Peer review

This paper reviews the results of published and unpublished experiments over 30 years looking at the effect on yield of planting density and row spacing, and of raised bed planting, of modern irrigated short wheat cultivars under the favourable climatic conditions of northwest Mexico. With optimum planting dates (mid Nov. to mid Dec.), regardless of row spacing, wheat yield was remarkably insensitive to planting density within the range normally studied (80?400 plants/m2). However yield was sensitive to row spacing: the most sensitive cultivars (erect dwarf cultivars) lost yield at spacing 30 cm and greater, while the least sensitive (some taller vigorous semidwarf cultivars) tolerated spacing up to at least 50cm without yield loss. Yield sensitivity to wide spacing was unaffected by density but varied between years, being less in favourable cooler years, and was markedly greater with later plantings (Jan.). The response to yield on narrow raised beds (2?3 rows per bed, 75?90 cm between bed centres), which have many agronomic advantages, could be explained by the aforementioned yield responses to spacing, in this case the furrow gap or distance between rows on adjacent beds being critical. Yield testing with a furrow gap of 44?50 cm appeared to select cultivars more suited to this system, but it wasn't clear whether such cultivars represented the best option under normal 20 cm row spacing in the absence of raised beds. When testing was extended to very low plant densities, yield was remarkably insensitive (maximum yield often achieved with only 16/m2), provided the planting date was normal and the planting arrangement did not deviate greatly from a square grid, with the honeycomb design giving an additional small yield advantage. These yield responses reflect the great plasticity in wheat imparted especially by tillering, and appear to be reasonably well explained by the notion that maximum yield requires close to full light interception no later than just before flag leaf emergence, being the onset of dry weight accumulation in growing spikes. Greater leaf greenness and probably higher radiation use efficiency with very low densities (and wide gaps), and associated delays in 50% flowering, may also assist in compensation for gaps. In contrast to much conventional wisdom, it is suggested that a worthwhile breeding objective would be selection for high per spaced-plant yield, with a view to achieving normal (or better) commercial yields at very low plant densities (< 20/m2), thereby opening up opportunities for new breeding and agronomic strategies in this era of precision cropping.

Text in English