The physiology of grass production under grazing. 2. Photosynthesis, crop growth and animal intake of continuously-grazed swards.
Abstract
Measurements were made to determine the balance between photosynthesis, animal intake and the losses of matter in swards maintained by 'lenient' continuous grazing by sheep at a leaf (lamina) area index (LAI) of 3.0 and by 'hard' continuous grazing at an LAI of 1.0. Gross photosynthetic uptake was greater in the leniently-grazed sward than in the hard-grazed sward. In both swards, a similar proportion of photosynthetic uptake was lost in respiration or by partition to non-harvestable parts. Thus, shoot production under lenient grazing was also greater than under hard grazing. Despite the lower gross photosynthetic uptake in the hard-grazed sward, animal intake in this sward was greater than in the leniently-grazed sward. This was because, in the hard-grazed sward, a far greater proportion of the shoot produced was harvested and a far smaller proportion remained unharvested to be lost to death. It is a major limitation to production under continuous grazing that the high gross photosynthetic uptake and high rate of shoot production seen in the leniently-grazed sward cannot be associated with a high efficiency of harvest and so give rise to high harvested yield. Maximum intake per hectare is therefore achieved in a sward maintained at an LAI which is substantially below the optimum for photosynthesis.<new para>ADDITIONAL ABSTRACT:<new para>Measurements were made to determine the balance between photosynthesis, animal intake and the losses of matter in Lolium perenne swards maintained by 'lenient' continuous grazing by sheep at LAI (lamina) 3.0 and by 'hard' continuous grazing at LAI 1.0. Gross photosynthetic uptake was greater in the leniently-grazed than in the hard-grazed sward. In both swards, a similar proportion of photosynthetic uptake was lost in respiration or by partition to non-harvestable parts. Thus, shoot production under lenient grazing was also greater than under hard grazing. Despite the lower gross photosynthetic uptake in the hard-grazed sward, animal intake in this sward was greater than in the leniently-grazed sward. This was because in the hard-grazed sward a far greater proportion of the shoot produced was harvested. It was concluded that a major limitation to production under continuous grazing was the fact that the high gross photosynthetic uptake and high rate of shoot production seen in the leniently-grazed sward cannot be associated with a high efficiency of harvest and so give rise to high harvested yield. Max. intake/ha is therefore achieved in a sward maintained at an LAI substantially below the opt. for photosynthesis.