Ohio State University Fact Sheet

Food, Agricultural and Biological Engineering

590 Woody Hayes Dr., Columbus, OH 43210

Yield Monitors and Maps: Making Decisions

AEX-550-97

Larry Lotz

"I Have a Yield Map ... Now What?"

Yield monitors are the first step for many producers into the age of precision farming. While their cost is reasonable, the commitment of time and resources required to effectively use this technology is a significant undertaking. A yield monitor, combined with Global Positioning System (GPS) technology, is simply an electronic tool that collects data on crop performance for a given year. The monitor measures and records such information as grain flow, grain moisture, area covered, and global location. Grain flow is measured by a sensor usually located at the top of the clean grain elevator. Mass of the grain thrown at the sensor by the grain elevator paddles is measured per unit of time.

Yield monitoring can be defined as "the measurement of the harvested portion of a crop over space and time and the summation of those measurements in graphical form" (1994 Pierce). Other definitions include:

1. Regular intervals where a harvested weight has been obtained along with a GPS reading. A display of the weight translated to bushels per acre or yield produces a yield map.

2. A spatially referenced, graphic representation of a crop yield component for a defined area.

Yield monitors come with various technical designs and features. Yield monitors alone do not make maps. Data obtained by yield monitors are stored on a PCMCIA (Personal Computer Memory Card International Association) card. This data, combined with mapping software and GPS, are capable of producing a colorful map showing variations in grain yield and moisture. However, colorful maps are not knowledge. If these maps are to be of any real value, data generated from them must be incorporated into the decision making, analysis, and overall planning process of the farm operation.

Mapping with GPS technology is revolutionary. Yield maps are a very important piece of information and one of the first things a farmer wants to see after harvesting a field. However, yield maps are not the only types of maps that can be produced using GPS technology. Corn moisture and combine speed can be mapped. Theoretically, any variable for which a sensor can be built can be mapped. Examples of other maps could include seed depth, fertility, plant population, compaction, weed populations, plant leaf analysis data, and even the operator's blood pressure while harvesting a field. If possible, do not destroy the original data recorded on the PCMCIA card during computer data management and mapping operational processes.

Yield Map Interpretation

The value of a yield map is in its interpretation. A yield map exhibiting yield variability will cause you to ask more questions than it will answer and can become a source of frustration rather than a source of information. A yield map only documents the spatial distribution of crop yield, not what caused the variation. A yield map does not indicate why yields vary, whether yield potential is reached anywhere in the field, or predict yield patterns in future years. A yield map is of value only when it leads to a management decision or validates management practices.

As yield maps are evaluated, sources of yield variability can be grouped into two areas--variability caused by producer management practices and naturally occurring variables.

Interpreting yield maps can be a challenging process. In the pictured yield map, yields range from less than 80 bu/ac to more than 200 bu/ac. Some of the known reasons for this variability include: A. corn hybrid change, B. surface drainage problems, C. low wet area, D. old woodlot recently cleared, E. end row compaction, F. change in soil type, G. a mechanical problem, and H. grass waterway.

I. Management Variables

1. Equipment/mechanical errors

Reliable equipment properly installed is a must. An accurate, dependable GPS signal is critical to getting good data. The loss of GPS differential signal results in wrong values for the location where the data were taken. Grain flow data problems can occur when one of the following situations occur:

a. when the combine is filling to threshing capacity,

b. when the combine has stopped moving and the threshing area is emptying,

c. at the beginning or end of a swath,

d. when the combine is plugging and during breakdowns.

Electronic devices such as cellular phones, CB radios, and other electronic equipment can also cause interference and loss of differential signal. Data from these points should be discarded. Combine operators should have a working knowledge of their equipment and the consequences of failure on yield map characteristics. They should be very familiar with field characteristics and plan ahead on how to treat end rows, grass waterways, and other field peculiarities.

Proper and timely monitor calibration is also very important. A well calibrated yield monitor will usually produce yield information with more than 97% accuracy. Don't skip on calibration! Recalibrate when field variables such as seed variety change. Questions the operator should ask, but not limited to are: "Where is the GPS antennae located in relationship to the combine head (centered?)? What header width was input during calibration (do you always use the complete header width)? and is the PCMCIA card formatted, charged, and ready for use?" Refer to your equipment operator's manual if you have questions about calibrating or for technical support information. Remember, you only get one shot at collecting and recording yield data.

2. Field history

Sometimes the variability in crop yield can be attributed to some historical event within the field. Look for patterns in your yield map. Patterns with straight lines tend to be artificially made. To interpret these patterns, a producer should refer to last year's management records and possibly the last ten to twenty years if they are available. Historical records are extremely important in answering questions of yield variability. Seek historical information from neighbors, past owners of the farm, and Court House records. Characteristics like old farmsteads and fence lines, manure, fertilizer and chemical applications, wood lots, feed lots, chemical spills, old tile lines, and compaction strips leave a long lasting effect on crop production. In addition, more recent practices such as crop variety, tillage, and previous crops should be considered. Be sure to record or map errors and variations in application of crop inputs. This may be valuable information in identifying yield variations in the map. Irregular patterns may reflect different soil conditions, soil types, drainage problems, and pest infestations such as weed, disease, and insects. Match pattern widths to sprayer and tillage tool widths.

3. Soil fertility

One of the first questions you will ask yourself when looking at yield map patterns will be the relationship to variability in soil nutrients. A gridded soil test map is a valuable tool in diagnosing the reasons for yield variability. Most often fertility will not be a cause of yield variability, but it is information we need to know. Data on pH, organic matter, CEC, phosphorus, and potassium can be very helpful. We are finding that pH in particular is a yield reducing factor in a number of situations and can effect certain soil/chemical reactions. Whether a field is gridded into squares (two to three acres) or by soil type depends on the producer and particular field characteristics. Look for areas where lower yields may be from areas that have high fertility. What could be the limiting factor(s) in these areas? Refer to OSU Agronomy Report 9302 "Grid Soil Sampling and Fertilization" for more information.

4. Compaction

Operating equipment on a wet soil can compact the soil, destroy its structure, and reduce crop yield. A compacted soil layer will generally have poor structure, and most of the voids in this layer will have been eliminated. Voids are open spaces between soil particles filled with air, water, or a combination of both. Soil water will tend to accumulate above the compacted layer because movement of water through the compacted area is severely restricted. Root growth is also restricted, which reduces tolerance to dry weather. If the compacted layer is located at the soil surface, very little water will enter the soil and much of the water will run off, potentially creating a flooding and/or erosion hazard. Compacted areas may be hard to define on a yield map, but keep in mind areas of heavy traffic and equipment operation in wet conditions.

II. Naturally Occurring Variables

1. Weather

Weather is the largest factor affecting crop yield. It is possible to receive a large amount of rain in one corner of a field and a smaller amount in another section. Consider investing in a weather station and collecting weather data by field or by area in very large fields rather than relying on a weather station some distance away. Compare several years of weather data. Temperature and Growing Degree Day data on a year to year basis are also valuable.

2. Soil Physical Properties and Water Management

Many times yield variability can be related to drainage and water management. Agricultural drainage is the removal of excess water from the soil surface and/or soil profile of cropland, by either gravity or artificial means. The two main reasons for improving the drainage on agricultural land are for soil conservation and enhancing crop production. Research conducted in Ohio and throughout the Midwest has documented many benefits of agricultural drainage improvements. Drainage probably causes more variability in yield than any other factor. Environmental conditions control a significant amount of the crop growth potential compared to what we can control. In Ohio, a good drainage system is our best economic practice for managing soil moisture. Refer to Ohio State University Extension fact sheet AEX-320 "Understanding Agricultural Drainage" for more information. Soil survey maps, topography, tile location, and drainage patterns are very important pieces of diagnostic information. Note compaction related problems from farming in wet years which can affect drainage patterns.

3. Pests

Maps or even general recorded information on weed, insect, and disease patterns in fields can be very valuable in yield map interpretation. Field scouting information of events that occurred during the growing season is an important piece of the diagnostic puzzle.

4. External Variables

What is adjacent to or in the vicinity of the field? Factors such as windbreaks, bodies of water, buildings, fence rows, and trees can all create effects that can influence crop yield.

In general, investigate the conditions at the highest and lowest yield points in a field. What are these conditions and can they be repeated? What is the size of these areas in relationship to the whole field and are they significant? Don't worry about all the little changes. Look for trends and not absolute bushels. Record and map all information. We learn just as much or more from a stress year than from a year with high yields. Don't be too quick to jump to conclusions. Involve others in this interpretation process. Remember, better information results in better decisions and the yield monitor is just one piece of the precision farming/information gathering system.

Yield Map Limitations

It is easy to place too much importance on a yield map. While yield maps show variability in a field, meaningful comparisons to base decisions on can be hard to come by. Furthermore, variability in yield can be the result of several characteristics rather than one. In some instances, it can take at least five years before a meaningful management decision can be made. Do not jump to conclusions! Some short term decisions can be made, but longer term decisions are tougher. The worst case scenario is that some fields may have several years of yield maps with no consistency from year to year. Also, yield maps of the same field from different mapping software companies can look very different. Changing the yield range (increase or decrease) assigned to a specific map color can also change the look and therefore the subsequent interpretation of a map dramatically. Some of the questions that need to be considered when purchasing mapping software include: system specifications, software installation and support, data handling, and map generation quality. Be certain the software/data will be compatible with newer versions or technologies as they are developed. When you buy a yield monitor, realize that this technology is in its infancy and improvements will come rapidly. Companies are working on the development of sensors that can measure physical grain quality such as cracks, splits, color, and such chemical properties as protein, carbohydrate, and fiber content.

Decision Making

The type, amount, and quality of data produced on the farm is dramatically changing. And, as precision farming technology becomes more developed and user friendly, there will be volumes of data available to the producer for decision making processes. Producers will be forced to sift through these data and decide what information is most relevant for their purposes. They will have to set priorities! Steps in the decision making process include:

1. data collection,

2. data interpretation,

3. decision making,

4. implementation of a plan, and

5. evaluation.

The yield monitor is involved in the first step of this decision making process. The yield map is involved in step two.

So, what strategy should be used to implement management practices based on a yield map? As producers contemplate purchasing yield monitors, they should first determine just how involved they want to become in this total precision farming effort, just how intensely they want to manage, and what their short term and long term goals are. Do you have or know your business priorities? Do you make decisions with priorities in mind? Do you usually "go with your gut"? Do you believe you are qualified to make management decisions? Do you believe you are qualified to make precision farming management decisions?

Change the obvious first. This could include better equipment maintenance to correct poor application of inputs like seed, fertilizer, and chemicals. Work primarily on the inputs you can change (not necessarily the ones that are very difficult) and the ones that have the most impact on economics such as nitrogen on corn or varying herbicide rates and product. If you are interested in learning more about decision making and farm planning, contact your local Extension agent concerning one of the management "Excel" programs.

Acknowledgments

Nathan Watermeier, OSU Extension; Randal Reeder, OSU Extension Engineer; Erdal Ozkan, OSU Extension Engineer; Jay Johnson, OSU Extension Agronomist; Tom Krill, Van Wert County Extension Agent; John Cardina, OARDC Horticulture and Crop Science; Dennis Wickham, Pioneer Seed; Dick Heilmann, Top Soil Testing Service; Mark Kingen, Terra Inc.; Ohio State Precision Farming Task Force Members