A precision agriculture approach to crop management requires producers to consider information about production units in the same manner as production inputs such as fertilizer or irrigation. As the technologies for measurement of important agronomic indexes improve, producers may have to evaluate precision management for all aspects of production systems. The following sections attempt to briefly address the potential for precision management strategies for factors that impact crop production. Each section deserves a detailed examination, but the intent here is to highlight possibilities, while leaving the in-depth examination to others.

Crop Genetics

Crop productivity depends on the genetic makeup of the plant variety and the response of the variety to its environment. Varieties are selected for particular genetic traits (i.e., drought tolerance, resistance to diseases and insects, and yield). In a precision agriculture scenario, producers try to match crop variety and populations with various conditions that exist in a field. The introduction and maintenance of transgenic varieties requires sophisticated management techniques. For example, the use of Bt-enhanced seed varieties (cultivars engineered to contain Bacillus thuringiensis, a bacterium that produces a protein toxic for insect pests) requires the planting of nontransgenic varieties in refuge areas to avoid the development of resistance. The requirements for refuge areas vary with the intended management practices, and precision agriculture techniques may predict the optimum location for each variety.

Similar potential exists for the use of herbicide-resistant varieties. If there is a yield penalty associated with the varieties containing the herbicide-resistance gene, the producer may wish to plant that variety only where weed problems exist. Variable-rate technology planters can change the variety being planted in each portion of the field. However, if resistant and non-resistant varieties are mixed in the same field, the planting sites for each would have to be recorded and used in any subsequent herbicide applications in order to ensure that susceptible plants are not sprayed. Changing varieties to manage for drought-prone soils has also been proposed.

Plant Population

Knowing when to change the plant population density for optimum yield in fields with known variability would benefit a producer and could be done by varying the seeding rate on a planter or grain drill. Preliminary data indicate that a positive net return can be achieved by varying plant population according to depth of topsoil (Barnhisel et al., 1996). If a producer expected fair-to-poor conditions for germination and emergence of seeds on productive soils, the seeding

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