The first attempt to set a scientific grazing start date for the native grasslands of the Northern Plains was determined by concensus of many highly experienced range scientists that selected 15 June as a result of interpretation of data collected on a four year grazing study conducted at Swift Current, SK (Campbell 1952) and a four year clipping study conducted at Mandan, ND (Rogler et al. 1962) that determined the percentage of herbage biomass reduction at 6 different defoliation start dates compared to the potential peak herbage biomass on an undisturbed control site.
The scientifically determined grazing start date for Northern Plains grasslands was 15 June until the late 1990's, when a physiological growth stage of grasses was found that had negative effects from defoliation before and positive effects from partial defoliation after. Scientists at the Mandan ARS station were conducting several years of very detailed grass growth stage studies to better understand grass development. They determined that second year young lead tillers cease production of leaf primordia in the apical meristem at the third new leaf stage and are actively producing flower primordia by the three and a half new leaf stage (Frank 1996, Frank et al. 1997).
Thus, before the third new leaf stage, grasses are negatively affected by partial grazing defoliation and after the three and a half new leaf stage, grasses are positively affected by partial defoliation that removes 25% to 33% of the lead tiller aboveground herbage weight (Manske 1999).
During 1983 to 1989, Manske conducted a grass phenological development study that measured leaf height and percent leaf senescence of every leaf of 10 ungrazed vegetative and reproductive tillers, and also recorded flower stalk growth stages biweekly from June through August on sandy, shallow, silty, and clayey ecological sites with both grazed and ungrazed treatments from seasonlong and twice-over grazing strategies and on the long-term ungrazed control.
This study was able to determine that the major native cool season grasses reached the three and a half new leaf stage just before, near, or on 1 June, prairie sandreed was the first warm season grass to reach the three and a half new leaf stage soon after 1 June, and the other native warm season grasses reached the three and a half new leaf stage by mid June. The grazing start date on Northern Plains native grasslands can be 1 June each year.
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