Browsing Journal of Range Management, Volume 42, Number 2 (March 1989) by Subjects
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Cattle nutrition and grazing behavior during short-duration-grazing periods on crested wheatgrass rangeDaily changes in diet quality, ingestive behavior, and daily forage intake were investigated using crested wheatgrass [Agropyron desertorum (Fisch.) Schult. and A. cristatum (L.) Gaertn.] range in a 3-year study to provide an understanding of how the rapid defoliation that occurs under the high stocking density of short duration grazing (SDG) affects livestock nutrition. A 10-paddock short duration grazing cell was stocked with yearling Angus heifers. Grazing periods in paddocks varied from 1 to 4 days. Dietary quality was assessed daily within pre-selected paddocks by determining crude protein content and in vitro organic matter digestibility of extrusa samples collected from esophageally fistulated animals. Three variables of ingestive behavior were measured concurrently, including ingestion rate, biting rate, and grazing time. Daily forage intake was estimated by multiplying ingestion rate and grazing time. There were large daily changes in diet quality, ingestive behavior, and forage intake during the grazing period within particular SDG paddocks. Diet quality declined significantly during the 2 or 3 day grazing period in all 3 years. Although not as consistent throughout the study, ingestive behavioral responses changed significantly, indicating declines in forage intake during the grazing period on a particular paddock. Ingestive behavior was correlated with several characteristics of the sward that changed as it was defoliated. Ingestion rate decreased with herbage availability, apparently causing the animals to compensate by increasing biting rate or grazing time. Ingestion rate and biting rate decreased as nutritional quality of the sward declined, as indicated by decreased crude protein content and digestibility, and increased fiber content. Based on the system studied, grazing periods in SDG paddocks should be no more than 2 days to maintain high levels of livestock performance on crested wheatgrass range.
Experimental evaluation of the grazing optimization hypothesisThe herbivore grazing optimization hypothesis predicts an increase in aboveground net primary productivity (ANPP) at a moderate grazing intensity. The hypothesis was tested by grazing controlled densities (0 to 145 individuals/m2) of big-headed grasshoppers (Aulocara elliotti Thomas) for short time spans (7 to 13 days) on enclosed swards (0.7 m2) of blue grama [(Bouteloua gracilis) (Willd. ex H.B.K.) Lag. ex Griffiths]. ANPP of each of 257 experimental enclosures was estimated following regrowth by using a standing crop index (the product of mean total blade length per tiller and percent basal cover) after the grazing period and clipping after the regrowth period. ANPP was not significantly reduced by grazing in any of the 5 short-duration grazing experiments. In 2 of the 5 experiments, ANPP increased significantly with grazing. In 1 of the other 3 experiments there was evidence for the grazing optimization hypothesis.
Growth dynamics of fourwing saltbush as affected by different grazing management systemsIndividual leaders of fourwing saltbush were permanently marked and their growth responses monitored during a 3-year study in a shortduration grazing system, a 4-pasture rotation system, and in ungrazed exclosures. Primary and secondary leader growth and numbers of secondaries were responses of interest. Plants continuously browsed by cattle were usually maintained in a hedged form and produced relatively little growth. There was little difference in growth responses between plants in the 4-pasture rotation and the shortduration system when the shortduration rotation cycle was 32 days. However, when the rotation cycle was increased to 64 days, there was a substantial increase of growth for plants in the shortduration system. Plants protected from browsing for 1 year also responded with progressively less leader production as length of protection time increased. We suggest fourwing saltbush plants respond to a 60-day deferment at the beginning of the growing season.