As a result of the lag in the spring rise in temperature behind the improvement in light conditions, crop growth in the spring is frequently limited by the rate at which assimilates can be used in growth rather than by the rate of production of assimilates in photosynthesis. Hence there is a loss of potential dry matter production in the spring, both in perennial crops such as grasses, and in spring sown arable crops. There is considerable genetic variation within and between crop species in ability to grow in cool conditions, and there are good prospects for achieving earlier growth of grasses and arable crops in the spring by breeding. Spring sown arable crops, such as sugar beet and potatoes, are slow to build up an adequate leaf cover and greater yields can be achieved in mild southwestern areas than in the traditional areas for these crops. Climatic conditions in lowland areas in the west are quite favourable for crop growth, and much land in these areas at present under permanent grass is potentially capable of producing high yields of arable crops. Some alternatives to the present extensive use of home grown barley and wheat for animal feed are discussed and it is shown that certain root crops produce higher yields of metabolizable energy (m.e.) and protein than cereals. Since silage is relatively rich in protein in relation to its content of m.e., luxury levels of protein have to be consumed and a re-examination of the cost of using silage for winter feed is required in the light of increased costs of nitrogenous fertilizers. On poor marginal land yields of animal protein from sheep are very low and such land would seem to be better suited for forestry, which can produce high yields of wood for relatively low inputs of soil nutrients.