FACTORES DEPENDIENTES DEL INDIVIDUO ( BIOLOGÍA HUMANA)

covering has had a chance to be completed. For the nozzle to remain a constant dis- tance above the ground, it has to either be mounted independently on its own height- gauging device (Fig. 12.8), or, if mounted directly on the opener, the latter has to have a positive height control of its own, which is necessary for adequate control of seeding depth anyway.

Even with adequate height control, there are other problems with spray noz- zles. The application rates of water that the manufacturers of herbicides recommend be applied per sprayed area are difficult to achieve because the narrow bands mean water application becomes concentrated on to a very small area for each nozzle. This requires very fine nozzles, which require

DM yield of drilled species (kg/ha) DM yield of resident species (kg/ha)

With fertilizer 141 1207

Without fertilizer 172 966

Table 12.6. Effects of fertilizer application at drilling on overdrilled ryegrass plants 12 weeks after

micro-filtration to avoid blockage by water impurities that would otherwise be accept- able to farm boom sprayers. Further, because these nozzles operate close to the soil (50–75 mm), they are subject to blockage from random soil splash and debris and damage through contact with stones, etc.

Hollow cone nozzles are most suited to single-nozzle band application, although fan-type nozzles have been used success- fully, largely because the inherent variations across the band are acceptable when the objective is only to suppress rather than kill all of the target plants. Hollow cone nozzles generally have a more uniform pattern from a single nozzle than fan-type nozzles.

An innovative method of applying banded herbicides has been used with the disc version of winged openers. Because this opener is equipped with two semi- pneumatic rubber gauge/press wheel tyres, the herbicide can be dripped on to the top of the tyres at low pressure and rolled on to the ground in much the same way as a lawn marker (Ritchie, 1986a, b). This avoids problems of blockage, micro-filtration, wind drift, the presence of tall plants and physical damage, common with small nozzles, and introduces the feasibility of ground-metering

of the herbicide. Figure 12.9 shows a drip roller arrangement.

Ground metering involves using a posi- tive displacement pump driven by the ground wheel of the drill in such a way that its output per metre of travel remains largely constant, regardless of travel speed or pressure. Such a system is impractical with pressurized nozzles because inevitable variations in ground speed cause variations in nozzle pressure, which in turn cause variations in band width because the width of the spray pattern from a nozzle is partly dependent on its operating pressure. With the drip roller system, the output pressure is very low and unimportant, since there is no pattern of spray to maintain and, even if there is, this is aimed at the top of the tyre, which then delivers the herbicide to the ground as a wet film on the bottom of the tyres, rather than directly as a jet.

In one respect, the rolling on of herbi- cide is a disadvantage because the tyres operate behind the opener and inevitably pick up soil, which quickly turns to mud on the wet tyres. Their use for this task is only possible with winged or double disc openers because of the minimal surface soil disturbance each of these openers creates.

Any soil contamination that does occur is countered by the improved efficacy of uptake of most herbicides from being rolled rather than sprayed on to the leaves. The result from many thousands of hectares of field-testing is that the 75 mm wide bands created by rolling on of herbicides works as well as spraying the same width of band and has a greater tolerance of the conditions under which it can be used.

Depth control and slot formation Control over the drilling depth of pasture, sports turf and many forage crop species is particularly demanding. Most drills designed expressly for pasture renovation have been promoted on a low-cost basis. Because of this, the control mechanisms for depth of seeding are generally primitive and sometimes non-existent.

For example, the simple low-cost drills that dominate the pasture drill markets in Australia and New Zealand are almost all equipped with ‘Baker Boot’ versions of sim- ple winged openers (inverted-T-shaped slot). While the choice of slot shape is appropri- ate, the ability of these openers to follow the surface is limited by the simplistic

drill designs to which they are attached, particularly the mechanisms for articulating each opener up and down. This causes the angles of the opener wings to change throughout their arcs of travel (see Chapter 4). To avoid complete loss of wing angle in hollows, the preset angle for level ground is about 10°. This relatively steep wing angle means that the shallowest this opener can drill and still maintain a true inverted-T- shaped slot without breaking through the covering surface mulch is about 25 mm.

In contrast, the more sophisticated disc version of winged openers is mounted on parallelogram drag arms, ensuring that the wing angle never changes. The preset wing angle is reduced to 5°to allow the wings to operate with integrity at depths as shallow as 15 mm. There is a major advantage, therefore, in being able to drill pasture with a machine equipped with similar techno- logy demanded for the more highly valued arable crops.

While many drill designers consider pasture and sports turf to be the most diffi- cult of media to drill, with winged openers the matted roots of pasture and turf plants provide a mulch medium of considerable elasticity and tensile strength, which can

be readily folded back and replaced while retaining the integrity of the inverted-T- shaped slots (Ritchie, 1988).

Seed metering

Most pasture and forage crop seeds are small, light and/or fluffy. Many pasture seeds also have awns attached. This presents several handling and metering problems.

First, they are difficult to meter accu- rately. Small-grain metering devices that commonly dispense several hundred kilo- grams of seed per hectare are often not well adapted to dispensing less than 1 kilogram of small seeds per hectare. Further, if the seeds have large awns or are fluffy, they will have a tendency to bridge above the metering device, interrupting the feed. This requires an agitator to be fitted to the drill to continuously avoid bridging. Often, drills for sowing small and/or difficult seeds use an auxiliary hopper designed especially for such seeds.

Many pasture species are sown as blends of two or more species. Common blends are clovers and grasses. Clover seeds are gen- erally round and dense. Grass seeds are gen- erally elongated and often fluffy and light. A previously mixed blend of such different seeds may partially separate into its indivi- dual components within the seed hopper of a drill in response to the continual vibration of the machine. To reduce separation and aid metering, the small seeds are often mixed with inert filling material such as sawdust or rice hulls to bulk up the material and reduce settling. Separation can be a problem with these mixes as well, especially if they are metered and dispensed by an air-delivery system. In these circumstances the high- speed airstream may blow some lighter, fluffier seeds out of the seed slot altogether before it has been covered.

Summary of No-tillage for Forage Production

1. Farming systems that depend on an