When to turn out cattle onto pasture in the spring is a critical decision that producers make. There are several factors producers should consider before spring turnout, including hay and feed reserves, timing and order of pasture grazing and pasture grazing readiness. Air temperature is the main environmental factor that influences the rate of plant development. Growing degree days (GDD) are the temperature or heat units that a plant needs to accumulate to begin leaf development. Pasture grazing readiness is based on the number of GDD necessary for grass species to reach the 3 to 3.5 leaf stage, and thus be better equipped to withstand grazing pressure. If graced too early, production for those pastures can potentially be reduced for the remainder of the growing season. The date when these growing conditions occur can vary by location, year and plant species.
Grazing Readiness Tool
The South Dakota Grazing Readiness Map was developed to help livestock producers understand the earliest and range of spring turnout dates on their operation (Figure 1) and compare these dates between two native cool-season grasses (Green Needlegrass and Western Wheatgrass), a native warm -season grass (Blue Grama) and an introduced cool-season grass (Crested Wheatgrass).
Data for the grazing readiness tool was calculated using the PRISM 40-year dataset. Pixel size for each map is 4 km2. For each year and each pixel, the date which the cumulative GDD reached the required amount for grazing readiness was calculated for each species (Table 1).
Base temperature for cool-season grasses is 32˚F and 40˚F for warm-season grasses. The average turnout date is the 40-year average day that enough GDD had accumulated for grazing. The required number of GDD for each species and the base temperature was based on previous work conducted at the US Department of Agriculture (USDA) Agriculture Research Service at Mandan, North Dakota.
|Table 1. The number of cumulative growing degree days (GDD) for Haun Development Stage for four common species found in SD|
|weed||3-Leaf Stage||3.5 Leaf Stage|
Users can select the grass species of interest using the tabs on top of the tool. Each map takes about 30 seconds to load and displays the latest, average and earliest spring turnout dates based on GDD for each 4 km2 pixel. Users can zoom into the map and toggle layers on and off for display. Upon clicking within a pixel on the map, a pop-up displaying the spring turnout dates for that grass species and pixel are displayed (Figure 2). This allows users to quickly identify the average spring turnout date for their location and the range of possible dates between species. The ‘earliest’ is the earliest date over 40 years that enough GDD have accumulated for grazing. The ‘Latest’ is the latest date over 40 years that enough GDD have accumulated for grazing. For instance, the pop display of the South Dakota State University (SDSU) Cottonwood Field Station shows the average turn-out date for Green Needlegrass pastures was May 29, but has ranged from May 15 as the earliest date to June 7 at the latest date . Depending on the year and growing conditions, the expected turnout date may vary by a month.
Overall, the South Dakota Grazing Readiness Map is one tool available to producers when they begin making plans for the grazing season. Additional factors producers might consider include management objectives. For example, producers may want to turn cattle out early in pastures to reduce cool-season invasive grasses. The key to utilizing this tool is outlining your management objectives and understanding the underlying species composition of pastures. Another way that producers can utilize this tool is to keep grazing records of when they turned into a pasture that was crested wheatgrass dominated or western wheatgrass dominated, etc. – by recording turn in dates every year, producers can begin to see trends on their individual operation and understand them in the greater context of the 40-year average, as presented by the South Dakota Grazing Readiness Map.
– SDSU Extension