Answer to Question 1
Rotating of crops in the same field over several years is an effective approach to weed control. Continuous and uniform cropping practices tend to select for weeds adapted to those systems. Using a diversity of crop species increases the effectiveness of rotation.
Corn versus soybean. Although both these crops are annual, the different heights result in selection for different groups of weeds. Soybeans shade the soil faster in the spring following planting and are overall more competitive against weeds than corn. However, corn is more competitive than soybean is with tall mature weeds such as velvetleaf. When herbicides are used in combination with crop rotations, broadleaf weeds are easier to control in corn than in soybean, and grassy weeds are easier to control in soybean.
Perennials versus annuals. Perennial crops like alfalfa provide a tremendous advantage in containing weeds compared to annuals like corn or soybean. In alfalfa systems with three or four harvests of the crop each season, flowering and reproduction of both annual and perennial plants are eliminated or greatly reduced. Perennial weeds like Canada thistle are killed because the removal of their herbage exhausts their root reserves. In contrast, some perennial weeds like quackgrass can adapt to frequent cutting and are best controlled in annual row crops like corn or soybean.
Plant population. Increasing the population of some crops increases the speed at which the crop foliage covers the soil and shades weed seedlings.
Cover crops. Some cover crops (see Chapter 12) can compete with weeds for light and moisture to suppress weeds. Winter rye grain is a widely used cover crop in the northern United States. It is typically planted following a corn crop in the fall and killed the following spring before planting the next crop, which is usually soybean. Rye is allelopathic and will kill or suppress weed seedlings that emerge.
Companion crops and intercropping. Small-seeded legumes like alfalfa are routinely established using companion crops like oat or barley. The larger-seeded grasses form large plants quickly and compete with weeds as the weaker legume gets established. Small grains reduce weed yields but sometimes also reduce establishment of the legumes.
Planting date. Planting dates can be altered to avoid times of greatest weed competition. Many weed seeds typically germinate in the early spring. Delaying planting until after the first weeds have germinated will allow a producer to control early-germinating weeds with preplant tillage or an herbicide. However, there may be a negative effect on crop yield with the shorter growing season.
Answer to Question 2
D
Genetic control of flower symmetry.
The control of breathing via respiratory centres in the mammalian brain.
The patient and his family meet with the nurse to discuss progress on the patient care plan.
A typical evaporative emission control system. Note that when the computer turns on the canister ...
Control of thyroxine secretion through negative feedback.
Quality Control Microbiology