Nitrogen management strategies on perennial ryegrass-white clover pastures in the Western Cape Province

[摘要] The response of perennial ryegrass and white clover, grown under controlled conditions, tofertiliser N rates applied under variable soil temperature (6, 12 and 18 °C), soil water potential(-10, -20, -25 and -35 kPa) and seasonal growing (June/July and October/November)conditions as well as field conditions, were evaluated. Primary- (PDM), residual- (RDM) andtotal dry matter (TDM) production (g pot-1) were recorded over the first- and second regrowthcycles as well as the accumulative DM production over the two regrowth cycles, respectively.Leaf N content (%) was recorded at the end of first and second regrowth cycles. Tiller/stolonnumbers and root dry mass (g pot-1) were recorded at the end of the second regrowth cycle.Soil ammonium-N and nitrate-N (mg kg-1) content was monitored after fertiliser Napplication.Decreasing soil temperatures resulted in decreased TDM production in both crops. Onlyperennial ryegrass was influenced by fertiliser N rate, with a general increase in dry matterproduction as fertiliser N rate was increased. Ryegrass TDM production did not differbetween the 100 and 150 kg N ha-1 rates but were both higher (P=0.05) if compared to the 0and 50 kg N ha-1 treatments. Soil nitrate levels 31 days after application of 150 kg N ha–1 werestill sufficient to stimulate ryegrass RDM production. The 173.8% increase in ryegrass TDMproduction measured at 6 °C where 150 kg N ha-1 was applied compared to the 0 kg N ha-1treatment illustrated the ability of ryegrass to respond to fertiliser N at low soil temperatures.Soil water potential of -20 kPa resulted in higher ryegrass PDM and TDM productioncompared to the -25 and -35 kPa levels. White clover PDM and TDM production werehowever not influenced by soil water potential or fertiliser N rate. Ryegrass TDM productionincreased (P=0.05) as fertiliser N rates were increased. The most favourable soil water levelfor both ryegrass and clover root development was found to be -35 kPa.Perennial ryegrass and white clover PDM, RDM and TDM production were higher during theOctober/November season compared to the June/July season. Increased fertiliser N ratesresulted in increased (P=0.05) ryegrass PDM and TDM production. White clover dry matterproduction was not influenced by fertiliser N rates. In the field study the effect of 0, 50, 100 and 150 kg N ha-1 applied as a single applicationeither in autumn, early winter, late winter, early spring or late spring on pasture dry matterproduction, clover content and selected quality parameters of a perennial ryegrass-whiteclover pasture were investigated. Soil nitrogen dynamics in the 0-100, 200-300 and 400-500mm soil layers were studied for 49 days following fertiliser N application.The effect of 50 kg N ha-1 on soil N dynamics was generally the same as found at the 0 kg Nha-1 applications and may therefore be regarded as a low risk treatment. The application of150 kg N ha-1 especially in autumn and early winter showed a tendency to exceed theabsorption capacity of the pasture and thereby expose fertiliser N to possible leaching andcontamination of natural resources.Increased fertiliser N rate resulted in a general increase in pasture dry matter production withthe highest yields recorded where N was applied in early and late spring and the lowest inearly winter. The application of 150 kg N ha-1 in early and late spring resulted in the highestTDM production, however, the 50 kg N ha-1 resulted in a more efficient conversion of Napplied to additional DM produced. In contrast to DM production, the clover percentagegenerally decreased as fertiliser N rate was increased. The effect of season of application wasinconsistent. Annual trends show that the clover percentage eventually recovered to the samelevels as the 0 kg N ha-1 treatments. Due to the above minimum levels recorded for mostmineral and quality parameters tested it is envisaged that treatment combinations as used inthis study will not be at any disadvantage to pasture and animal productivity.The study has shown that the use of fertiliser N to boost perennial ryegrass-white cloverproductivity and thereby minimising the negative effect of the winter gap on fodder flowmanagement during the cool season in the Western Cape Province, may be an importantmanagement tool. Except for late spring applications, all seasons of application reduced thenegative impact of the winter gap on fodder availability. It is concluded that regression linesas summarised in Tables 7.2 and 8.2 show great potential to be instrumental in developingregression models, accurately predicting the effect of fertiliser N rate on pasture performance.Other factors to be considered includes the productivity of the pasture, initial clover content,expected clover content at the end of the first regrowth cycle after fertiliser N application andthe quantity of additional fodder required. Additional requirements will be to maintain and 150 kg N ha-1) in winter, as the N uptake capacity of the pasture could be exceeded andthereby increasing the risk of N leaching, resulting in environmental pollution. The Nresponse efficiency of the pasture is also the lowest at the 150 kg N ha-1 rates, therebyreducing the profitability of these treatments.

[发布机构] Stellenbosch University