Critical dates are timely monitoring points in annual management cycles. Current and predicted forage resources are the primary focus of critical dates.
Each critical date should have an action plan which clearly states target points for initiating the plan.
Target points may be based on carrying capacity of current forage or a percentage of average precipitation, ie, 75%.
Action plans primarily focus on financially and ecologically efficient ways to change stocking rates at one or more times during the year.
"There is a point when a high percentage of the feed that’s going to grow that year has already accumulated. And it’s not August 15th, it’s before that. And our inclination, human nature says well if I just wait a little longer, surely we’re going to get a rain. But the effectiveness of that rain in late July and early August is very small compared to the total feed supply." Roger Gates
Selecting Critical Dates
Selecting critical dates and developing action plans requires an understanding of how environmental variables and grazing management affect herbage and animal production in your area.
Midpoints of rapid-growth windows for dominant cool-season and dominant warm-season grasses are excellent critical dates for implementing different phases of drought management plans. Precipitation and soil moisture reserves are most important just prior to and during the rapid growth windows of dominant forage species.
The term “windows” is used to emphasize the fact that rangeland plant species are not capable of growing rapidly over the entire grazing season. These windows are primarily defined by air temperatures and cumulative heat units (degree days).
Air temperature and soil water must be simultaneously favorable for rapid growth. Optimum temperatures are different for different species. Critical dates will be earlier for cool-season forage resources compared to warm-season species.
Rangeland plant communities are naturally composed of mixtures of species that primarily grow at different times during the spring or summer. Grasses are classified as cool-season or warm-season species generally based on their growth response to air temperature.
Maximum growth rates of most cool-season (C3) species occur when air temperatures are 65 to 75F compared to 90 to 95F for many warm-season (C4) grasses. Considerable variation occurs in the range of air temperatures over which measurable growth occurs within each season-of-growth category.
Rapid growth occurs in grasses when air temperatures and soil water are simultaneously favorable. Rapid rates of herbage production are associated with stem elongation. Optimum air temperatures differ among species. For example, 50 to 80 percent of the annual herbage production of most species occurs during a 30-day time period for mid- and tall-grasses on semiarid rangeland.
These rapid-growth windows are best defined by growing degree days. The sequence of rapid-growth windows for different species is the same each year; however, initiation of rapid growth may change by one to two weeks as cumulative degree days change from year to year.
In the Great Plains, plant rapid growth (recovery) period lengthens from west to east as average annual rainfall increases, and from north to south as length of growing season increases.
Many semiarid rangelands are composed of mixtures of cool and warm-season species. Consequently, it is often helpful to select two critical dates when most species of each growth-season category have headed to evaluate the contribution of each component to total herbage production.
How do you know when you’ve reached a target point? By gaining a solid understanding of your ranch’s precipitation patterns, how much forage your pastures/ forage resources can produce, when the dominant forages in your pastures are growing, and how many AUM’s your livestock enterprises require. You may need to consult with an advisor to help you complete this step.
In general, drought management plans for semiarid regions are implemented when cumulative plant-year precipitation is 20 to 25% below average on critical dates. Livestock producers in subhumid regions may select precipitation deficits of 30 to 35% because of relatively high yield responses to precipitation.