Factors influencing the estimation of the nutritive value of the diet selected by cattle fistulated at the oesophagus

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Abstract
SUMMARY: Four experiments were conducted to examine the extent to which some sampling procedures may bias the estimation of the nutritive value of herbage consumed by cattle which have been fistulated at the oesophagus.In one experiment the nitrogen, sulphur and calcium contents (g/100 g organic matter (OM)) and organic matter digestibilities (OMD %) of 15 herbage samples were determined and compared with those of extrusa collectedfrom Hereford cattle fistulated at the oesophagus. The extrusa samples were analysed both as total extrusa, which included saliva, and as extrusa which had been squeezed through muslin and the solid and liquid fractions separated. The herbage samples covered a wide range of nutritive values (0·78–4·92 g N/lOOg OM; 0·08–0·27 g S/100 g OM; 0·17–1·43 g Ca/100 g OM and 34·4–85·7% OMD).It was concluded that satisfactory estimates of nitrogen, sulphur and calcium contents and digestibility of herbage prehended by cattle can be made from oesophageal fistula extrusa and that the precisions of the estimates will not be increased if extrusa are separated into solid and liquid fractions immediately following collection. Corrections are not necessary for estimates of herbage sulphur, calcium and digestibility made from total extrusa but may be necessary for nitrogen values in excess of 2·74 g N/100 g OM. If extrusa are squeezed it is necessary to apply correction factors for nitrogen and sulphur but not calcium or digestibility.In the other experiments three factors were examined in relation to the nitrogen content and digestibility of extrusa. They were the effects of (i) previous grazing experience of the pasture (ii) fasting for 0, 3, 6 and 23 h prior to sample collection and (iii) collection of extrusa during the morning or afternoon.It was concluded that the lack of previous grazing experience of apasture or the length of a pre-collection fast up to 23 h will not significantly bias the estimation of the nutritive value of extrusa, but that it is preferable to collect extrusa samples from cattle in the morning and that sampling on rainy days should be avoided.