Every year at green-up, grass managers must make decisions about when and where to begin grazing. Following a relatively open winter, hay reserves are higher than many years, so delaying turnout probably won’t require additional feed purchases. The tradeoff to be examined is between continuing to feed, delaying the impact of grazing on pastures versus turning out, relieving the cost of feeding. The cost of turning out too early is the reduction in forage production which results. North Dakota research suggests that beginning to graze too early can reduce total production for the year by more than 50 percent. In terms of the total feed budget, it may be prudent to continue feeding as an investment protecting future pasture production.
Research suggests that grass plants are most vulnerable before they have formed three new leaves. For introduced grasses, this stage generally corresponds to the accumulation of about 500 growing degree days (GDD, base 32 degrees after March 1). For many native cool-season grasses, about 1200 GDD are required to reach the three-leaf stage.
The calendar date when these growing conditions occur varies considerably from one location to another and from year to year. For example, examining climate data from two weather stations located in southwest South Dakota, the average date at which 500 GDD accumulate is April 28 at Oral and May 3 at Nisland. At Oral, 500 GDD accumulated as early and April 8 recently. The normal date for 1200 GDD is May 28 at Oral and June 1 at Nisland. This has occurred recently as early as May 11 at Oral and May 21 at Nisland. 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 the South Dakota State University Website. This site allows users to specify parameters for the calculation of growing degree days.
Knowing how many growing degree days have accumulated provides a general “rule-of-thumb” about plant development. It is more precise and may be easier to examine the important plants in your own pasture. For tame pastures, examining crested wheatgrass or smooth bromegrass plants might begin as early as mid-April. Mid-May might be a reasonable starting date to start examining native cool-season grasses such as western wheatgrass and green needlegrass. Access to pastures planted with introduced cool-season grasses provides early season flexibility and avoids early grazing on native pastures which may compromise production later in the season.