Effect of nitrogen fertiliser rates and defoliation regimes on the vertical structure and composition (crude protein content and digestibility) of a grass sward

The vertical sward structure (proportions of lamina and sheath) and sward composition (crude protein and digestibility) have an effect on animal behaviour and performance, but their variation through management is not well known. We studied the effect of defoliation regime and nitrogen supply on these sward characteristics. There were six treatments, two nitrogen fertiliser rates combined with three defoliation regimes. Two of the latter mimicked a long growth duration (up to 7 weeks) and one mimicked a short period of regrowth (2 weeks). The swards were cut 7 cm from ground level. We found that there was a single relationship between sheath length and sward height, regardless of treatment and sampling date. For the treatments which mimicked a long growth duration, crude protein and digestibility decreased linearly from the top to the base of the sward when these decreases were expressed per unit of herbage mass. Conversely, for the treatments which mimicked a short growth duration, crude protein and digestibility fell quickly between the upper layers and those close to the ground. This discontinuity was the result of a smaller number of laminae per tiller than for the other treatments, whereas the composition of the sheath was the same. For resting periods longer than 20 days a model was proposed to simulate the effect of sward defoliation depth on the chemical composition of the diet selected by grazing animals according to available herbage mass before and during grazing. From a practical point of view the sward height alone could not be used to predict the sward composition from the top to the base of the sward and its effect on diet selection by grazers. The length of the resting period needs to be known in addition to the residual sward height. Copyright © 2003 Society of Chemical Industry