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Predict Grass Growth

04/08/2014 @ 1:02pm

Predicting grass growth for the season could be as simple as tallying April, May, and June rains. If these months yield a normal amount of moisture, expect normal grass growth on rangeland. If moisture is below normal, take steps to manage for drought.

This is a particularly apt rule of thumb for the northern Great Plains. A study of century-long precipitation records for South Dakota shows spring is the season bringing the most moisture.

The records also show drought is common for the region. Below-normal, normal, and above-normal spring precipitation occurred 29%, 48%, and 23% of the time, respectively. The below-normal moisture of drought was categorized by rainfalls providing 75% or less of an average amount for spring.

More recent analyses show how annual grass growth correlates with this spring rainfall. One 15-year study showed relationships between spring precipitation and forage yield from continuously grazed pastures stocked at light, moderate, and heavy grazing densities.

“Across the grazing treatments, average forage yield in years of spring drought was 420 pounds per acre less than in years when there was no spring drought,” says Sandy Smart, South Dakota State University range scientist. “This translates to an average decrease in stocking rate of 0.18 AUM (animal unit month) per acre.

“Because research shows such a tight relationship between spring rainfall and forage production, you can anticipate grass growth for the season if you know what precipitation is for April, May, and June,” he says. 

On the northern Great Plains, spring precipitation plays a greater role than summer in determining annual grass growth. That’s because cool-season grasses typically dominate the plant community and reach their peak by July. “One set of data shows that 91% of annual forage on the northern Great Plains is produced by July 1,” notes Smart.

An accounting of spring rain can trigger steps in a drought-management plan. “If rainfall is below normal by May 1, you can expect to run into some reduction in grass growth for the season,” he says. “By anticipating decreased grass growth as early as the first of May, you have time to make management adjustments to allow for drought. The earlier in the season you can make these decisions, the better.”

July 1 offers another trigger date. If rainfall remains below normal, you have time to implement even more stringent drought-management tactics, like destocking or weaning calves early.

The degree to which drought reduces grass growth depends on long-term stocking densities. “In a year of spring drought, pastures historically lightly stocked will experience a 20% reduction in forage,” says Smart. “Pastures moderately grazed will see a 27% drop in forage; pastures heavily stocked will produce 34% less growth.

“Our data show that during years of spring drought, pastures historically lightly stocked had 1.9 times more forage than heavily stocked pastures,” he adds. “That means there is more water-use efficiency occurring on well-managed land. Pastures with a history of being lightly stocked have more resilience.”

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