Exploring potential irrigation management strategies for annual ryegrass using the SWB-Sci model

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RATIONALE

Irrigation uses about 62% of South Africa’s surface and ground-water resources (DWAF, 2004). Irrigated agriculture is facing fierce competition for this substantial share of water as the water demand for industrial, domestic, municipal and other activities are increasing rapidly. There is a need to increase water (and land) productivity, to meet the increasing demand for animal protein as human populations increase and diets become more affluent (Smil, 2002). Natural veld cannot fulfil this need alone and must be supplemented with irrigated and fertilised planted pastures. This requires intensive use of fertilisers and water, which leads to a higher cost of production and a greater risk of environmental pollution. Thus, farmers are under pressure to decrease their share of water and fertiliser usage, whilst at the same time, produce sufficient pasture for animal production to supply the protein demand of a growing population more efficiently.

IRRIGATED PASTURE PRODUCTION IN SOUTH AFRICA

More than 80% of South Africa is arid to semi-arid, with unreliable rainfall. This makes most of the country unsuitable for intensive agriculture such as dairy farming under dryland conditions (Gertenbach, 2006). Grasses are often grown under dryland conditions, however, there is a trend towards greater use of irrigation by farmers to improve the reliability of yield of pastures. It is estimated that the total area utilized for irrigated pasture production is approximately 16% of the total area under irrigation in South Africa. The most common irrigated pastures are ryegrass, kikuyu (Pennisetum clandestinum) and lucerne (Medicago sativa). Irrigated ryegrass and dryland kikuyu with supplemental irrigation, are the primary sources of feed in the pasture based dairy industry and are mostly grown in the relatively higher rainfall areas, particularly in the Natal Midlands, the Eastern Highveld, the Eastern Cape and in the winter rainfall areas of South Africa (Dickinson et al., 2004).

Irrigation guideline

In semiarid regions, water is the primary contributor to grassland production (Whitney, 1974). The development of well-established pasture requires favourable growing conditions with no water stress. This leads to higher yields and good nutritive valued pasture. In some situations, irrigation may give little or no advantage, especially in humid areas. Under hot climatic conditions, water deficits, even for short periods, limit metabolic processes, which may reduce growth rates. Therefore, the aim of irrigation management is to maintain a favourable supply of water within the root zone between the extremes of excessive dryness or wetness.

Water use efficiency

Water use efficiency (WUE) can be defined as harvestable biomass per volume of water used (Wallace, 2000). It includes the total amount of water needed for plant growth, including water lost through evapotranspiration from the soil and plant surfaces (Van Vuuren, 1997). Atmospheric demand, soil water availability and other cultural practices such as fertilisation, different cultivation practices and defoliation methods can influence water use of pasture. Nevertheless, water use of grasses is strongly affected by growth rate, length of season and soil surface coverage.

Acknowledgments
Declaration
Abstract
Chapter 1: General introduction
1.1 Rationale
1.2 Irrigated pasture production in South Africa
1.2.1 Irrigation guideline
1.2.2 Water use efficiency
1.2.3 Nitrogen guideline
1.2.4 Effects of excessive nitrogen applications
1.2.5 Nitrogen use efficiency
1.3 How can nitrogen and irrigation water use efficiency be improved?
1.3.1 Irrigation scheduling
1.3.2 Allowances for nitrogen mineralisation and carryover
1.3.3 Adaptive management
1.3.4 Modelling
1.4 Hypotheses and objectives
Chapter 2: Improving nitrogen and irrigation water use efficiency of annual ryegrass through adaptive management
2.1 Introduction
2.2 Materials and methods
2.2.1 Site description and crop management
2.2.2 Treatments
2.2.2.1 Fixed nitrogen application rates
2.2.2.2 Nitrogen mass balance
2.2.2.3 Adaptive nitrogen
2.2.2.4 Adaptive water
2.2.3 Data collection and calculations
2.2.4 Statistical analysis
2.3 Results and discussion
2.3.1 Forage yield and quality
2.3.2 N rate and nitrogen use efficiency
2.3.3 Water use efficiency
2.3.4 Potential leaching
2.4 Conclusions
Chapter 3: Nitrogen application and critical soil solution nitrate concentrations for optimum yield and quality of annual ryegrass
3.1. Introduction
3.2 Materials and methods
3.2.1 Site description and crop management
3.2.2 Treatments
3.2.3 Plant Sampling and quality analysis
3.2.4 Soil nitrate sampling and analysis
3.2.5 Calculations and statistical analysis
3.3 Results and discussion
3.3.1 Forage yield
3.3.2 Forage quality
3.3.2.1 Crude protein, true protein and non-true protein
3.3.2.2 Fibre
3.3.2.3 Metabolisable energy
3.3.3 Critical soil solutions nitrate concentrations for yield and quality
3.3.4 Nature of the trade-off between yield and quality parameters
3.4 Conclusions
Chapter 4: Improving water management of irrigated annual ryegrass using SWB-Pro model
4.1 Introduction
4.2 Model description
4.3 Materials and methods
4.3.1 Site description and crop management
4.3.2 Treatments
4.3.2.1 Irrigation strategies
4.3.2.2 Evapotranspiration measurement using the shortened energy balance method
4.3.3 Data collection
4.3.4 Model reliability test
4.3.5 Model application
4.4 Results and discussion
4.4.1 Model calibration
4.4.2 Model validation
4.4.2.1 Forage yield and leaf area index
4.4.2.2 Soil water deficit
4.4.2.3 Evapotranspiration
4.4.3 Model application
4.4.3.1 Water requirement
4.4.3.2 Irrigation calendars
4.5 Conclusions
Chapter 5: Simulating water and nitrogen balances of annual ryegrass with the SWB Sci model
5.1 Introduction
5.2 Model description
5.2.1 Crop nitrogen uptake
5.2.2 Organic matter turnover
5.2.3 Inorganic nitrogen transformations
5.3 Materials and methods
5.3.1 Site and treatments description
5.3.2 Treatments
5.3.2.1 Cedara
5.3.2.2 Hatfield
5.3.3 Data collection
5.3.3.1 Weather
5.3.3.2 Soil analysis
5.3.3.3 Soil water content
5.3.2.4 Crop growth, yield and nitrogen uptake
5.3.3.5 Soil solution nitrate concentration
5.3.4 Model parameterisation and testing
5.4 Results and discussion
5.4.1 Model calibration
5.4.2 Model validation
5.4.2.1 Forage yield
5.4.2.2 Leaf area index
5.4.2.3 Forage nitrogen uptake
5.4.2.4 Soil water content
5.4.2.5 Soil nitrate concentrations
5.5 Conclusions
Chapter 6: Exploring potential irrigation management strategies for annual ryegrass using the SWB-Sci model
6.1 Introduction
6.2 Materials and methods
6.2.1 Model description
6.2.2 Model modification
6.2.3 Model input parameters
6.2.4 Model output parameters
6.2.5 Scenarios simulation analyses
6.3 Results and discussion
6.3.1 Forage yield and water use
6.3.2 Water and irrigation use efficiency
6.3.3 Nitrogen leaching
6.4 Conclusions
Chapter 7: General conclusions and recommendations
7.1 Overview of the study
7.2 Balancing forage yield and quality using adaptive N and water management
7.3 Estimating water requirements and developing irrigation calendars using simple
web-based SWB-Pro model
7.4. Exploring potential N and water management strategies using SWB-Sci model
7.5 Recommendations
References
Appendix

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Using adaptive management and modelling to improve nitrogen and water use efficiency in crop production: A case study using annual ryegrass