Forage quality in relation to long-term grazing history, current-year defoliation, and water resource
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Authors: D. G. Milchunas, A. S. Varnamkhasti, W. K. Lauenroth, and H. Goetz
Date: 1995
Journal: Oecologia
Volume: 101
Number: 3
Pages: 366-374
Summary of Methods: The effect of grazing intensity (i.e., ungrazed, light, or heavy) on the short-grass steppe of the Central Plains Experimental Range in north central Colorado was assessed in terms of forage nitrogen concentration, nitrogen yield, and in vitro digestibility. Studies were conducted on pastures for which the long-term 50 year grazing history could be categorized as light or heavy. Portable cages were placed on the low-lands of each long-term grazing treatment, each cage received one of two water treatments (supplemental water or control i.e., no additional water in a year of average precipitation). Paired plots within each water treatment received one of two defoliation treatments (clipped to a height of 1 cm or unclipped). Water was supplemented (20, 26, 26, and 26 mm) biweekly from last week of June to 2nd week of August to simulate a wet year. Clippings occurred three times during the growing season. Reference plots were established outside the treatment cages. All plots were harvested for live-plus-recent-dead biomass at the end-of-season peak standing crop. End of the season standing crop was combined with the clipped biomasses for defoliation-treatments.
Article Summary / Main Points: In general, light grazing in the current year improved forage quality as measured by nitrogen concentration and digestibility. However, areas with a long-term history of grazing and conditions of above average precipitation had lower forage quality than sites without significant past grazing and average precipitation. Supplemental water, to simulate a wet year, decreased forage quality (i.e., nitrogen concentrations) regardless of the defoliation treatment. Forage nitrogen content, an estimate of nutritive value, was generally higher in lightly grazed plots compared to heavily grazed plots. During a simulated wet year, nitrogen yield and forage digestibility was greatest under no grazing. Defoliation improved digestibility in a wet year, but grazing levels did not affect digestibility under conditions of average precipitation. Forage digestibility increased under light grazing but not under heavy grazing. Forage digestibility increased with increased water on the ungrazed site while forage digestibility decreased with increased water on the heavily grazed site. The effects of grazing on forage quality are therefore inherently tied to annual precipitation levels.
Vegetation Types: Shortgrass Prairie
MLRA Ecoregions: 67A Central High Plains, Northern Part 67B Central High Plains, Southern Part
Agrovoc Control Words: Rangelands Grazing Stocking rate
Article Review Type: Refereed
Article Type: Experimental Research
Keywords: grazing intensities, defoliation, precipitation, nutritional quality, grazing history, nitrogen
Annotation: The results of this study are applicable to short-grass prairie species; it is unclear how widely these results can be applied. This was a well replicated and designed long-term study evaluating the effects of 50 years of consistent grazing management. When applying these results keep in mind that there were several interactions and confounding factors making the interpretation of the results variable and difficult to apply to other settings.
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