Among cereals, maize (Zea mays L.) is third in economic and food importance after wheat and rice. In botanical terms it is a graminaceous plant with a root system that is bundled but not basket shaped. The secondary roots are essential for water absorption and are concentrated in the upper 50-70 cm of soil. The plant favours loose soils and dislikes poor soils; this is highly important in choosing the ideal irrigation method. For germination and growth phase warm soils with a high nutrient content are preferred. The best are soils rich in phosphorus, nitrogen and organic matter, and with high cation-exchange capacity. In crop rotation maize takes the role of spring-sown renewal crop, needing deep working and leaving high quantities of residue in the soil.
Maize is a typical C4 photosynthesis plant found in warm climates, and has developed mechanisms to conserve water in dry conditions. It has a very low water coefficient (around 300 kg of water per kg of dry product), but as its brief life cycle takes place in a critical time period for the availability of water, irrigation plays a determining role in crop production.
On average, water consumption is estimated at around 5000-6000 cu.m./ha. over the whole season for maximum production levels. This figure obviously varies according to water availability in the area, climatic conditions and growing methods.
The irrigation schedule should meet all the crop’s water needs during the period between the appearance of the tassel (around two weeks before flowering) and the milk-wax stage (around 5-6 weeks after flowering), a total of approximately 50-60 days.
Water is always negative but can be exceptionally harmful if it occurs in specific phenological stages such as flowering. In fact, during this phase, a lack of water would cause the failure of the pollination process (no pollination or aborted ovules), resulting in a reduction, sometimes total absence, of grains per ear. For this reason water resources need to be rationalised so that the greater part of available water is reserved for the flowering period (lasting about a week), in order to prevent irredeemable loss of production.
Throughout the growing period, irrigation cycles and volumes of water used should be calculated taking into account the pedoclimatic conditions in order to rationalise water resources and avoid any stress to the plants, which translates into withering of the foliage, in some cases resulting in a production loss estimated at 6-8% per day.
Estimated water requirements should take into account potential evaporation in the field and the appropriate crop coefficient (Kc), which for maize is equal to:
0.30 – 0.40 for the first two weeks
0.60 – 1.00 for the subsequent 4 weeks
1.10 – 1.20 from earing to milk phase
0.80 – 0.90 during subsequent ripening phases
0.60 -0.30 until harvest.
Water volume should be calculated so that the surface layer of soil is watered to a depth of around 0.70 m.
Until the beginning of this millennium, maize had always been irrigated with highly inefficient methods. Still today, in many parts of the world, maize is watered by flooding or lateral infiltration of the furrows. The most commonly used method is sprinkling by self-propelled machines which need high flow rates and energy, working at pressures in the order of 10-12 bar. This method gives water use efficiency levels between 60 and 70%.
Thanks to growing awareness of environmental issues, decreasing water supplies and increasing energy costs, in recent years the use of drip irrigation has also been developed for this crop. The advantages of drip irrigation for maize are many and considerable:
- on average crop yield can be increased by 30 – 40%;
- higher quality grain thanks to fewer periods of water and nutrient stress; this translates into a reduction in attacks by micro-toxins;
- water saving in the region of 30%, partly due to the lack of effect of water loss by wind and evaporation loss with the drip method;
- compared to sprinkler methods, drip irrigation requires a working pressure of around 1 bar or less, giving energy savings of 60%.
for fertigation, nutrients can be distributed at the appropriate time and intervals to best meet the needs of the maize. Nitrogen, the most important element for growing, can be distributed during the phases when it is most needed (from the fourth to the tenth week), using simple fertilisers and avoiding overdose in the sowing phase or the use of costly slow-release products. Nitrogen-enriched fertilisers can be distributed evenly and at intervals, reducing loss through leaching and thus reducing environmental impact. Fertigation allows nutrient distribution to be reduced by 30%.
with micro irrigation the water is delivered slowly and constantly, creating the ideal balance of water and oxygen in the rhizosphere and benefiting the maize plants which dislike over-compacted soil; while with sprinkler irrigation the beating action of the water is inevitable.
with sprinkler irrigation, the crop is watered only a few times over the year (4-6), and water volume is high (40-60 mm/ha, equal to 400-600 cu.m./ha.); with micro irrigation the correct volume of water can be divided into more interventions, thus maintaining the ideal level of soil moisture and avoiding the water stress which is so dangerous for this crop. This translates into higher yield and better quality product.
- by avoiding wetting the plants, attacks by fungi are less likely.
- in the initial phases of growth when the plants are still undeveloped, the complete lack of damp in the soil reduced the growth of weeds among the crop.
- the use of lightweight driplines allows unproductive areas to be reduced, by cultivating the strips where the wheels of the sprinkler system would pass.
- the characteristic border effect of sprinkler irrigation is avoided, giving a visibly uniform and even growth over the field.
The irrigation of maize using the drip method uses appropriate lightweight driplines fitted with coextruded plastic labyrinth and turbulent flow drippers of varying capacity and distance apart. For this crop it is generally advisable to use an hourly flow rate of 1.0 – 1.5 litres, with a distance of 30 – 40 cm between drippers.
One dripline can be installed for each row of plants, but several studies and experiments have come to the conclusion that one dripline for every two rows is adequate. In this case and with a distance of 75 cm between rows, the driplines should be installed every 150 cm. With dual rows it is always recommended to install a dripline for each pair of rows. Irritec supplies a wide range of lightweight driplines developed specifically for this crop: P1 at 16 mm, P1 ULTRA at 22 mm, P1 MAXI at 25 mm, P1 EXTRA at 29 mm, ISIPLASTTAPE at 16 and 22 mm.
Greater diameters allow uniform irrigation of rows up to 600 metres long, using only 0.7 bar of pressure.
Special towed machines permit the installation of 3 – 5 driplines at the same time. The same machinery can be adjusted to facilitate picking up the lines after harvest.
Subsurface irrigation is also being used increasingly in maize cultivation: in this method the driplines can be buried at a depth of around 40 cm, with a distance of about one metre between the lines, depending on the characteristics of the soil and the crop to be grown. This technique allows the highest levels of efficiency to be achieved, and reduces amounts of nutrients needed by delivering fertilisers directly into the rhizosphere. The lines are installed by machine, and with this method thicker driplines are needed than those used for surface irrigation. Subsurface irrigation of maize allows growing operations to be modified in order to reduce environmental impact to a minimum and preserve the structure of the soil. Indeed, the combination of subsurface irrigation, minimal tilling and sod seeding is classic.
Micro irrigation of maize allows the task of handling digested slurry from biogas production to be transformed into a resource for fertigation.
Fertigation with injection of digested slurry brings a series of benefits hitherto not fully evaluated:
- the manure is allowed to penetrate the soil evenly and only into the stratum occupied by the crop’s root system, thus reducing percolation into underlying layers which may impact on the water table;
- drip irrigation reduces ammonia emissions, to the benefit of the environment;
- large amounts of nitrogen can be distributed quickly and in a form which can be easily assimilated;
- nitrogen delivered in this way is rapidly mineralized.
The use of digested slurry in fertigation is subject to preliminary analysis of the properties of the product to be distributed and the use of appropriate filtering equipment to reduce or eliminate the risk of blockage in the drippers, which handle low hourly flow rates.
Considering the importance of fertigation techniques with liquid manure, Irritec is carrying out research to determine the correct level and filtering combinations for water added to the sludge, the ideal proportions of water and sludge depending on its provenance, and the results and effects of the use of digested slurry in the fertigation of maize and other crops.