10.07.2015 Views

Crop yield response to water - Cra

Crop yield response to water - Cra

Crop yield response to water - Cra

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

the onset of stem elongation <strong>to</strong> the end of anthesis, when stress may limit the number of fertileflorets produced from the florets initiated, and the proportion of the fertile florets pollinatedand set. During the grain filling period, <strong>yield</strong> is also reduced by <strong>water</strong> stress strong enough<strong>to</strong> inhibit assimilation, leading <strong>to</strong> reduced weight per grain. In terminal drought situationscommon <strong>to</strong> the Mediterranean climate, <strong>yield</strong> loss is most likely the combined result of fewergrains resulting from early abortion of young developing embryos, and reduced weight pergrain. In addition <strong>to</strong> reducing s<strong>to</strong>matal opening and pho<strong>to</strong>synthesis, terminal drought alsoaccelerates leaf and hence canopy senescence, causing premature cessation of assimilateproduction. Consequently, harvest index is reduced because vegetative biomass is formedmostly in the early part of the season when <strong>water</strong> is not as limiting whereas grain biomassderives largely from that assimilated during the late part of the season under drought.Yield is also significantly affected by stresses during the early part of the grain filling periodwhen potential grain size is being determined and grain number may be reduced by abortionof the developing embryos. Before the onset of stem elongation, the detrimental effects ofstress are due <strong>to</strong> either poor stand establishment and/or slow tiller and canopy development.Either a poor stand or slow canopy development reduces radiation captured for pho<strong>to</strong>synthesis,and hence, biomass accumulation rate. Slow development of tillers also reduces the numberof heads per unit of land area. That is why for instance an early control of weeds is essential<strong>to</strong> avoid <strong>yield</strong> penalties.Various aspects of temperature effects have already been discussed under Growth andDevelopment. An additional effect is that on grain filling. In most temperate conditions grainfilling occurs when the weather is getting hotter. High temperatures (above 30-32 ºC) accelerateseed development <strong>to</strong>wards maturity, shortening the grain filling period and hence reducesweight per grain at harvest. The effect is noticeable even if periods of high temperature areshort (even only 3 days). Another possible negative effect of high temperature late in theseason is acceleration of canopy senescence, although this may partly be the indirect effect of<strong>water</strong> stress induced by high transpiration under high temperature. In addition <strong>to</strong> lower <strong>yield</strong>s,stresses over grain filling may reduce grain quality for malting, by increasing the proportionof grains smaller than 2.5 mm, increasing protein percentage and reducing the malt extract.Salinity is a prominent stress for barley production in some irrigated areas. However, barley isconsidered <strong>to</strong> be the most salt <strong>to</strong>lerant of cereals. Regarding mineral nutrients, soil fertilizationrequirements can only be determined in relation <strong>to</strong> achievable <strong>yield</strong> and soil type and nativefertility. As a rough guide, for each <strong>to</strong>nne of <strong>yield</strong> per hectare, barley needs <strong>to</strong> take up about30 kg of N, 5 kg of P and 20 kg of K. The amount of actual uptake depends on the availabilityof the native and fertilizer nutrients as well as the crop uptake efficiency. Broadly speaking,uptake efficiency is seldom higher than 0.6. The crop N status not only influences <strong>yield</strong> butalso grain protein percentage. Low protein percentage is more suitable for malting, whereashigh percentage is more suitable for animal feed.Irrigation practiceBarley is usually grown under rainfed situations. In some cases, however, full or partial irrigationmay be applied, especially when barley is grown for malting or where double cropping ispractised, with an early-maturing barley followed by late-sown maize (or soybean).barley 139

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!