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The Western Free State is a major potato producing region in which 38 producers produce approximately 51.1% of South Africa’s commercial potatoes on 6 871 ha (2024 harvest year). The main cultivars produced for commercial consumption (table and processing) are Mondial (32%) followed by Sifra (28%) and Panamera (20%). Approximately 91% of potato production in this region is under irrigation.
Kroonstad is located in South Africa’s summer rainfall region. This region’s moderate climate is characterised by very hot summers (warmest in December/January) to cold winters, with frost occurring from April.
The Kroonstad cultivar trial was laid out in sandy loam soil in a randomised block design with three replications per cultivar. Relevant technical information relating to the trial is summarised in Table 1.
Influence of growing periods
Cultivars with short and long growing periods were included in the cultivar trial which means growing periods can influence the eventual yield of certain cultivars. The length of growing periods is subject to the nature of a given season but is regarded as the time that passes from emergence to natural leaf senescence. Table 2 outlines how growing periods can differ from cultivar to cultivar.
Environmental factors and management practices also influence the different growth phases and when it commences. The ideal practice would be to harvest cultivars once they are harvest ready, but this is not practical when conducting a trial on a commercial farm.
The density and haulm count of each tuber influence its sizes and yield. The number of axillary buds per tuber is cultivar dependent and determines the number of sprouts a tuber can yield. In this regard, the plant readiness of tubers is important since ideal plant readiness usually results in better sprouting and more stems per sprout. Table 2 indicates the plant readiness of tubers at the time of the trial as well as the stand density percentage and haulm count observed later in the growing period.
Cultivar performance
The evaluation of new cultivars such as in the Kroonstad cultivar trial delivers results regarding, among others, yield and marketing index. The marketing indices of the relevant cultivars are calculated by classing and sorting each cultivar according to quality and size groups (such as Class 1 large or Class 2 large-medium). Prices were then compared to market prices at harvest time.
The performance of new cultivars cannot be based on the results of one particular season alone, since climate can vary from one year to the next. For this reason cultivars are preferably tested across several seasons.
Rainfall and climate trends
As with any crop, temperature, water availability (good irrigation scheduling or rainfall), as well as heat units are important factors with a significant influence during the potato plant’s growing period. These factors are therefore taken into account when evaluating cultivar performance.
Relevant daily and long-term weather data was obtained from a nearby Hortec weather station as well as a selected Agricultural Research Council (ARC) weather station located as close as possible to the trial site.
The rainfall trend for the 2024 season indicates a rainfall figure that is substantially lower than the normal long-term average. In a dryland trial such as this, the low rainfall that continued throughout most of the growing season was catastrophic in terms of yield. All rainfall up until the end of March was very low and isolated, thus limiting the difference rain could have made. Exceptionally high rainfall was recorded towards the end of the growing season, but the 147 mm recorded in April fell over just a few days at the beginning of the month (Figure 2).
Influence of heat and temperature
Figure 3 illustrates minimum and maximum temperatures recorded during the trail. Throughout winter, from the beginning of June to middle August, days with temperatures below 0°C were recorded. During the season maximum temperatures above the long-term average temperatures were recorded, with a few days recording temperatures above 35°C.
The collection of heat units during a growing period is an important factor in the development of a plant. The trend of available heat units for the cultivar trial during this season was significantly higher than the long-term data trend for heat units (Figure 4). This makes sense, given the persistent dry and warm conditions experienced.
Yield
Yield data collected during harvest day were statistically processed using the GenStat® programme. The mean was separated using the Tukey test of least significant differences (LSD).
The cultivar effect during this trial (Figure 5) was statistically significant (p<0.05) while the coefficient of variation (CV) was high (25.8%); a typical acceptable cut-off point for field trails is 25%.
The high CV value is indicative of the challenges posed during the trail which led to substantial variation in the cultivar yields. Therefore, the results cannot necessarily be regarded as very reliable. The yield of each cultivar is divided by the trial average and the average of all the cultivars is regarded as 100%. This creates a yield index and each cultivar’s performance in terms of yield is regarded as a percentage of the trial average.
The average yield of the trial for the 2024 season was 14.66 t/ha, which is much lower than the trial average recorded between 2018 and 2023.
Many cultivars experienced problems with regard to yield and density, which can be attributed to the dry and warm conditions during the month after planting (Figure 5). Lady Luce and Palace fetched the highest marketing index, which can be attributed to a good yield in large and Class 1 tubers.
Size group distribution and grading are the evaluations conducted when studying a cultivar’s marketability (Figure 6 and 7).
Just as seasons tend to fluctuate, so does the performance of cultivars from one season to the next. This is simply because the climate never the same from one season to the next. Therefore, it is important to consider consistent cultivar performance across a number of seasons. Mondial currently exhibits the least variation in the 2018 to 2024 Kroonstad cultivar trial (Figure 8).
Finally, when observing the internal quality of potatoes it is also possible to evaluate processing characteristics. To comply with processing requirements, cultivars have to comply with a chip colour norm of >50 and a specific gravity (SG) of ≥1.075 (Table 3). Eight of the 14 cultivars met both the chip colour and SG requirements. – Enrike Verster, Potatoes SA
Special thanks to the farming enterprise, co-workers, trial participants and the Western Free State working group. For more information, send an email to enrike@potatoes.co.za or laryssa@potatoes.co.za