How to monitor and sample your crop

Crop monitoring and sampling protocol

Key points

  • After planting but before 1st plant emergence identify a minimum of three representative sample areas of the crop.  These areas will be used to record plant emergence, ground cover and yield.
  • At each sample area, record plant emergence in order to calculate the date of 50% plant emergence.
  • Ground cover must be measured weekly using a grid or the NIAB Network app from one week after 50% plant emergence to haulm destruction/harvest.
  • The first crop sample needs to be taken 50 days after 50% plant emergence, for maincrops or 35-40 days after 50% emergence for salads.
  • The sample area will depend on row/bed configuration and crop type. Do not bulk samples before grading.
  • How you grade the tuber samples will depend on how you plan to use the data.

Initial field set-up and measurement of crop emergence and ground cover

Number of replicates and sample location

Crop growth and yield are spatially variable and this is particularly true for potato crops.  To ensure that the measurements of ground cover and yield samples are representative of the entire cropped area, then the crop must be sampled in a minimum of three positions (Figure 1).  It is also important that sample points are positioned in areas that appear representative of the majority of the crop.  Avoid atypical areas (nematode/diseased areas, fertiliser/herbicide misses or overlaps etc.). Also, avoid headlands, spray/irrigation-wheeling or rows adjacent to these wheeling, edges of the field, previously-sampled areas and any other areas which are obviously unrepresentative of the crop. If the field is irrigated by rain gun, ensure that the sample point is at least 50 m from the edge of the field along the ridge.

If more than one seed size/stock has been planted in a field, ensure that the sample replicates are representative of the relative areas planted with different stocks. Alternatively, treat each seed size/stock as a separate ‘crop’ and take a minimum of three samples from each area planted with the same seed. A decision should be made at the beginning of the season as to which plantings within a field will be sampled and then measurements always be taken from that planting.  In many cases, it will be sufficient to sample the planting that occupies the majority of the area of the field.

Likewise, if areas of the field are considered distinct, it may be necessary to treat each distinct area as a separate ‘crop’ and always take the samples from within these crops. Do not combine samples from distinct areas within a field. Reasons to consider areas within the field as different crops include: more than one variety, differences in use of polythene, sprouting, date of planting, field cropping history etc.

Figure 1. Example of crop sampling points within a field. 

Measurement of emergence

It is important to know the date when 50% of plants have emerged since, amongst other things, this allows prediction of when tuber initiation occurs and hence when scab control irrigation regimes should start. After planting, but prior to emergence, use flexi-canes to mark out a minimum of three 10 m runs of bed (or pairs of ridges) and record the location of the marked areas (a GPS fix of each location can be useful). Emergence counts must be started as soon as the first plant is visible and then count and record the number of plants emerged within the 10 m long sample area. These counts need to be repeated at least once a week until the crop is fully emerged (i.e. no change in the number of  emerged plants). Once the crop is fully  emerged, estimate the date of 50 % plant emergence by calculating the date on which 50 % of the final number of plants had emerged. This date must be entered onto the NIAB Network for the yield model to work.

Measurement of ground cover

Ground cover is a measurement of the percentage of the  ground that is covered by green, productive potato leaves.Regular measurements of ground cover  development are useful for the timely identification of problems, to estimate  yield production and to schedule irrigation.Measurements of ground cover are made in the same areas used to measure  emergence and should be made weekly from one week after 50 % emergence onward  to haulm destruction/harvest. Ground covers may be measured using a grid (see Appendix 1 on how to build and use a  ground cover grid) or by using the NIAB Network app (see NIAB Network app user guide).

Crop sampling to measure tuber fresh weight yield, numbers of plants, stems and tubers

The exact protocol used to sample crops will depend on several factors and include:

· How the data will be used subsequently

· The necessary compromises between data accuracy, the number of fields to be sampled and staff availability

· Availability of specialist equipment (e.g. tuber grading systems and drying ovens)

Location of crop samples

The crop should be sampled near to the areas used to measure crop emergence and ground cover. However, avoid taking samples from where field staff have walked to and from the sample locations.

Timing of crop samples

The ideal time for the first sampling is usually 50 days after 50% crop emergence, for maincrop potatoes or 35-40 days after 50% crop emergence for salad potatoes. If the initial samples are taken too soon, then estimates of the total tuber population may not be  reliable. Forecasts of the change in tuber size distribution (and crop value), with the change in yield, will not be accurate. However, if samples are taken too late, then the benefits of early yield forecasting are reduced.(Sampling within a couple of days either side of the planned date is deemed OK for weather/staff availability reasons, so long as the actual date of sampling is recorded.)

Sample area

In order to get an accurate estimate of plant, stem and tuber populations,the sample area needs to take into account the planting configuration and the average plant spacing. Some guidelines are shown in Table 1, but local circumstances (for example differences in growth between the left- and right-hand rows within a bed) must also be considered.

Table 1. Variation in sampling strategy depending on planting configuration


Mark out the sampling area using a tape measure.You will need to ensure the ends of the sample area are between plants – if  not move the sample area up or down the row as appropriate. After the sample area has been marked out, record the number of plants and stems in the sample area. Stems can be recorded as the total number of above ground stems or as the number of main stems and secondary stems. It is good practice to note the size of the mother-tuber (if still intact) as a check that the sample is being taken within the correct part of the field. Carefully collect all tubers > 10 mm and retain for grading. Be careful not to ‘poach’ any tubers from adjacent plants outside the harvest area. Discard any rotten tubers and replace with the same number of tubers of a similar size dug from additional adjacent plants. Record the number of tubers with rots. Do not bulk samples from the individual sample areas.

Tuber grading and data recording

How you decide to grade the tubers will depend on what crop  you are growing and how you plan to use the data.Use your usual method of grading as individual templates can be used for the inputting of yield digs.

Modelling of total yield

If the data is to be used to produce forecasts of total (e.g. > 10 or 20 mm) yield, then only the number and weight of tubers above a lower-size limit (e.g. 10 or 20 mm) need be reported. However, if forecasts of changes in tuber size distribution are needed, then the sample will need to  be graded into several size grades.

Modelling of tuber size and distribution

In order to accurately model the change in tuber size  distribution with change in in total yield, the sample will need to be graded into a minimum of five grades. It is important that these grades are more or less equally spaced and the grades are selected so that there are some tubers in each of them.

Some generic grading profiles are available on  the NIAB Network site but if these are not suitable than you can create your own. An example of a completed grading sheet is shown in Appendix 2. Once grading has been completed the data is  then entered on the NIAB Network web-site –see the User Guide on how to do this.

Table 2.  Suggested grading profiles for seed and salad and main and processing potato crops

Tuber dry matter content or specific gravity

For some sectors of the potato industry (e.g. crisping or chipping) information on the tuber dry matter concentration (or specific  gravity) at the time of sampling is important. End-users of the crop will have their own protocols and these should be followed.

Appendix 1.  Construction and use of a ground cover grid

Make up a wooden grid frame from 40-50 mm x 12-15 mm lathe, the internal dimensions C and E are shown in Figures 3 and 4 below. Drill holes at centre distances D and F marked below. Note: there must be 100 squares or rectangles! String grid using nylon braided cord (orange shows up well against potato leaves). You will require approximately 20-30 m of cord.

Place grid over a representative area of plants in field. Keep grid level with top of plants (i.e. do not squash plants so that leaf area distorts).The left-hand edge of the grid (A) should be aligned directly over the wheel furrow, with the right-hand edge (B)  immediately over the central furrow of the bed (or centre row if using 3-row  beds) (Figure 2).

Figure 2.Using a ground cover grid in a single-row planting configuration.

This positioning will then take account of any ‘staggered’  bed arrangement (e.g. where the two rows within a bed are closer together than the two rows either side of the wheel-furrow).Try to keep your eyes directly over each square when assessing area  covered by leaf, since this will reduce the error created by parallax when viewing the squares at an acute angle. Count each square with 50% or greater of their area covered with leaf  (not stem) material as 1%. Count squares which are less than 50% full as zero. Do not add up fractions of squares to make an entire one.The use of the grid is especially important at ground covers less than 80 %, since operator error using visual assessment without a grid can create large variations.

Figure 3.Measuring ground cover in a two-row bed  system.

Figure 4.  Measuring ground cover in a three-row bed system.

Appendix 2.  An example of a completed grading recording sheet

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