The Effect of Physiological Status on Diet Selection by Grazing Ewes

Abstract

The effect of the physiological status of grazing ewes on diet selection was studied, using ewes with oesophageal fistulae. The diets of dry ewes grazing Phalaris, annual grasses and subterranean clover pastures, over a range of availabilities, were compared with those of ewes in various stages of pregnancy and lactation. There were no significant differences at any stage of reproduction in the botanical composition of the diets. However, small differences in diet digestibility, soluble carbohydrate and nitrogen content were found during pregnancy and lactation. These occurred at all levels of availability of pasture. The differences did not appear to have nutritional significance in the particular pasture ecosystems studied because levels of N, soluble carbohydrates and digestibility of the diet were such that selection for survival was unlikely to be necessary. tt is concluded that dry fistulated ewes can be used to obtain samples for assessing diet N and soluble carboliydrate contents for ewes in other physiological states. The use of in vitro digestibiiity tests of diets of dry ewes to assess diet digestibility for ewes in other physiological states may be subject to error.

The effect of herbage allowance on herbage intake and performance of ewes and their twin lambs grazing perennial ryegrass

“…The digestibility of the herbage eaten by the ewes was estimated by in vitro analysis (Tilley & Terry, 1963) of extrusa samples collected from oesophageal fistulated wethers and lambs. Since diets selected from mixed swards by ewes in different physiological states were shown not to differ significantly by McManus, Arnold & Ball (1968), it was assumed that wethers would give samples representative of the diet selected by ewes, particularly on a single species sward. The fistulated lambs were brought on to the experiment in the 7th week.…”

Dietary self-selection by animals.

“…The poorest performance reported during gestation found pregnant rats (Tribe, 19S4b, 1955) and pregnant sheep (Gordon & Tribe, 1951) unable to select rations that would allow them to bear and nurse normal young. McManus, Arnold, and Ball (1968) used an oesophageal fistula to study the nutrient composition of foods selected by free-grazing sheep during pregnancy and lactation. No significant differences in selection were noted, but the authors speculated that the high nutritional plane and the homogeneity of the pasture areas accounted for the failure to observe dietary selection.…”