When the rate of dry‐matter production of a grass crop is determined only by the incoming light energy, it increases exponentially until a closed green surface is reached. Thereafter the production is linear for some time, but soon it decreases and finally drops to zero. This decrease is partly caused by an increase in the rate of respiration relative to photosynthesis. However, there are still unknown factors affecting the rate of production, especially in the second half of the season. When a sward receives an optimal supply of water and nutrients the organic plant constituents at a given growth stage remain fairly constant throughout the season. It would be advantageous, however, to harvest grass with a higher sugar and a lower protein content. This seems to give better rumen digestion and, besides, would improve the regrowth of the sward. Since plants with a high rate of dry‐matter production usually have a fairly low reserve level, it is unlikely that selection could provide a strain with both a high reserve level and a high rate of product