How can you make well-informed decisions for summer grazing? We all know that 2020 was a drought year in Colorado. Soils are dry and will need to be refilled. When do we need the moisture for summer grass? How can we plan for the 2021 grazing season?

Cooperative Extension research across the High Plains found that cumulative precipitation up to 30 days before peak grass growth has the most impact. For Eastern Colorado, peak grass growth is in June and July. That means the moisture received to mid-June sets the stage for summer grass growth. This is also true for SW Nebraska and Western Kansas as well as short grass prairie and sandy sites in southeast Wyoming and the southern part of the Nebraska Panhandle. Where Needle and Thread and Western Wheat grass dominate the range in southeast Wyoming/northeast Panhandle, peak growth shifts to May-June. Thus, the critical cumulative precipitation date moves up to mid-May.

But it would be helpful to anticipate what might in store before May and June. Results from 70 plus years of research from the Agricultural Research Service range in Nunn may help. It turns out that long-term climate trends such as La Niña/ El Niño and the Pacific Decadal Oscillation (PDO)**, are important. In fact, these ocean temperature cycles explain 70 percent of yearling weight gain differences!

Researchers used this data to create a decision tree to help them make decisions before the growing season. They watch the El Niño/ La Niña and PDO cycles and moisture conditions through the winter. Then in early April, they use the decision tree as part of their stocking discussion.

Let’s look at the decision tree for 2021. The Pacific Decadal Oscillation is still in the warm phase. Forecasters report that La Niña is strong. (La Niña is correlated with below average winter moisture for Colorado.) The decision tree suggests that the stocking rate should be decreased relative to moderate. Ocean activity indicates that drought conditions are likely to persist.

Another tool ranchers can use is Grass-Cast. Grass-Cast, or Grassland Productivity Forecast, has over 30 years of historical data about weather and vegetation growth. It compares that data with current year precipitation to create three production forecast maps. Each map indicates the expected grass growth based on above-normal, near-normal or below-normal summer rain.

The first Grass-Cast maps for 2021 will be released in April. These maps can identify areas where there are early signs of opportunity or challenges. The bi-weekly maps become more accurate as more rainfall is recorded. By May 30, the average accuracy of the maps improves to 70 percent.

The decision tree, understanding timing of moisture and grass growth and Grass-Cast are all useful tools to help ranchers make timely grazing decisions. Responding early to drought can help protect financial and rangeland resources into the future.

Learn more about the drought decision tool here: https://bit.ly/3it5AsL or by typing “early warning stocking decisions” into an internet browser.

Learn more about Grass-Cast at https://grasscast.unl.edu/.

** La Niña/ El Niño is also known as El Niño-Southern Oscillation or ENSO. It results from changing sea surface temperature of the Pacific Ocean off South America near the Equator. El Niño (the warm phase) tends to give wet conditions on the plains. La Niña (the cool phase) is usually dry. The neutral phase does not drive precipitation on the Plains. Phases can last less several months up to three or more years.

Pacific Decadal Oscillation refers to changing sea-surface temperatures in the more northern Latitudes of the Pacific Ocean. The warm phase consists of a boomerang of warmer-than-average seas along the coast of western North America with a pocket of colder-than-average waters in the central North Pacific. The cool phase is the opposite. Phases switch on a 10-30-year cycle.