Analysis of the effects of cropping system diversification on weed suppression using stochastic periodic matrix models.
Paula Westerman, Matt Liebman, and Philip M. Dixon. Iowa State University, 2501 Agronomy Hall, Ames, IA 50011
We used stochastic periodic matrix models to examine how contrasting crop rotations affect two weed species, velvetleaf (Abutilon theophrasti) and giant foxtail (Setaria faberi). The models were parameterized with estimates of emergence, plant survival and fecundity for both weeds derived from an experiment in which a 2-yr rotation system (corn-soybean) managed with conventional rates of herbicides was compared with 3-yr (corn-soybean-triticale + red clover) and 4-yr (corn-soybean-triticale + alfalfa-alfalfa) rotation systems that received 71% and 78% less herbicide, respectively. Data were available for 4 replicates in 3 years. Bootstrapped mean population growth rates, λ, were 0.97, 0.92, and 0.93 for velvetleaf and 0.78, 1.39, and 1.07 for giant foxtail in the 2-yr, 3-yr and 4-yr rotations respectively. For giant foxtail in the 3-yr and 4-yr rotations, the increase in population growth rate caused by herbicide reduction in the corn and soybean phases could not be offset by the reduction in population growth rate caused by the addition of triticale and the forage legumes, which were less conducive to weed growth. The growth rates just described were calculated using the assumption that no seeds were lost due to seed predation; field measurements indicate, however, that weed seeds were indeed destroyed by predators. Elasticity analysis identified seed predation rate as a key demographic rate affecting weed populations that are growing (i.e., λ>1). Giant foxtail populations would decline if 75% and 20% of the seeds were lost to predators in the 3-yr and 4-yr rotations, respectively. Further analyses indicated λ was particularly sensitive to changes in seed mortality in the soil, especially in declining weed populations (i.e., λ<1). However, this demographic parameter is difficult to manipulate.