Animal Nutritional Response to Sward Structure Under Short Duration Grazing Management

A ten-paddock short duration grazing cell was stocked with yearling heifers at a stocking rate of 0.7 ha per animal unit month and a stocking density of 0.14 ha per animal unit. A continuously-season- long-grazed (CSLG) pasture was used as a control. It was stocked at the same stocking rate, but at a stocking density of 1.4 ha per animal unit. Grazing periods in SDG paddocks were two or three days. Dietary quality was assessed by crude protein · content and in vitro digestibility of esophageal fistula estrusa samples. Three variables of ingestive behavior were measured, including ingestion rate, biting rate, and grazing time. Daily forage in take was estimated by multiplying ingestion rate by grazing time. Animals in CSLG gained significantly more weight in 1983, no statistical differences were detected in 1984, and, in 1985, animals gained more in SDG. No differences were detected in diet quality between SDG and CSLG throughout the study. No treatment differences were de tee ted in ingestive behavior variables in 1984, but ingestion rate was significatantly higher and grazing time significantly less in SDG during 1985. Differences in diet quality and ingestive behavior be tween SDG and CSLG at the beginning and end of the grazing season were evaluated for indications of possibly extending the season of nutritious forage. Such differences were few and inconsistent. Daily changes in diet quality and ingestive behavior during the grazing period within SDG paddocks were large. Diet quality declined significantly during the grazing period in all three years. Ingestive behavioral responses changed significantly, including declines in ingestion rate and increases in grazing time. Forage in take declined during the grazing period on a particular paddock. A model was developed that related behavioral responses to sward characteristics. Ingestion rate and grazing time were predicted from available biomass and herbage crude protein content. The model indicated that declines in biomass and herbage crude protein content translate into rapid declines in ingestion rate, and thus, forage in take. Based on the system studied, grazing periods in SDG paddocks should be two days or less to maintain high levels of livestock performance