Selecting the Right Season to Maximize Grazing Effects
Plant phenology, or how plants grow through the season, should be considered when using grazing to manage vegetation. A plant’s growth stage will determine how it responds to grazing. For example, most grasses and forbs tolerate early-season grazing, a time when soil moisture and nutrients needed for regrowth are abundant. Apical meristems are close to the soil surface at this time and less likely to be removed by herbivores, so leaf growth from stems or shoots can continue unabated after grazing.
Early in the growing season, plants need fewer nutrients because they are smaller with fewer leaves and stems. Losing leaves and reducing the ability to capture sunlight early in the season is less damaging than later in the growing season when energy demands are higher. For these reasons, grazing early in the season may have little effect on the plant community. However, many perennial plants have large root systems to support. Spring may be a poor time for controlling invasive herbaceous plants unless they grow and mature early in spring. The effects of early-spring browsing on shrubs are less well researched than for grasses and forbs. As with herbaceous plants, shrubs often tolerate earlyseason grazing because water and nutrients needed for regrowth are readily available.
Plants are most likely to be damaged by grazing at specific stages of development. Generally, a plant has the most difficulty recovering if it is grazed or browsed between the time when the flowerhead is ready to emerge (boot or bud stage) and full bloom. Grasses are most susceptible to grazing in the boot stage when the developing, elongating flowerhead is causing the stem to swell, often bulging where the flowerhead is forming. For example, wheatgrasses grazed after the stem starts elongating and the flowerhead begins to emerge produced fewer new shoots the following year than when grazed earlier in the season, although the exact time when grazing is most detrimental varies by species.10, 15 Likewise, forbs are most susceptible to grazing when stems are elongating and exposing the developing flowerhead – called the bolting stage.
Annual grasses require seeds to develop new plants. Defoliating grasses to limit seedstalk production can help reduce the numbers of seeds in the soil (the seedbank) and may decrease their density in the vegetative community. Biennial plants have a rosette stage in one year followed by a seed production stage the next year. As with annual plants, biennials need regular seed production to maintain populations. However, plants with a long-lived seedbank can be more difficult to control because the seeds can remain dormant in the soil until environmental conditions are favorable for emergence.
If the newly formed flowers and seeds are removed, the regrowth a plant needs to regain its ability to capture sunlight and synthesize carbohydrates must come from expansion of existing leaves or from new stems and leaves initiated by axillary buds. In many parts of the arid West, defoliation during the boot or bolting stage can damage plants because it coincides with a time in the growing season when water and nutrients required for regrowth are becoming limiting. This window of susceptibility for grazing target plants – generally in the boot stage for grasses and the bolting stage for forbs – typically occurs six to eight weeks before seed set.