The use of sheep fitted with oesophageal fistulas to measure diet quality

Abstract

Ewes and wethers with oesophageal fistulas were compared with normal sheep in several experiments over three years. The grazing behaviour, herbage intake, liveweight changes, and ability to bear and rear lambs were altered little by fistulation, provided fistulas were less than 5 cm in length and closure of the fistulas was good. This was so even at a relatively high stocking rate of G ewes to the acre. These findings infer that studies with fistulated sheep provide information validly applicable to normal sheep. Fistulas that allowed good closure control (<5 cm in length) did not always allow complete recovery of ingested material. However, with incomplete recovery representative diet samples were obtained. Sampling procedures for obtaining adequately representative diet samples for individual sheep, and for obtaining estimates of diet composition for flocks of sheep on different pastures were established. The routine that must be followed to avoid obtaining untrue diet samples is to sample sheep when they are accustomed to a pasture, without prior fasting, and at the time(s) of the day when sheep are normally grazing. An hour's sampling period will collect about l/l0th of the total daily intake and longer periods may cause rumen dysfunction. Pooling data from once-a-day samples for two successive days for an individual sheep seemed to give accurate values for the whole diet over those two days for sheep set stocked on pasture. Variation between-sheep, between-days, and within-days in the nitrogen and soluble carbohydrate content and botanical composition of extrusa samples was studied. The contribution of these sources of variation to total variation differed for different diet constituents. For nitrogen, between-sheep variation was greater than between-day variation but for soluble carbohydrates the position was reversed. Within-day variation was much the same as between-day variation in a constituent. The standard deviations of nitrogen and soluble carbohydrate contents and botanical composition were large and a true mean value for any of these parameters for a flock of sheep, could only be obtained by pooling data for several sheep on several successive days.