Prevailing conditions often are classified as conditions of soil, climate, geophysical characteristics, and biotic stresses. With these four major prevailing condition areas in mind, the next step in wisely selecting a grass, legume or other species to use as forage is to determine the things that will definitely limit the choices. These things are limitations or factors so important that they will remove certain species as options. Limitations can be either fixed, meaning that they cannot easily be changed or variable, meaning that they may be changed with inputs and/or management. Deciding whether a limitation is variable or fixed is sometimes simple and sometimes complicated. It can vary from farm to farm and person to person in some cases. Let's look at some examples.

Fixed limitations are factors that cannot be changed. This would include climatic elements such as: snow cover, temperature, humidity, day length, and length of growing season. Although they may vary from year to year, there are definite patterns and the farmer/rancher can do little to change them. Geophysical elements of elevation and aspect often are fixed as well. Determining the fixed limitations means to specifically list the prevailing conditions.

For example:

  • Temperature range: 15 - 100 degrees F (-9.4-37.8 degrees C)
  • Snow cover: sporadic
  • Relative humidity: low
  • Average day length: 12 hours
  • Average growing days: 211 days
  • Average maximum temperature: 100 degrees F (37.8 degrees C)
  • Average minimum temperature: 10 degrees F (-12.2 degrees C)
  • Average precipitation: 36 inches (91.4 centimeters)

Limitations also can be variable, which indicates that they can be changed and may include biotic stresses, fertility, and pH. Variable limitations often are management related: adding fertilizer, grazing or mechanical harvesting, frequency of herbage removal, use of skills and/or equipment. A farmer/rancher needs to be realistic about his/her abilities to handle variable limitations. The farmer/rancher needs to know the cost in money, equipment, and time of handling a variable limitation. Experience also will factor into a wise decision. The predisposition of the forage manager is important to consider. What he/she is comfortable with and/or knowledgeable about will determine, in large part, what is accomplished.

Carefully determine if limitations are fixed or variable. Rainfall may sound fixed but with irrigation it is possible that the amount of water the plants receive can be variable. The range of pH can be variable because adding lime to soil can change the pH. Fertility can be improved with added nutrients, crop rotation, manure, and other options.

Some factors can be variable or fixed depending on further details. Rainfall can be fixed if irrigation is not possible because of cost or water rights. Liming may change pH but the cost or results may not be economical and so pH becomes a fixed limitation. Temperature may be fixed in most cases, but snow cover can insulate plants and low temperatures may not be as extreme as they may seem, so snow cover may influence a selection that seems unrealistic for the temperature range.

In general, all fixed limitations must be considered in selecting a forage species. Variable limitations may be changed with inputs or management but the costs must be considered.

Wise selection also recognizes that climatic and biotic conditions usually cover quite an area but soil and geophysical conditions can vary greatly from farm to farm. Some farms have quite a variety of soil and slope conditions. Understanding the local insect, disease, and weed situations is important as well. Pests and their damage potential often are geographically distinctive. Weeds are more widespread but selection and proper establishment of a forage crop can reduce many weeds. Forage crops are harmed by weeds that cause thinning or weakening by competition. But forages can be selected to be more aggressive in seedling growth, to tolerate herbicides, be more resistant to clipping, even to thrive under a canopy of weeds that covers the forage crop causing excessive shade. Plant breeders have worked to develop forage varieties that can specifically deal with almost any limitation.

Knowing the specific traits of the 30 - 40 forage species is a great beginning to selection. Make a list of the fixed and variable limitations. The chart below will help you set up the pertinent information.

  • Species Characteristics, (life cycle, growth habits, nutritional attributes)
  • Location Characteristics,(slope, flooding possibilities, traffic potential)
  • Tolerance,(frost, flooding, drought)
  • Optimum, (for precipitation, sunlight, snow coverage)
  • Average, (precipitation, snow coverage, yield, pH)
  • Maximum/Minimum (precipitation, growing days, pH)


Use the list of fixed limitations to eliminate species that cannot grow productively under these fixed factors. From the variable limitations, determine if any should be moved to the fixed limitations. From that foundation selection continues to be even more specific with the study of species variety.