The energy requirements of sheep for maintenance and gain. II. Grazing sheep

Abstract

The intake of grazing sheep in terms of digestible organic matter, DOM, has been measured by faecal index methods, employing chromic oxide to measure faecal output and faecal nitrogen for digestibility of pasture. A regression equation for the prediction of digestibility from faecal nitrogen (organic matter basis) was developed from several series of digestibility trials with pasture, the equation beingdigestibility of organic matter = 30·84 + Four trials to measure maintenance and gain were conducted, as follows:(i) Thirty-eight sheep, comprising twelve wethers of mean weight 112 lb., thirteen small or thin Romney ewes of mean live weight 110 lb. and thirteen large or fat Romney ewes of mean live weight 166 lb. were grazed all together for 17 weeks on a rye-grass-white clover sward at an intensity such that live weight was just maintained. Corrections to intake were made for small live-weight changes.Daily DOM intake (lb.) for maintenance = 0·062 maintenance for a 100 lb. sheep becomes 1·48 ± 0·08 lb. DOM/day.(ii) Thirty-six weaned Romney ewe lambs of mean initial live weight 56 lb. were grazed for 8 weeks on a rye-grass-white clover pasture and allowed to gain in weight. Regression of intake on gain gave the equationdaily DOM intake (56 lb. lamb) = 0·93 (±0·07) + 0·90 gwhere g = gain in lb./day. Combining this lamb data with that of the previous trial yields the equationDOM intake (lb.) for maintenance = 0·052 (iii) Forty ewes, comprising eight Southdowns, twelve Romneys, eight Border Leicesters and twelve Border-Romney crossbreds were grazed together on short pasture (rye-grass-white clover) for 12 weeks. The ewes were of widely differing live weight and the mean gains or losses of the groups over the period varied from + 1·5 to −4·5 lb.Daily DOM intake (lb.) for maintenance = 0·061 which for a 100 lb. sheep becomes 1·63 ± 0·13 lb. DOM.(iv) Seven Romney ewes were grazed in high and eight ewes in low intake groups for 8 weeks. Regression of intake on gain gave the equationdaily DOM intake (130 lb. ewe) = 1·58 (±0·06) + 1·98 g.For a 100 lb. sheep this becomes 1·36 (± 0·06) lb. DOM/day for maintenance.The three estimates of maintenance of a grazing sheep of 100 lb. live weight 1·48, 1·63 and 1·36 lb. DOM/day are greatly in excess of the estimate of 0·92 lb. DOM/day for pen-fed sheep. It is believed that this difference is outside experimental error and represents a true increase in energy cost due to grazing. It is suggested that the cause of this derives from the energy costs of walking and harvesting the pasture together with climatic factors wind, cold and rain. The exponent of live weight agrees well with the estimates of other workers based on pen-fed animals. The energy cost of gain is less than that of pen-fed sheep and it is suggested that this is due to the maintenance cost decreasing with increasing availability of pasture and with increasing gain.