Weed specialists and agents at the University of California’s Integrated Pest Management (IPM) program advise growers to control weeds before corn plants reach 6 to 8 inches high. At that height, weeds will begin to affect corn yields because they start to compete with the plant for nutrients, water and light. Weeds that grow later in the growing season seem to have less effect on yield. Unfortunately, insects such as thrips and armyworms can hide in those late-season weeds, where they will eventually feed on the corn.
Management of weeds is important not only because of their yield damage but because they can reduce silage feed quality, the university’s weed specialists indicate. Weeds slow down harvesters, causing the wheels to slip or clog. They also may increase grain moisture content, and weed seeds may infest future crops grown in fields.
Because growers shouldn’t rely on one weed control plan to fight infestation, the IPM program recommends growers establish a weed management initiative that focuses on cultural, mechanical and chemical controls. They suggest growers follow a program that relies on proper seedbed preparation, variety selection, seeding rates, fertilization, irrigation, cultivation, pest control and crop rotation. If growers initiate this plan, they can reduce weed competition and infestations.
Part of a good management plan involves monitoring corn crops properly and identifying any germinating weeds in fields. The specialists with the California IPM program advise growers to count any emerging weeds as they walk a field so they can determine the weed species present, or at least contact their local Cooperative Extension agent to properly identify the weeds. He can help growers figure out what growth stage the weeds are in and their concentrated numbers in the field.
As mentioned already, good cultural practices are important, too. Growers are wise to cultivate in a timely manner to bury weeds before they become established and remove weeds from fence lines, ditches and waste areas. Weed scientists advise growers to clean their tillage equipment and harvesters thoroughly before they go to the field. This helps to cut down on the spread of weeds to a certain field or from one field to the other.
A good cultural practice before planting is to use a field that is free of weeds, to prepare the seedbed of any large soil clods so seeds can germinate more freely, and to apply a preplant herbicide if necessary. Also, choose a rapid growing variety so it can quickly overshadow weeds and reduce weed competition. The scientists of the IPM program indicate that corn plant population density of 30,000 to 34,000 plants per acre can assist in providing enough shading and reduce mid- to late-season weed growth.
They suggest another good cultural tool is to use a sweep to cultivate weeds and grasses such as johnsongrass, bermudagrass and nutsedge. Let’s look at them.
According to the California IPM program, johnsongrass appears coarse and clumpy. The seedling of johnsongrass is egg-shaped, and its seed is a dark, reddish-brown to black. Its first leaf blade runs parallel to the ground and has a smooth edge. Its midvein is a white color. Thick stems are erect and reach about 7 feet tall. Once grass matures, its leaves spread. They will roll inside the bud. Flower blooms are initially green or greenish violet, but once they mature the blooms turn to a dark reddish or purplish brown.
Stems of the perennial bermudagrass are erect and flattened, according to the California IPM program. The leaf blades look flat or folded and appear to be flexible when moved. They can roll in the bud and grow less than 2-1/3 inches in length. The leaf tips are pointed. Growers will notice the flower heads are made up of about three to eight spiky branches at its flower cluster.
Yellow and purple nutsedge looks like a grass and are hard to control. Because of that and because of their denseness, this perennial can reduce corn yields. At first, growers will notice that seedling leaves are small and fine-looking. More mature leaves will start to emerge together and fold and appear thick and stiff. They may look V-shaped and in sets of three at the base. They also will see that the stem base is triangular and stands erect. A nutsedge will grow from tubers underground.
After planting corn, weed scientists advise growers to destroy weeds in the row middles using rolling cultivators or sweeps. However, rolling cultivators may cause less damage to roots that have spread into the furrow than sweeps. Unfortunately, the scientists indicate they are less effective on johnsongrass, bermudagrass and nutsedge. Weeds around the row may need more attention. They advise to cultivate at least 4 inches from the corn plant.
If a large population of weeds are present or if growers are fighting hard to control weeds, then herbicides may be necessary. The California IPM program suggests that herbicides can cut down on these large populations and any weeds that arrive early in the field or those that propose a threat to future plant crops. They recommend growers check into using a preplant, preemergent or postemergent herbicide to control these tough weeds. They warn growers when selecting herbicides they should consider the cost, weeds that have been identified, stage of growth they are in, soil type in their fields, crops they plan to rotate with and any crops adjacent to the fields.
They indicate that postemergent herbicides work well after planting corn. Contact your local Extension agent or sales representative for these brand names. Follow label directions carefully.