Spring development of pasture grasses
April 27, 2009
With the most recent “Earth Day” still a very recent memory, the phrase “suddenly green” may bring to mind very different images among Tri-State area ranchers than it does for more urban Americans. Nonetheless, I’ve been most impressed this year with the rapid response of vegetation to changing growing conditions. In a matter of weeks the landscape has gone from white to green, with very few days of brown intervening. As springtime demands and opportunities propel us into “activity overload,” it would be prudent to consider the possibilities the coming growing season will provide and make flexible plans that will allow us to adapt rather than react.
Following a long, hard winter, with hay supplies dwindling or exhausted, the incentives to turn livestock out to pasture is strong. Exposing grassland vegetation to grazing without a sufficient “head start” can compromise total growing season production. Pasture plants depend on energy stored in the previous growing season to initiate spring growth. New leaf tissue will soon develop the capacity for photosynthesis and the ability to capture the energy of sunlight as “fixed carbon.” Just like my checking account, however, the deposits must meet or exceed the withdrawals for there to be a balance. Starting grazing before the plant is in a positive energy balance will reduce season long production. Research suggests this impact can be on the order of 35 percent.
On recent “strolls,” I’ve been looking at the development of grasses, primarily smooth bromegrass, anticipating inquiries about when it’s appropriate to turn out on pasture. Most of the plants I’ve looked at have three fully formed leaves. Research indicates that grass plants are in a positive energy balance when four leaves are fully formed. For tame cool-season grasses, like smooth bromegrass or crested wheatgrass, formation of four leaves generally requires about 500 growing degree days (base 32 degrees). The results of my “strolls” are consistent with the weather data from the Rapid City Airport. Through April 23, we had accumulated 482 growing degree days. In contrast, heat accumulation at the Cottonwood Research Station was already 550 growing degree days. Both Rapid City and Cottonwood were slightly ahead of normal accumulation. On the other hand, accumulated growing degree days at the Antelope Research Station in Harding County were only 379, somewhat behind normal.
Native cool-season grasses like western wheatgrass and green needlegrass develop more slowly and generally require about 1,200 growing degree days to reach the four-leaf stage.
Growing degree day data for South Dakota weather stations can be accessed using the SDSU Climate web site. A “Growing degree days querying module” is available at: http://climate.sdstate.edu/climate_site/archive_data.htm. This site allows users to specify parameters for the calculation of growing degree days. Similar climate data can be accessed for adjoining states.
Calculation of growing degree days provides a helpful “rule of thumb,” but the best evidence will be a stroll through your own pasture, examining plants to see how many leaves are fully formed. Turn out decisions should be based on average conditions, not the exceptional, early developing plant.
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A second possibility that may require adaptation of management plans is a late freeze. Research data from the Cottonwood Research Station indicates that for cool-season dominated native pastures, freezing temperatures in late spring can substantially reduce season-long production. The later the freeze occurs, the greater deficit in production results. Production is reduced almost 50 pounds per acre for each day later in the year that a freeze occurs. Having a stocking plan that will adapt to reduced forage production due to a late freeze should be in your planning repertoire.
Just as a late freeze reduces forage production, rainfall deficits reduce vegetation growth. While moisture supplies have been adequate or better for the last 18 months in much of the Tri-State region, severe drought is always an impending possibility. Research from both the Cottonwood Station and the USDA-ARS Ft. Keogh Station in Montana demonstrate that for the cool-season dominated native rangelands in our region, season-long production is highly dependent on rainfall in April, May and June. Producers need to follow rainfall accumulation through this period closely and make stocking adjustments early in summer, rather than postponing a decision until it becomes a crisis.
Fortunately, current conditions are favorable. The most recent Drought Monitor displayed D0 (unusually dry) conditions only in extreme northwest North Dakota, and D0 designation was removed from the Nebraska Panhandle. Favorable growing conditions this year might also warrant a flexible plan. Consider ways that you might merchandise abundant vegetation growth if it materializes. Leasing pasture, with or without management, purchasing yearlings, developing replacement heifers or leasing cows might be possibilities. Thoughtful planning and clear negotiation should lead to a written contract for any lease or rental arrangement.
If you’ve read this far, thanks for your attention. Thanks also if you’re one of the land stewards who husband our country’s grazinglands so attentively. In contrast to some Americans who celebrate Earth Day once a year, I’m indebted to ranchers who practice Earth Day all year round. I brag on you every chance I get.