INTRODUCTION

Soils generally do not contain sufficient nitrogen (N) for high grass yields. Therefore farmers are obliged to rely on fertiliser N to achieve high production. Responses of grass swards to fertiliser N are often linear for N rates up to 300-400 kg/ha/year. Research at Cedara has demonstrated that 375 kg N/ha applied to irrigated Italian ryegrass may produce 1 400 kg livemass gain/ha, while in the case of dryland kikuyu in the Mistbelt 300 kg N/ha may produce up to 1 000 kg livemass gain/ha.

Sound fertiliser N management practices are a key to efficient animal production off grass pastures. Unlike nutrients such as phosphorus and potassium, which are strongly held against loss on clay particles, a number of pathways exist whereby N may be lost from the rooting zone and thus to uptake by the plant. The fact that N fertilisers are frequently the most costly component in pasture production systems underlines the importance of a sound approach to N fertilisation.

NITROGEN RECOMMENDATIONS

The Cedara Fertiliser Advisory Service, in making N recommendations for grass pastures, caters for three production levels, viz. low, medium and high levels of production. Expected yields of various pasture species at these three production levels are listed in Table 1.

The figures given in Table 1 represent general guidelines on N usage. Actual yields achieved at a given N input are dependent on numerous factors, some of which are discussed later.

OPTIMIZING RETURNS FROM NITROGEN

To optimize returns from N fertilisers attention should be focused on the following aspects.

• Levels of other growth factors, in particular, water and other nutrients.
• Nitrogen losses by leaching, volatilization and denitrification.
• Nitrogen reserves in the soil.

It should be borne in mind that, unlike phosphorus fertiliser, N is not "fixed" in soils. However, in general, only 50 to 60 % of applied N is recovered in herbage. There is thus considerable scope for improving the efficiency of N utilisation in pastures. Encouragingly, the farmer has direct control over a number of other growth factors affecting N efficiency.

Other growth factors affecting the efficiency of N utilisation

Maximum response to applied N will only be achieved where other growth factors are not limiting.

• Acidity problems together with nutrient insufficiencies should be corrected prior to topdressing with N. Data presented in Table 2 demonstrate how pasture production under high N fertilisation may be influenced by unfavourable acidity and nutrient levels.

• Available soil moisture has a profound effect on pasture response to fertiliser N. For pastures grown under dryland conditions, N requirement is closely related to rainfall. The higher the rainfall, the higher the production potential and thus the higher the level of N required to realize that potential.

Where moisture supply is limiting, N inputs should be reduced accordingly. Obviously, where moisture conditions are optimized through the use of irrigation, relatively high N rates are required.

TABLE 1. Dry matter production levels of various pastures species (in tons dry matter per ha)

TYPES OF N FERTILISER

"Straight" solid N fertilisers that are available commercially are listed below.

• Limestone Ammonium Nitrate (28 % N) is a mixture of ammonium nitrate and lime. The latter to some extent offsets the acidifying action of the ammonium. The N component is water soluble.

• Ammonium Sulphate (21 % N; 24 % S) is readily soluble in water.

• Ammonium Sulphate Nitrate (27 % N; 13,2 % S) contains three parts N as ammonium and one part as nitrate. It is water-soluble.

• Urea (46 % N) is highly water soluble.

These fertilisers generally differ little in price when compared on a "per kilogram N" basis. Urea is the most concentrated form of N, with the result that transport savings may be appreciable when this source is used.

Other factors that should be considered when selecting a N source are presented below.

• Urea, being concentrated and highly water soluble, is particularly suitable for application through irrigation systems.

• Urea is poisonous to animals. Stock losses have been reported on urea-fertilised pastures. This has resulted from animals eating solid lumps of urea remaining on the pasture after fertilisation or as a result of animals consuming urea-contaminated drinking water.

• Overseas reports suggest that ammonium sulphate and urea are more efficient N fertilisers on very wet or waterlogged soils. The reason for this is reduced N loss by denitrification.

• Both ammonium sulphate and ammonium sulphate nitrate contain large amounts of sulphur (S) which is an essential plant nutrient. Sulphur is also present in superphosphate. Where superphosphate has not been applied to pastures for some years, a dressing of ammonium sulphate or ammonium sulphate nitrate is advisable in order to ensure an adequate supply of sulphur for plant growth.

• Nitrogen fertilisers accelerate soil acidification. The sources, however, differ markedly with respect to the extent of acidity generated. The most acidifying sources are ammonium sulphate and ammonium sulphate nitrate, while the least acidifying is limestone ammonium nitrate.

TABLE 2. The effects of varying soil fertility levels on the dry matter production of irrigated Italian ryegrass over a season in the Highland Sourveld of Natal. Total N applied at all fertility levels was 336 kg/ha.

Liquids

Liquid N fertilisers are not widely used on pastures in Natal. However, in recent times there has been some interest in the use of urea ammonium nitrate solution (UAN). As yet little research on this product has been carried out in South Africa. However, overseas research has indicated that the efficiency of liquids can equal that of solids. Particularly uniform pasture growth usually results where N in solution is applied through correctly calibrated spray equipment.

FERTILISER NITROGEN SCHEDULING AND APPLICATION

As indicated earlier in this leaflet, N differs from other important plant nutrients in that there is no mechanism for long term storage of fertiliser N in soils. Consequently, N additions should be commensurate with immediate pasture requirements. This means that the total N requirement must be applied in the form of split dressings over the growing season.

The amount of N to apply on each occasion depends on the level of production required, on the interval before the next N application and the time of the year. At establishment, dressings of from 30 to 70 kg N/ha are required; the lower rates apply where soil N reserves are high or where maximum early production is not a priority. For seeded species this dressing may be applied with basal fertilisers prior to planting or soon after seedling emergence. In the case of vegetatively (roots, stolons) propagated species, N dressings should be delayed until planted material displays clear evidence of growth. On established pastures, topdressing of from 40 to 75 kg N/ha generally give near maximum yields in each regrowth period.

The following points should be borne in mind with regard to N scheduling.

• Since dry matter production is closely related to the amount of N applied, N applications may, within certain limits, be used strategically to optimize fodder flow on the farm. Thus, for example, an N application could be withheld from kikuyu in mid-summer when feed is generally not limiting, and the same nitrogen could rather be applied to a tall fescue pasture to promote the production of foggage for winter use.

• On irrigated Italian ryegrass pastures farmers frequently attempt to stimulate production during the critical mid-winter period by regular, heavy N dressings. It must be recognized that growth in mid-winter is restricted by low temperatures and not by N insufficiency. In view of this, N dressings should be drastically reduced (or terminated) in mid-winter and the bulk of the N applied in spring and late summer/autumn.

• Research findings emphasize the need for light, frequent N applications. Nitrogen losses by leaching, denitrification and volatilization increase with increasing N application rate. Furthermore, infrequent heavy dressings may result in excessively high protein and nitrate levels in herbage. In practice, therefore, a total seasonal N rate of 300 kg/ha should be applied as six dressings each of 50 kg N/ha, rather than as three dressings of 100 kg N/ha. Four- to six-weekly intervals are optimum for nitrogen topdressings.

• There is evidence to suggest that on pastures having a dense mat of surface material the initial response to N is very slow, with a part of the applied N being absorbed by bacteria living in the organic matter of the mat. The continuance of N applications overcomes this initial defect.

• Heavy N applications to grass swards in periods leading up to seed harvests are likely to promote lodging and thus reduce seed recovery.

Application of N to pasture may be as solids, in spray solution or through irrigation water. The uneven growth of many pastures is evidence of the poor distribution of fertiliser nutrients. The following aspects regarding the application of N to pastures should be borne in mind.

• The different solid N sources tend to have different flow characteristics and therefore varying spreading widths. Careful calibration of fertiliser distributors is thus essential.

• Solid N fertilisers applied to wet grass may "burn" (scorch) the grass. Severe burn, particularly in the case of temperate tufted species, may result in individual plants or the whole pasture being destroyed. Care should be taken, therefore, to ensure that herbage is completely dry prior to fertiliser application.

• The application of N through irrigation water, commonly called fertigation, has become an established practice in Natal. This is not surprising since it is generally the most convenient and inexpensive method of applying fertiliser. In addition, fertigation offers tremendous flexibility with regard to the rate of fertiliser application. A major disadvantage of this method is that fertiliser distribution is determined by water distribution, which, in most cases, is far from even. Drift of water as a result of wind is also a complicating factor. Sound irrigation management is thus called for in order to ensure efficient N application. If N is applied to the pasture through fertigation then drinking troughs should not be connected to the irrigation system. Injection of N should commence at the start of the irrigation cycle in order to wash N into the soil and to flush N out of the system to prevent corrosion of irrigation pipes. Some fertiliser will remain on plants after irrigation, but because of the extreme dilution, leaf-burn is not usually a problem.

• Application of N solutions by means of spray equipment may cause leaf-burn if dilutions are not correct or if the solution is sprayed onto herbage in the heat of the day. Use of a "dribble bar" which trickles the solution through flexible polythene tubes in contact with the ground minimizes the risk of burn.

In dealing with N requirements of pastures, mention must be made of nitrogen contributions from legumes. Provided legume-only swards are effectively nodulated no fertiliser N is required for maximum yields to be realized. In legume/grass pastures the legume may contribute from 50 to 250 kg N/ha to the pasture. This is insufficient to obtain maximum dry matter production from the grass component and for this reason fertiliser N is frequently applied to grass/legume pastures. Considerations to be borne in mind in managing grass-legume pastures are listed below.

• Fertiliser N applied in small doses continuously throughout the growing season, may simply replace legume-N (i.e. there is a reduction or even a termination in N fixation by the legume). However, N applications at specific and carefully chosen times, when the growth of the legume is not at a maximum, may be used to improve productivity from the sward as a whole.

• Research on Italian ryegrass/clover pastures has shown that it pays, in terms of animal performance, to sow clover in the sward, even when fertiliser N is to be used heavily. Under these conditions clover-N fixation is negligible. However, a superior quality herbage is produced and the clover extends the grazing period when compared with a pure Italian ryegrass pasture.

• Where N fixation by the legume component of a mixed sward is relied upon to supply N to the pasture, an important consideration is that the cycling of N through excreta is the principal avenue whereby N from the legume is made available to the grass. Thus "cutting-and-removal" management, or those grazing systems which result in the animals depositing their excreta off the pasture, is likely to limit grass growth through N deficiency.