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Pictorial Evaluation Tools - PETs

- Crop Yield Assessment Manuals, based on photo-indicators

PET- Crops Yield Assessing Methodology

1. Methodology

1.1  The PET- Crops protocol to estimate crop performance through identifying yields at harvest time comprises the structured application of increasing intensities of observation, best described diagrammatically as an inverted pyramid as shown in Figure 1, beginning with observations from a vehicle, then walking and finally made within fields during discussions with farmers.

Figure 1. Observation Levels

Inverted pyramid showing transects for crop assessment

2. Vehicle Transects

2.1 At the first level, observers identify the approximate yield of all fields seen through the windshield of a vehicle travelling at a slow, steady speed. At such a distance and using the photo-indicators (annotated PET photos) of the fields from-a-distance, on the appropriate page in the manual for the crop in question, the proportion of harvestable crops passed on the journey that fit into the 3 categories, BLUE (low), YELLOW (medium) or RED (high) are identified.

2.2 The 3 colour categories have been established to ease rapid approximation of yields during transects. Indeed the range of yields should be regarded as contiguous - NOT as three preset isolated discontinuous clusters into which all high, medium and low crops must fall. The colours provide a convenient initial grouping, but do not preclude assessments of levels between each band (i.e. red-yellow; or yellow-blue). Also in some years it is possible, although unlikely, that there may be no crops in the red band. In such a year, the highest crop may be found in the yellow band. Other years there may be no crops in the blue band, the lowest crops being found in the yellow band, again equally unlikely, but possible and such instances will be recorded in the system.

2.3 Every small field, or each regular section of large fields passed during the journey (transect) is classified as blue, yellow or red on prepared sheets by noting single ticks in columns of boxes under the corresponding heading that may be added-up at the end of the transect to enable weighted average yields of each crop under assessment to be prepared.

3. Walking Transects

3.1 On arrival at a designated area, or, an area of specific interest as determined by the results of the initial observations, or at predefined regular intervals, observers leave the vehicle and switch to walking transects which enable the fields to be observed more closely and allow crops to be categorised at low, medium or high levels within the colour bands and at levels between, below or above the colour category already determined during the driving transect, allowing more accurate assessments of yield to be made using the more detailed annotations linked to the photographs of close-up (1 sq m) and photos of the harvested product of that 1 sq m.

3.2 When sufficient fields have been closely observed and scored during the walking transect, the sequence of actions is completed by conducting detailed case-studies of a small number of available farmers involving a) semi-structured interviews in his/her field using a common pre-tested/proven checklist, and b) taking representative samples of the field, threshing, winnowing and weighing the product to cross-check your use of the photo-indicator.

3.3 At the end of each day, each team summarises the weighted returns for the vehicular and walking transects and prepares summary sheets from the completed interview check lists. Samples are re-bagged, stored safely ready for drying later to determine dry-matter content: and the day's data cross-referenced to check for inconsistency.

PET Methodology - further details

Emphasis is placed on the facts that:-

  • PET is a tool for rapid assessment using Optimum Levels of Accuracy (OLA);
  • As such speed of application, analysis and reporting is essential;
  • The PET manual provides the formula to be applied to assess yields, but it is the rigour, consistency and stamina of the teams using the manual that make the approach successful;
  • PET provides estimates of yields per hectare at harvest time; it does not provide forecasts for immature crops, therefore the timing of assessment missions applying PET needs careful planning. PET estimates of crops just prior to harvesting need to be adjusted for moisture content (by drying to a constant weight);
  • PET (the manual) does not provide a ready-reckoner for area; area should be assessed by collecting data from a variety of sources including a) theoretical estimates from previous surveys; b) direct data from well-positioned and involved key informants including regional/district MoA extension workers, elders, tractor drivers, tractor owner/managers, threshing-machine operators and grain traders; c) cross-checking such data against each other and against field observations made during PET transects may be accomplished by overlaying approximate farm/field densities on maps prepared with suitably-sized grids to determine closeness of fit;
  • Combining the yield estimates from PET with the best-fit area estimates provides a rapid and direct estimate of production with which to identify food security strategy options at an early stage in the marketing year in prospect.

Selection of the farmers with whom to conduct semi-structured interviews and cross-check PET estimates connects to farmer availability at the time of the transects:-

  • Just before harvest time when the missions take place, farmers are found in or near their fields, if not bird scaring then watching events;
  • Farmers selected should be those farmers who are amenable, willing to spend time to talk, and happy to allow their fields to be entered and samples taken*;
  • The relative positions of the chosen farmers compared to other farmers should be judged before starting interviews by comparing their fields with all others seen during the walking transect- are their fields better or worse that those surrounding them?
  • A copy of the interview check list/summary sheet used with all recommended techniques may be found in the CFSAM guidelines (FAO/WFP, 2008);
  • Sampling details- why? when? and how? to be applied are included in the PET - manual;
  • Regarding grains sampled a) If the sample is taken when grain is harvest-dry (grain snaps when bitten) the sample should be weighed, the weight carefully recorded; and the grain returned to the farmer; or b) If the grains were still wet, as detected by a soft bite, the grains should be weighed fresh, a purchase price for the sample negotiated and the grains placed in a strong manila envelope, labelled with all details, and taken back to base for drying to constant weight. (Procedures to follow are included in the PET- Crops manual).


* In my experience, approached correctly, all farmers are fascinated to know what you think they have got .


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PET- Crop Assessor's Course





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