The slow road to full adoption of "precision farming" practices contains many potholes, among which is the growing recognition that interpreting yield variability throughout a field can be very difficult. Part of the difficulty lies in the uncertain reliability of the data that are measured and massaged by the current generation of "precision farming" tools, including yield monitors. Part of the difficulty lies in the reality that crop yields are influenced by a vast array of yield limiting factors, many of which interact together in their influence on grain yield.
Yield limiting factors (YLFs) are simply those factors, biotic or abiotic, that limit the actual yield of a field or part of field. Some influence yield directly while some interact with others to influence yield indirectly. Some YLFs occur every year, others do not. Some influence different crops differently. Most importantly, weather interacts with almost all of the YLFs.
Because of the opportunities for interactions among different YLFs plus the influence of weather on their effects, the YLFs that are important for this crop in this field during this year may not be the same ones that influence yield for the same crop in the same field in a different year. Similarly, the YLFs that influence corn yield this year in this field may not influence soybean in the same field next year even if the weather conditions are similar.
Perennial YLFs are those yield limiting factors that occur every year. Examples of perennial YLFs include soil nutrient and pH levels, soil drainage patterns, and patterns of perennial weed growth. Perennial YLFs can be easily identified and managed with site-specific technology simply because they exist every year in pretty much the same areas of a field.
For example, intensive soil sampling techniques can delineate a fairly accurate "picture" of the variability for soil pH throughout a field. Subsequent variable-rate applications of agricultural lime can correct those areas of the field with stressfully-low soil pH levels while leaving those areas with acceptable pH levels untouched.
Sporadic YLFs are yield limiting factors that occur unpredictably over the years. Examples of sporadic YLFs include disease and insect infestations, pesticide injuries, and stressful weather conditions. Additionally, since weather conditions often influence the severity of other YLFs, the effects of perennial YLFs such as soil nutrient levels may seem sporadic. Site-specific technology can help identify sporadic YLFs when they occur, but not necessarily prevent their reoccurrence since one cannot reliably predict the timing of their reoccurrence.
The reality of "precision farming" may be that we will only be able to fine-tune our crop production to a limited extent with site-specific technology. Such technology will identify and correct obvious YLFs such as soil fertility, pH, and drainage. We may also be able to identify and attack certain YLFs "on the go" during the season (e.g., developing insect infestations). But, we may be limited when trying to site-specifically manage those YLFs that interact heavily with seasonal weather patterns. At least, until we can better predict next years weather!