Western Free State cultivar trial under irrigation at Bultfontein in 2025

Estimated reading time: 7 minutes

The Western Free State is a major potato production region where approximately 6 871 ha of potatoes (15.1% of the country’s potatoes) are planted by 38 producers. The most prominent cultivars produced in this region, roughly 91% of which are irrigated, are Mondial (32%), Sifra (28%), and Panamera (20%).

Bultfontein is located in South Africa’s summer rainfall region (Figure 1) and has, over the past 23 years, recorded an average annual rainfall of about 580 mm. This region has a moderate climate with very hot summers – particularly in December and January – and cold winters, with frost occurring from April to August.

Trial design and details

The cultivar trial at Bultfontein is laid out in a random block design with three replications per cultivar. Table 1 contains extra information relating to the trial. The cultivar trial includes cultivars with short and long growing periods, which can influence the final yield of certain cultivars. The length of growing periods is subject to the nature of the seasons, but is generally regarded as the time that passes from emergence to natural leaf senescence. Table 2 sets out how these growing periods differ among cultivars.

Environmental factors and management practices influence the different growth stages and their onset. In addition, the density and haulm count of each tuber influence its size 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 than tubers that are not yet ready for planting. Old seed tubers produce too many stems and small tubers.

The plant readiness of tubers at the time of planting, as well as the density and haulm count observed later in the growing season, are presented in Table 2. Representative soil samples were taken prior to planting in order to determine the soil nutrient profile of the trial site (Table 3).

Marketing-related results

The evaluation of cultivars, as in the Bultfontein cultivar trial, provides results on yield and marketing index.

Prices were then compared to market prices obtained at the time of harvest. However, the performance of cultivars cannot be based solely on a single season’s results, as climate varies from year to year. Therefore, cultivars are preferably tested across several seasons.

Climate and weather conditions

As with any crop, factors such as temperature, water availability (whether through effective irrigation scheduling or rainfall), and heat units significantly influence the potato plant during its growth period. These factors are considered when evaluating cultivar performance. Daily and long-term weather data are obtained from a nearby Hortec weather station as well as the closest Agricultural Research Council (ARC) weather station.

Rainfall was exceptionally above average during most of the active growing period of the 2025 season. Frequent showers kept the soil profile continuously saturated. Rainfall in April amounted to 204 mm, setting a new record. The below-average yield of this trial can be attributed to the above-average number of cloudy days and the resulting reduction in photosynthesis during the active growing period. Persistently wet conditions also contributed to quality problems.

Minimum and maximum temperatures are illustrated in Figure 3. The 2025 season experienced particularly large fluctuations in maximum temperatures. Numerous days with temperatures above 35°C were recorded during the first month after planting. Temperatures dropped below freezing towards the end of May, which coincided with natural leaf senescence.

The collection of heat units during a growing period is an important factor in the development of a potato plant. The trend of heat units available for this cultivar trial appears to be consistently higher than the long-term data trend for the season as a whole (Figure 4), despite the high rainfall and cloudy days.

Yield and cultivar effect

Yield data recorded during harvesting is subjected to statistical processing using the GenStat® program. The Tukey test of least significant differences (LSD) was used to separate the mean. The cultivar effect during this trial (Figure 5) was statistically significant (p<0.05), and the coefficient of variation was low (9.7%). These factors indicate that the trial was well executed, and the results are therefore reliable.

The yield of each cultivar is divided by the trial average, with the average across all cultivars set to 100%. This is used to determine a yield index, with each cultivar’s yield performance expressed as a percentage of the trial average.

The average yield of the trial in the 2025 season was 44.41 t/ha – somewhat lower than the previous five seasons’ trial average of 50.3 t/ha. The cultivars EXP B, EXP C and Palace statistically produced the highest yields. Palace, EXP B and Sound achieved the highest marketing index, which can be attributed to the excellent yield of large and Class 1 tubers (Figures 5, 6 and 7).

Size group distribution and grading evaluations in potatoes are essential for determining a cultivar’s marketability.

Cultivar performance will vary from one season to the next simply because the climate varies. Thus, it is important to consider consistent cultivar performance across multiple seasons. Of all the cultivars included in the trial, Palace currently shows the least variation in the Bultfontein area over the past three seasons (Figure 8).

Lastly, when considering the internal quality of potatoes, processing characteristics can also be evaluated. To adhere to processing requirements, cultivars must meet a chip colour standard of >50 and a specific gravity (SG) of ≥1.075. Only Noya met both the SG and chip colour requirements (Table 4).

– Enrike Verster, Potatoes SA, and Izak Cronjé, producer

Special thanks to the relevant farm and collaborator, as well as the trial participants and the Western Free State working group. For more information, send an email to enrike@potatoes.co.za.