Centre for Conservation Farming Charles Sturt University. Wagga Wagga. NSW 2678
Currently, considerable effort is being put into encouraging farmers to participate in objective crop monitoring programs. What does this mean, why is it required, what does it involve and what are the benefits? This paper discusses these issues in a broad sense in relation to wheat production. The general principles, however, are applicable to most crops and particular instances are noted.
What is objective monitoring?
Objective measuring involves taking physical measurements/observations of relevant crop parameters for comparison with known benchmarks or targets to assist with crop management and to facilitate the evaluation of crop performance.
Typically this involves the recording of information relating to climate, nutrition, pests and diseases, crop development, water use, crop husbandry and crop performance parameters. This information can be used to establish the yield potential of a crop. The other factors indicate whether the crop is likely to fulfil this potential or identify the management required to rectify identified problems. Objective monitoring relies on comparison of the situation with established benchmarks and targets to determine the best course of action, as opposed to subjective decision making, which may involve carrying out a particular activity because that is the way it has always been done.
Why is objective monitoring necessary?
Australian farmers have always known about the variability in our climate, particularly rainfall. This variability has resulted in considerable effort being put into the understanding of crop water-use requirements and the development of drought tolerance in crops and pastures.
While the water-use efficiency concept (measuring grain production per mm of water used) was developed many years ago, it was work by French and Schultz during the 1970s/80s in South Australia which highlighted the relationship between water-use and crop yield. They found that after allowing 110mm for evaporation, wheat was able to produce 20 kg of grain per hectare for every mm of water used by the crop. This then enabled researchers and farmers to calculate how a crop should perform in a particular amount of growing season rainfall (GSR) under South Australian conditions (French and Schultz, 1984). Thus
Yield ( kg/ha ) = ( GSR - 110mm ) x 20
While this type of work indicated an almost linear relationship up to a certain GSR, investigation of typical farmer yields shows a far from linear trend. Cornish and Murray (1989) found that for the Wagga Wagga region, where stored moisture at sowing is likely to be higher and evaporation lower than in South Australia, potential yield can be approximated by the equation:
yield ( kg/ha ) = 15 W -1000
where W is crop water use and estimated by
W ≅ 1/3 (January to sowing rainfall) + (sowing to maturity rainfall)
They also found that during 1980-84 where moisture was the limiting factor, experimental yields displayed a linear relationship, whereas farmer yields in the Wagga Wagga district plateaued at less than half the potential yield calculated for the average GSR as shown in Figure 1.
Figure 1. Actual vs Potential wheat yields (Cornish and Murray, 1989)
n = Experimental yields, u = Wagga district farmer yields
Clearly, crop performance was influenced by factors other than moisture such as:
• timeliness of operations
• pests and diseases
• plant population
• other climatic factors
While climate may determine potential, it is management which influences whether it is achievable.
There is an excellent understanding of many of the factors which determine the performance of a crop. It is therefore possible to compare the measured or observed characteristics with expected or target levels determined as being necessary to achieve a particular yield/product quality. Management can then be adjusted accordingly. This is the basis of objective monitoring.
What does objective monitoring involve?
The first step in an objective monitoring program is to establish the potential yield. This can be used actively during the growth of the crop to influence management, or passively after the crop has been grown, as an indicator of how the crop should have performed. Once potential has been estimated, the crop's likely requirements can be calculated and incorporated in its management. As the season progresses, monitoring of the relevant characteristics will allow for the potential yield estimates and management to be adjusted accordingly. In monitoring the crop there are several key times when particular observations/measurements need to taken.
Establishing crop potential will initially involve the use of average rainfall figures to determine yield potential of an average year. Using the Cornish and Murray estimate, average potential yield for the Wagga Wagga region is approximately 4.2 t/ha, assuming an appropriate sowing time. As sowing is delayed, this potential can decrease by up to 4% per week. While this estimates the potential for an average year, some crop characteristics cannot be adjusted during the year if the season is above average, and allowances therefore need to be made for this possibility. In the Wagga Wagga region wheat yields of 6 t/ha are frequently achieved in good years and this can form the basis for many crop decisions. Assuming an average potential of 4.2 t/ha, with the possibility of 6 t/ha, several crop requirements can be calculated.
Crop establishment - A yield of 6 t/ha requires around 600 heads per square metre which, ideally, comes from 200 plants per square metre. The sowing rate will need to provide sufficient seed for this plant population.
Nutrition - Any nutritional planning should aim to be replacing all of the nutrients that are to be removed by a crop. The nutrients likely to be removed in the largest quantities are phosphorus, sulphur and nitrogen. The low solubility of phosphorus means that total requirements need to be provided prior to, or at, sowing and that unused P will remain at the soil surface. Generally, 4 kg of P is provided for each 1 t/ha of expected yield. Similarly, the smaller sulphur requirement of 1.5 kg/t means that it is most economically applied at or before sowing.
Nitrogen is very different as it is required in large amounts and can be leached from the root zone. It can, however, be applied during crop growth and, therefore, the amount supplied can be adjusted to suit the season. However, it is important to note that the earlier the N is applied, the more efficient the uptake will be. The level required will depend on the grain protein and efficiency of uptake and is in the order of 35 kg N/t at 10% protein and 40 kg N/t at 11.5% protein. Nitrogen management is one of the key reasons for objective monitoring. Once the likely requirements have been established the potential soil supply can be determined from soil testing to calculate the amount and type of fertiliser needed.
Soil water - In years with average or below rainfall, the amount of stored water at sowing can play an important role. The measurement of soil water at this stage can give some indication of paddock potential, should the season be below average, and allow paddocks to be prioritised. It will also provide the starting point for calculating the water use efficiency of the crop.
Once the crop has emerged (at the 2 to 5 leaf stage) it is useful to evaluate crop establishment, with plant counts showing if the target population has been achieved. If it has not, the reasons need to be determined. Has the seed germinated, was sowing depth correct or have soil factors restricted emergence? This will identify areas for improvement in future years and, if the population is very low, allow for an adjustment in management. Application of nitrogen may be considered to stimulate tillering to compensate for the lower plant population.
At this time the type and number of weeds can be determined and recorded. This will not only help in planning control during the current year, but assist with planing in the following crops.
If the crop establishment is within an acceptable range, tiller numbers become the next crop characteristic to be considered. If the plants do not tiller as expected then most often nutrition, particularly nitrogen, will be the cause. From early tillering, tissue N levels can be monitored using Sap-Nitrate or NIR to compare actual levels with known desirable concentrations. If these readings indicate a lower than expected concentration then nitrogen can be top-dressed. This technology is rapidly developing, particularly with regard to grain protein, and is also beginning to be used for estimating the nitrogen requirements for crops other than wheat.
At the same time, these measurements need to be considered in conjunction with other crop features. For a crop to be responsive to nitrogen top-dressing, research and experience has shown that tiller number and the presence of disease and or weeds also needs to be considered. The response to 40 kg N/ha at decimal growth stage 30 is shown in Figure 2.
The desirable level will depend on the expected potential which needs to be adjusted as the season progresses, substituting the average rainfall figures for actual values as it occurs. This may be assisted with use of the rainfall deciles which can be used to evaluate GSR in terms of whether the season is above or below average. Decile 1 represents the driest 10% of years, decile 10 the wettest 10% of years. At April 1, the total GSR is predicted for different various deciles, as shown in Figure 3. At the beginning of each subsequent month the actual cumulative rainfall is added to the prediction for the remaining GSR. At the beginning of the season there will be a large difference between deciles 1 and 10, but as actual rainfall is substituted in the range it is reduced making this process useful for estimating potentially available crop water.
Figure 2. Wheat response to 40kg N/ha at DC30 ( Angus et al. 1989).
Figure 3. Predicted and actual cumulative rainfall
Once the crop has commenced flowering there are several important issues.
Head numbers - If head numbers are lower than tiller counts, this may be indicative of nutritional limitations, particularly nitrogen.
Weed control - The weed control program will have been completed and can be evaluated. This is important to identify possible existence of herbicide resistance and the general performance of the control measures used.
Pests and diseases - The presence of pests and diseases needs to be identified to ensure that action, where appropriate, is carried out to reduce their impact. Recording their occurrence is also vital for future planning. Without regular checking, considerable losses may be incurred.
Soil water - Measurement of soil water at flowering will also give an indication of the amount of rainfall required to achieve the anticipated yield.
Harvest is the final stage of the crop program and there are several parameters which need to be observed and recorded.
Head numbers - The number of heads at harvest will allow for the calculation of tiller survival. A low survival post- flowering generally indicates moisture stress or the presence of disease or insects.
Grain yield and quality - While it may seem obvious, it is necessary to record both yield and quality to fully evaluate the performance of a paddock. Even if a paddock yields extremely well, protein below desired levels indicates that there is room for further improvement.
Soil water - Measurement of soil water at harvest allows for the calculation of water use efficiency which in turn provides an excellent means for comparison of crop performance between paddocks and years which may have had greatly varying rainfall.
Throughout the season it is also necessary to be recording the dates at which activities are carried out and the rates of the various inputs being used. Recording rainfall is a vital component of crop monitoring and provides very useful information as opposed to interesting information, which is how it is often seen.
There are any number of ways in which this information can be recorded. The State Departments of Agriculture in NSW, Victoria, South Australia and Western Australia are promoting the use of the same 'MEYCheck' format which allows farmers to easily compare their results with other farmers. These forms also provide useful information about the various targets and benchmarks relating to crop monitoring. There is, however, no right or wrong way to record the information as long as it contains the necessary details and is easily understood.
What are the benefits of objective monitoring?
The primary benefit of objective monitoring will be the greater understanding of how a crop is progressing and the resulting improved management, with decisions based on quantifiable parameters combined with the experience and 'gut feelings' of the manager. Following from this is the ability to evaluate the performance of a particular crop. Without objective monitoring and recording the factors which contribute to the success or failure of a crop may go unnoticed, delaying the recognition of these factors as being critical and preventing the deliberate rectification of problems or repetition of successful management practices.
With improved understanding of paddock performance, planning can be more productive, thereby improving management efficiency and reducing the opportunity for environmental degradation.
Future directions for objective monitoring
As farmers become more proficient at monitoring there will be a change from paddock generalisation to more site specific monitoring and management with areas within a paddock being treated as required. This type of monitoring/ management is being facilitated by the development of global positioning systems and computer technology and will provide for considerable gains in crop productivity/ efficiency.
1. Angus, J.F., van Herwaarden, A.F. and Fischer, R.A. (1989). Predicting nitrogen response of wheat to top-dressed nitrogen. Proc. 5th Aust. Agron. Conf., p.550 (Perth).
2. Cornish, P.S. and Murray, G.M. (1989). Low rainfall rarely limits wheat yields in southern New South Wales. Aust. J. Exp. Agric. 29, 77-83.
3. French, R.J. and Schultz, J.E. (1984). Water Use Efficiency of Wheat in a Mediterranean-type Environment. II. Some Limitations to Efficiency. Aust. J. Agric.Res. 35, 765-75.