Modelling crop yield using growth parameters and fall armyworm (Spodptera frugiperda J. E. Smith) leaf damage under different NPK regimes for Cameroon Highland Composite (CHC) maize

In the highlands of Cameroon, where the air is cool and the soil is rich, maize is more than just a crop; it is a lifeline. For decades, this grain has shifted from a simple staple for home cooking to a vital cash crop that feeds cities and industries. Yet, growing it successfully is a constant balancing act. Farmers must manage the soil's hunger for nutrients, primarily nitrogen, phosphorus, and potassium, while simultaneously defending their fields against a relentless enemy: the fall armyworm. This moth's larvae can strip a field of leaves, threatening the harvest. For years, a common belief has persisted in agriculture: that feeding a plant more fertilizer makes it grow stronger, but also makes it tastier and more attractive to hungry insects. The question facing researchers in the North West Region was whether this trade-off held true for the specific maize varieties grown by local farmers, and if so, how it affected the final harvest.

A team of scientists from universities in Cameroon and The Gambia set out to test this relationship in a field experiment near the town of Bali. They focused on four specific varieties of maize known as the Cameroon Highland Composite, which have been a staple for small-scale farmers for thirty years. The researchers planted these seeds in a carefully organized grid, dividing the field into sections to test different amounts of fertilizer. Some plants received no fertilizer at all, while others received small, medium, or large doses of a standard nutrient mix. They watched closely as the plants grew, measuring their height, the thickness of their stems, and the size of their leaves. They also kept a sharp eye on the fall armyworm, using a simple scoring system to rate how much damage the insects had done to the leaves, ranging from no visible harm to severe destruction.

The results painted a clear picture of how the plants responded to the soil's nutrients. As the fertilizer doses increased, the maize grew taller, developed thicker stems, and produced larger leaves. This was true for all four varieties, showing that the plants were eager to use the extra food to grow. However, the type of seed mattered more than the fertilizer when it came to how many plants actually survived to harvest. One variety, known as CHC202, consistently produced the most plants and the tallest, healthiest stalks, regardless of the fertilizer level. It emerged as the clear champion of the group, outperforming the other three varieties in almost every measure of growth.

The relationship between fertilizer and pests, however, was more complex. The study confirmed that as the fertilizer levels rose, so did the damage from the fall armyworm. The insects seemed to prefer the lush, well-fed plants, with the highest fertilizer doses leading to the most severe leaf chewing. The variety that suffered the most damage was CHC204, which looked ragged and torn under heavy feeding. Yet, a surprising twist emerged when the researchers looked at the final harvest. Despite the heavy insect damage, the plants that received the most fertilizer still produced the most grain. The CHC202 variety, which had the least amount of leaf damage and the best growth, yielded the highest amount of fresh maize, reaching four tons per hectare. This was a significant improvement over the typical yields seen on local farms.

The scientists then turned to mathematics to understand what truly drove the harvest. They built several models to see if they could predict the amount of grain produced based on the plant's health, the number of surviving plants, and the level of insect damage. They found that simply counting the damage caused by the armyworm was not enough to predict how much food the field would produce. In fact, the damage scores alone told them very little about the final yield. The best way to predict the harvest was to look at a combination of factors: how vigorous the plants looked, how many survived to the end, and a small, barely noticeable contribution from the insect damage.

The study concludes that while feeding maize more fertilizer does invite more insect attacks, it does not necessarily ruin the harvest if the right variety is chosen. The CHC202 variety proved to be a resilient choice, managing to produce a high yield even when the insects were present. The researchers suggest that farmers should focus on planting this specific variety and using appropriate fertilizer levels, rather than fearing that feeding the crop will inevitably lead to a loss. By understanding that the insect damage alone is not the main driver of yield loss, farmers can make better decisions to secure their harvest in the face of these persistent pests.