(A) At the beginning of a graze period, all of the forage represented by the shaded area could be consumed in a short period and meet the animals’ requirements, maximizing harvest efficiency for that level of nutrient intake. (B) As a graze period progresses, higher quality components are selectively removed, decreasing the proportion of quality remaining and the amount still available that will meet the animals’ requirements. Figure is after Rittenhouse and Bailey (1996).
To change the differential effects among plants resulting from selective, severe and/or repeated defoliation, the timing, frequency, intensity, and/or distribution of defoliation among plants must change. If changes in infrastructure and/or management DO NOT change one or more of these parameters in the correct way, and to a sufficient degree IN THE AREAS WHERE SEVERE OR REPEATED DEFOLIATION OCCURS, no changes in plant composition, productivity or ecological function should be anticipated. However, with sufficient recovery between graze periods, even when degree of use is severe, differential and beneficial responses in the plant community have been noted as a result of enforced growing season grazing deferment of plant communities (e.g. Reardon and Merrill, 1976; Teague and Dowhower, 2003; Teague et al, 2011; Ash et al, 2011). In areas where repeated or severe defoliation of plants is not occurring (e.g. very low stocking rates with equitable distribution in time and space across a landscape), few or no detrimental effects on plant communities would be anticipated as a result of grazing, regardless of management strategy (Norton, 1998), though to achieve this type of distribution and degree of use on diverse landscapes, would likely require intensive management of distribution that may be costly in labor, infrastructure development, or both.
Severe defoliation (> about 50% of leaf material for many species evaluated in the scientific literature) causes a decrease in photosynthetic material that can affect the plant’s ability to compete with neighboring plants that have not been defoliated to a significant degree (Briske 1991) and, in many cases, loss of root mass (e.g. Crider, 1955).
Forage demand and AVERAGE grazing intensity for a paddock is a result of stocking intensity (forage demand per unit of forage in a paddock for the length of a grazing period) (Gammon, 1984) as well as stocking rate (forage demand per unit of area for a grazing unit over the length of a grazing season), both of which are a function of the number of animals per unit area (or unit of forage) and the time they are on that area. Therefore, forage demand for a paddock over the course of a grazing period is a function of the relative rates of change in stock density compared to graze period length (Steffens et al, 2009). However, very palatable plants will probably be severely defoliated when encountered, even when forage demand is relatively low, especially if exposed to grazing animals for extended periods of time. Therefore, the frequency with which these plants are severely defoliated, particularly if they are found in preferred areas of the landscape, is probably a function of the length of grazing period and the number of grazing periods in a growing season or year.
Timing of defoliation can increase the effects of defoliation on a plant and varies with species of plant, with defoliations that occur after grasses elevate the apical meristem generally considered to be the most critical point during the growing season in North America and dormant season defoliation being a less-critical period (e.g. Caldwell et al, 1981; Mullahey et al 1990, 1991; Reece et al 1996).
Following severe defoliation, a period of recovery will be required – that varies with severity of defoliation, plant species, and growing conditions – for a plant to re-establish photosynthetic tissue and regain lost vigor (Trlica et al 1977; Caldwell et al, 1981; Menke and Trlica; 1983).
In order to recruit new individual plants into a plant community, young plants need time to establish without severe defoliation that can cause mortality (Kothmann; 2009 ). The length of time required for germination and establishment may be relatively short in more mesic environments (e.g. Limb et al; 2011), but limited rainfall and brief, erratic periods of significant growth in more arid environments (Torell et al, 2011) may require much longer recovery between grazing periods, both to improve vigor of existing defoliated plants and to facilitate event-driven recruitment events (e.g. Woodmansee and Potter; 1971 ). This type of management prepares a system to respond to unusual climatic events and facilitates the improvement of the resource as described by Watson et al (1996).
Free roaming grazers do not use landscapes randomly, as a result of past experiences (Provenza 2003a,b), concentrations of nutrients/palatable plants (e.g. Senft et al; 1985), topography and proximity to water (e.g. Roath and Krueger 1982), etc. and will frequently sample large areas of the landscape, often intensely, to find and, when found, possibly repeatedly defoliate particularly palatable plants if allowed (Bailey and Brown; 2011 ). This can create areas on the landscape where desirable plants are routinely and intensely used (thus increasing the effective stocking rate on those areas). These intensely used areas often become focal points for resource degradation that may take the form of lower productivity, encroachment of toxic, low quality, or woody species, increased bare ground and erosion, etc. These areas of degradation may increase in scale over time (Ash and Stafford-Smith; 1996), even when average stocking rate for the landscape or stocking intensity for a paddock are moderate.
The ability of an animal to meet its nutrient requirements over a period of time is a function of the quality and quantity of forage available to the animal over that period of time. Animals can mix plants of differing quality in an attempt to meet their requirements and will normally consume a diet of higher quality than the average of the plant community available to them if given the opportunity for selection.
Nutrient intake must be sufficient to meet nutrient requirements if animals are to meet target individual performance objectives. Therefore, forage demand compared to quantity, quality and diversity of herbage must allow adequate selection and intake to meet performance goals, both within and among grazing periods and paddocks, when employing multiple paddocks per herd. Animal performance declines at higher stocking intensities and improves at lower stocking intensities. Stocking intensity for a grazing period will increase when the grazing period decreases less, proportionally, than paddock size. However, if grazing period decreases faster, proportionally, than pasture size decreases, stocking intensity decreases and nutrient intake over time can be improved (Steffens et al; 2009), provided that animals have learned how to perform in that environment (Provenza 2003b, 2008) and recovery between defoliations is sufficient for plants to again grow sufficient herbage before being grazed again.
If animals use a greater proportion of the landscape (Norton; 2003) or learn to select a greater variety of plants or plant parts (Provenza et al. 2003), and plants are given sufficient recovery between defoliations to maintain or improve vigor, stocking rate can be sustainably increased while maintaining adequate performance with any grazing strategy.
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