US - Graziers striving for developed pastures in Dakota this spring can follow the three leaf rule.
Choosing when to turn cattle out is a multifactorial decision but can be informed by waiting for plants to grow three new leaves, say advisers.
Research shows plants before this developmental stage are at their most vulnerable, with South Dakota State University advising that native cool-season grasses need more time than introduced varieties.
Turnout can be timetabled according to dominant species if growing degree days (GDD) are known, according to Professor Pat Johnson.
Generally, native cool-season grasses require 1200 GDD and introduced varieties need about 500 GDD.
“Knowing how many growing degree days are required to reach the 3-leaf stage provides a general “rule-of-thumb” about plant development,” said Mr Johnson.
“The date that grasses reach the 3-leaf stage varies considerably, so examining the important plants in your own pasture is recommended.”
One method takes the number of growing days from April 1 onwards as determined by days when temperatures are above freezing – 32 F.
However, even within the state of South Dakota there can be over a week’s variation as to when this is reached.
Professor Johnson said: “Climate data from the weather station at Oral, SD indicates the average date at which 1200 GDD accumulate is May 28 For Nisland, SD, average date for 1200 GDD is June 1.
"Recently, 1200 GDD accumulated as early as May 11 in Oral and May 21 for Nisland.”
He also warned producers to avoid grazing the same pasture at the same time in concurrent years due to “defoliation pressure”.
While he acknowledged some operations have little choice, it is a good rule of thumb to vary where cows and calves are.
“If a pasture is grazed at the same time every year, the vigour of plant species which are most vulnerable at that season will be reduced and they may eventually be eliminated from that pasture,” concluded Professor Johnson.