The Energy Retention of Lambs Grazing S37 Cocksfoot, S23 Ryegrass and S24 Ryegrass at Low Grazing Pressure

A grazing experiment to measure the feeding values of 4‐week‐old regrowths of S37 cocksfoot, S23 ryegrass and S24 ryegrass is described. A low grazing pressure was maintained on the pastures to ensure that animal production was not limited by herbage availability. Each sward was grazed by a group of 12 lambs under “worm‐free” conditions. Increases in empty body weight, dry body weight and energy were measured by the comparative slaughter technique. The liveweight gain of the cocksfoot‐fed lambs was the same as the ryegrass‐fed groups during the first half of the experiment but was lower in August and September. The final liveweight gains on S23 and S24 ryegrass were 14% higher than on S37 cocksfoot, but in terms of energy retention S23 ryegrass was 18% and S24 7% better than S37 cocksfoot. Digestibility measurements of herbage cut from the plots being grazed showed a positive correlation with energy retention by the lambs. Energy retention was negatively correlated with the proportion of acetic acid in the rumen adds, a fraction which was itself correlated with the soluble carbohydrate content of the herbage, but not with the digestibility of the herbage. When interpreting animal‐production results, intake, digestibility and rumen volatilc‐fativ‐acid proportions, should be taken into account. Because of the inadequacy of present methods of estimating the herbage intake of grazing animals, measurement of body‐energy gains is not justified and empty‐liveweight gains are sufficiently precise to measure relative energy retentions at pasture.