye has proven more effective at scavenging nitrogen from the soil profile than other popular cover crops, thereby reducing nitrate contamination of groundwater (Wagger, Cabrera and Ranells 1998). In addition, rye mulch contains several known allelochemicals and has proven effective at controlling weeds in multiple crops (Smeda and Weller 1996; Worsham and Blum 1992; Barnes and Putnam 1983). Unfortunately, the weed control has not been consistent from year to year (Nagabhushana et al. 1997). Inconsistent weed control by cover crop mulches is not surprising. Allelochemical production is a poorly understood phenomenon. In particular, the interaction between genetics and environment has received little scientific attention.
Two years ago, we began examining whether 9 commercial varieties of rye differed in allelochemical production. In a field study, different levels of weed control were found between varieties, even after the amount of biomass was adjusted to be equivalent (Reberg-Horton et al. 2000).
The effect seen in the field also shows up in petri dish bioassays. Whether tissues from the nine varieties varied in their inhibition of weed seedling growth was tested in the bioassays. While rye tissue was found to be toxic to the weeds in general, differences in toxicity between varieties were found. Pigweed and goosegrass, two important agricultural weeds, were variably suppressed by different rye varieties. Furthermore, growing rye with differing levels of fertilizer made no difference in toxicity. Our results suggest genetics play the primary role in allelochemical production.
Differences between varieties in their ability to scavenge nitrogen were also studied. Nitrates are a leading source of ground water contamination. Rye is particularly adept at preventing this contamination by capturing the nitrates before they reach the groundwater. However, the variation between rye varieties in this ability has never been studied. The nitrogen recovery rate was compared for all 9 varieties, under four rates of N fertilization (0, 20, 40, 80 lbs. N/acre). Soils were sampled at four depths, three times during the growing season. Some varieties showed better scavenging ability than others did. The most efficient scavengers were the deep-rooted varieties.
The encouraging results stimulated interest in breeding for a cover crop rye at North Carolina State University and the development of this SARE proposal. Researchers from various disciplines formed the Rye Breeding Committee to take the first steps towards a breeding program. We have two primary selection criteria. First, the ideal rye should produce large quantities of allelochemicals. The ultimate goal is for enough allelochemicals to be produced to provide weed control for a spring planted crop and replace the use of preemergent herbicides. Second, the rye should excel at scavenging residual nitrogen in the fall. This will reduce nitrate contamination of groundwater and lead to higher nitrogen use efficiency for the farming system as a whole.