Time of Giant Ragweed Emergence

Chris Boerboom

Estimate answers to these questions that compare giant ragweed that emerges either at the same time as corn or 4 weeks after corn.

1. Giant ragweed at 13 plants/100 ft2 can reduce corn yield by ______.

a) 10%

b) 30%

c) 60%

d) 90%

2. Giant ragweed that emerges 4 weeks after corn may have _____ corn yield loss as compared to ragweed emerging with the corn.

a) the same

b) 20% less

c) 40% less

d) 80% less

3. Giant ragweed produces ______ seed when it emerges 4 weeks after corn as compared to ragweed emerging the same time as corn.

a) the same amount of

b) 30% less

c) 60% less

d) 90% less

Giant ragweed is definitely a problem weed in Wisconsin because of its early emergence, rapid growth, and highly competitive nature. Some previous research might suggest that giant ragweed should not be a long-term problem for the following reasons. Giant ragweed often produces less than 5,000 seeds per plant compared to other weeds that can produce 10,000 to 100,000 seeds per plant or more. Furthermore, studies have found that 35 to 86% of the seed produced by giant ragweed is damaged by insects or is not viable even before it is shed from the plant. Other research suggests that up to 90% of the giant ragweed seed will not survive in the soil past one year. Considering all these facts, it would appear that giant ragweed should not be a persistent weed problem. However, we know it is not easy to quickly reduce giant ragweed populations.

It is difficult to obtain complete control of this large seeded broadleaf weed with current preemergence herbicides. As a result, some giant ragweed may emerge later in the season. It is logical to question how competitive these late emerging giant ragweed plants are and how much of a problem they may pose in the future if not controlled. To answer these questions, I will review a recent research report from Kent Harrison and his fellow weed scientists in Ohio. They studied giant ragweed that emerged at the same time as corn and ragweed that emerged 4 weeks after corn (at the V5 corn stage) at three densities of ragweed. They measured corn yield loss and giant ragweed seed production.

As you would suspect, giant ragweed was very competitive in their experiments and reduced corn yield by just over 60% at a density of 13 plants/100 ft2 when the ragweed emerged with the corn. Giant ragweed was much less competitive when it emerged 4 weeks later. The corn yield loss ranged from 76 to 87% less with the late emerging ragweed compared to the early ragweed across the densities and two years of their experiments. In other words, if corn yield loss was 60% with early emerging ragweed, the same density of ragweed emerging 4 weeks later may only cause an 8 to 14% yield loss. This level of yield loss is not acceptable, but this information may change our management decisions at low densities if seed production can be limited.

The researchers collected seed that was shed from the early and late emerging ragweed and found two key points. First, the giant ragweed that emerged 4 weeks later produced 86 to 91% less seed than the early emerging ragweed. Second, only about half of the seed produced was viable (Table 1). The other seed was either empty or infested with insect larva that were eating the seed. These larva were from fruit flies, weevils, and a moth. Unfortunately, late emerging ragweed produced the same percentage of viable seed as early emerging ragweed.

Table 1. Fate of giant ragweed seed that was shed from plants grown in corn.

*Many showed evidence of earlier insect feeding

Knowing the level of reduced competition of late emerging giant ragweed, combined with the large reduction in seed production, can help us with our decisions on how to manage late emerging giant ragweed. We will want to control higher densities of ragweed if they are early or late emerging. However, at very low ragweed densities, it probably will not pay to spray. This decision should be easier to accept knowing that these plants will not be producing much viable seed.

Source: Harrison and others. 2001. Competition and fecundity of giant ragweed in corn. Weed Science 49:224-229.

March, 2002