Grain yield is the product of plant density, tiller number, number of spikes per plant, number of spikelets per spike, number of kernels per spikelet, and kernel weight. Plant density is determined by seeding rate, germination percentage, and the number of seed- lings that emerge and survive. Tiller number per plant depends on plant density, culti- var, sowing date, availability of moisture and nutrients, and temperature. The number of spikelets that can form is determined by when stem elongation is initiated; stress (weed competition, heat, cold, drought, nutrient deficiency, diseases) during this period reduc- es the number of spikelets that are formed. Florets are initiated during the stem elonga- tion stage. The small grain plant is not able to produce enough photosynthate to allow development of all florets. The fastest-growing florets have first access to the available carbohydrate, nitrogen, and other nutrients and are the most likely to produce mature seed. Good growing conditions during stem elongation favor development of the maxi- mum number of florets. The cells of the endosperm accumulate starch and protein dur- ing grain filling. Any stress or damage that reduces photosynthetic output or interferes with the transport of carbohydrate between flowering and hard dough stage will reduce kernel weight.
Timing of Field Activities
Table 2 gives approximate dates when important crop growth stages occur for wheat produced in key growing regions of California. The dates and corresponding growth stages are for common wheat cultivars sown at optimal planting dates with sufficient soil moisture to initiate germination. Seasonal weather conditions are considered aver- age. Depending on the cultivar selected, crop development may vary 7 days on either side of the dates specified. Similarly, even under the most variable weather conditions, crop development will be within about 7 days of the dates specified. For barley, heading and grain-fill dates will be advanced by about 2 weeks. Herbicides for early-season weed control should be applied prior to the onset of tillering, when weeds are small (follow the label). Herbicide applications for later-season weed control should be made when the crop is fully tillered. Fertilizer top-dressing should be made at the mid-tillering stage. Nitrogen top dressing to improve grain protein should be made at heading or flowering in conjunction with an irrigation. If fungicide applications to control foliar diseases are planned, they should be applied to protect the flagleaf and penultimate leaf (the leaf that emerges prior to the flagleaf), and thus should be made just prior to boot stage.