Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)


Wildland Resources

Department name when degree awarded

Range Science

Committee Chair(s)

John C. Malechek


John C. Malechek


Philip J. Urness


Joseph C. Street


James A. Gessaman


Donald V. Sisson


Forage intake by animals is an important factor in determining production of livestock products from rangelands. However, relatively little is known of effects of such forage variables as availability and distribution in space upon intake. Even less is known about how the grazing animal modifies its feeding tactics when confronted with diminishing or limited supplies of available forage and how such altered behavior may affect the animal's energetic cost for existence. Forage intake, body weight gain, grazing time and biting rate of Angus heifers was related to forage availability and plant height on semiarid crested wheatgrass rangeland during the late summer.

Forage availability was estimated within 10 percent of the mean (P≤0.10) by use of hand-clipped 1m2 plots. Forage intake was estimated from data on fecal production and in vitro digestibility of forage. Fecal production was determined by total collection, using fecal collection bags, and by a single-dose marker technique, used to estimate fecal production indirectly. Digestibility was determined by an in vitro procedure. Additionally crude protein and cell contents of forage were determined. Grazing time was measured by mechanical grazing clocks (Vibracorders) mounted on animals' necks. Biting rate was determined visually using a stop watch to time specific grazing intervals during which all bites were counted.

There were a total of four 4-day trials during 1977 and five 4-day trials during 1978. Crude protein content within years was significantly higher during the third period in 1977 and during the fifth period in 1978 when regrowth occurred. Within years, in vitro digestibility was significantly higher only for the fifth trial during 1978. Cell contents decreased as grazing progressed only during 1978. Crude protein content, cell contents and in vitro digestibility were higher during 1977 than 1978.

Forage intake of heifers did not vary significantly among successive grazing trials as forage availability declined from 919 to 143 kg DM/ha. Heifers apparently compensated for the diminishing forage availability by increasing grazing time from 380 to 656 min/day and biting rate from 37 to 50 bites/min.

Grazing time was inversely related to forage availability and was expressed by the relationship y = 676.8 - 0.3x; r2 = 0.93. Biting rate was less correlated with forage availability and was described as y = 50.4 - 0.02x; r2 = 0.86. However, biting rate was more closely correlated with plant height as described by the relationship y = 53.0 - 0.48x; r2 = 0.95.

Heifers maintained or gained 0.1 to 0.7 kg/head/day weight during all trials except the last trial in 1978 when they lost approximately 1.1 kg/head/day. The weight loss for this trial was apparently not a result of restricted forage intake or limited forage quality but partly due to extra maintenance energy expenditures attributable to increased grazing time and biting rate. Correlation between estimates of fecal output by the single dose marker technique with total fecal collection were not significant (P≤0.05) using the model y = a + bx. More sophisticated two- or three-compartment models estimating the lagtime between injection of indicator and first appearance in feces and observations on the ascending limb of the excretion curve must be considered.