Here’s some useful advice on choosing crops for your rotation system, to ensure you get the most advantages.

Crop rotation is a practice whereby different crops are planted in different seasons on the same piece of land. The aim is to enhance sustainable production and increase yield potential, compared to monocultural practices.

Crop rotation systems should be planned to suit the farm and its business and to grow healthy, vigorous crops offering economically viable yields and good-quality grains.

These systems should also reduce insect and disease pressure. Since specific pests and diseases are associated with specific crops, rotation with an alternative crop reduces the potential build-up (which often happens in monoculture) over time.

Improved soil quality as a result of a rotation, and the addition of nitrogen from legumes, enhance growing conditions for the follow-up crop. Diversifying crops also reduces yield risk and makes a field less vulnerable to economic fluctuations than if only one crop was planted.

The choice of rotation and its sequence depends on the nature of the soil and climate; together they determine the type of crops that may be cultivated.

What we are looking at here is information about rotation systems, and crops that may be considered for them, when farming in the summer rainfall region of South Africa’s North West province.

CROP VALUE

Markets control the demand for crops, so it is important to identify market demand before planting. In other words, identify price and quality requirements and minimise risk.

Farmers should contact research institutions and agricultural advisory services to find out about specific environments and end use for specific crops that are in demand.

SOIL BUILDING

• Soil becomes depleted when the components that contribute to fertility are removed and not replaced or maintained.
• This in turn leads to poor yields.
• Excessively intense cultivation and inadequate soil management can lead to depletion and damage soil structure.
• Overuse of inputs such as synthetic fertilisers, pesticides and herbicides could lead to residue build-up and inhibit micro-organism and soil fauna activity.
• Always growing the same crop in the same place eventually depletes the soil of nutrients such as nitrogen.
• A crop that depletes the soil of one kind of nutrient should, during the next growing season, be rotated with a crop that returns that nutrient to the soil or extracts a different ratio of nutrients.
• For example, cowpeas – which can fix nitrogen – could be followed by maize.
• Rotate deep-rooted crops, such as sunflower, with shallow-rooted ones, such as maize.
• Doing this helps to make better use of fertiliser that could have leached to deeper soil layers and become inaccessible to the shallow-rooted crop.
• Farmers who have access to irrigation should plant winter cereal crops, such as wheat and barley, as cover crops that occupy the land between growing cycles.
• This will reduce soil erosion and increase organic matter content, which improves fertility and water-retaining capacity.
• Leaving the land bare makes it easier for weeds to grow freely and for water to wash away soil and important nutrients.

CONSERVING NITROGEN

Nitrogen (N) is the mineral most demanded by plants, but substantial quantities of phosphorus (P) and potassium (K) are also needed.

That’s why these 3 elements are usually included in commercial fertiliser mixtures. The content of each is indicated on the bag of fertiliser; for example, a 3:2:1 (22) fertiliser bag will contain 22% N, P and K at a ratio of 3:2:1.

• Proper growth cannot take place if there is not an adequate supply of N, and plants may remain stunted and underdeveloped.
• Plant growth and crop yield usually increase when N is added, even if there is N in soils, as most of it is stored in the organic matter in forms plants cannot access.
• On the other hand, if the supply of N in sandy soil is always greater than the crop demand, excessive nitrates (NO3-) can leach out and contaminate drinking water to the point at which it becomes a health risk.

An effective way to replenish nitrogen in soil is by planting legumes such as cowpeas and soybeans in sequence with cereals. Rhizobium bacteria, which live in symbioses with legumes, convert atmospheric nitrogen (N2), which plants cannot use, into forms (such as nitrates (NO3-) and ammonium (NH4+)) plants can absorb and use to help them grow and develop.

WEED AND PEST CONTROL

• Weeds compete with cultivated crops for growth factors such as light, nutrients and soil moisture.
• They interfere in farm operations (time and labour) and in effect increase the cost of production.
• This is why modern agriculture relies on synthetic chemicals to get rid of these unwanted plants.
• Using good-quality seed will ensure rapid, even germination and emergence that will provide a competitive advantage over weeds that emerge later by shading these weeds.
• Grow crops, such as sunflower, that have big canopies; they could grow more densely and suppress weeds.
• They will also help blanket the soil so that competing weeds don’t get sunlight and are therefore less competitive.
• Legumes have other benefits in addition to fixing nitrogen.
• For example, they can interrupt insect and disease life cycles.
• Hence, populations of certain pests and diseases can be limited by planting fields with crops that don’t promote their development.

Irrigation farmers should grow crops, such as wheat, that are poor hosts for root-knot nematodes for at least one season to reduce the quantity of these nematodes in the soil and make it possible to grow a susceptible crop, such as maize, the following season without needing to fumigate the soil first.

Root-knot nematodes are a serious problem on some plants in warm climates and sandy soils and can severely damage crop productivity.

Use a rotational system that will enhance the use of a wider range of herbicides and take note of the waiting period for each herbicide before planting the next crop. For example, don’t plant broad-leaf crops such as sunflower or cowpea in a field after using atrazine on maize.

Also read: “Modern agriculture to blame for plagues”

RESEARCH RESULTS

The South African Agricultural Research Council – Grain Crops Institute (ARC-GCI) conducted field studies in the communal areas of Gelukspan and Vryhof to assess 6 crop rotation systems at 3 fertiliser levels (none, 50% and 100% of the recommended fertiliser rate).

They used maize ( PAN 6479), cowpea (Glenda) and sunflower (Agsun 5551) as test crops. The rotation systems included one year of a legume crop followed by one year of maize.

It was found that the maize yield varied significantly and increased by 11% and a marginal 1%, following cowpea and sunflower respectively, compared to monocrop maize.

Also, where only 50% of the recommended fertiliser rate was applied to maize, this crop following sunflower and cowpea yielded 30% and 52% more maize than in monoculture.

If you don’t have a proper programme, it can be difficult to remember which crop is to follow which. It could also be complicated to decide on a rotational programme when intercropping is carried out. Table 1 shows potential rotations and the challenges associated with the choice of each.

LABOUR AND EQUIPMENT

One of the advantages of growing different crops is that when there is diversity of activity, the workload is distributed over the season. When large areas are planted to one crop, pressure at harvest time is extreme. A diversity of crops on the farm will help spread the workload during the cropping season.

During the transition years, a farmer should consider labour and equipment requirements for different crops in the rotation system to ensure that there won’t be a time crisis. Some farm machinery is used for limited periods during a season, so great savings can be made if farmers share equipment.

Also read: Reduce risks to the maize crop with cowpea

• This article was written by Cedric Baloyi and first appeared in Farming SA.