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Can intercropping legumes with sorghum improve sustainability?

Mark Marsalis and Sangu Angadi Published on 11 November 2009

The landscape of the plains of eastern New Mexico and west Texas has changed considerably over the past two decades.

Although the region has had considerable numbers of cattle through such industries as stocker and beef feeding operations, it has experienced a massive dairy boom to the point where there are now nearly 425,000 milking cows hungry for high-quality feed.

In addition, driving through the region today compared to just 10 years ago would portray another stark change – one of thousands of acres of forage sorghum grown for silage. Grain sorghum has been grown in the region for many years; however, large-scale utilization of forage-type sorghums is a relatively new occurrence.

While corn is the silage crop of choice for most dairies, declining water resources are forcing the acceptance and use of sorghum silage more and more every year. Under restricted irrigation conditions in this semiarid climate, forage sorghum has been shown to yield more than corn in most years.

However, the adoption of forage sorghum for the most part has been one of necessity and not preference. Forage sorghum has had a bad reputation in the past due to the oftentimes lower feed value and standability compared to corn.

In more recent years, though, forage sorghum nutritive value has been improved substantially through the introduction of new varieties and the brown midrib (BMR) trait, which is characterized by lower lignin (indigestible component of the plant cell wall).

Scientists at New Mexico State University’s Agricultural Science Center at Clovis are investigating yet another way to improve nutritive value and resource use efficiency of forage sorghum silage systems in the region. The study consists of intercropping legumes with forage sorghum and growing the two crops together in a complementary system.

The goal of this USDA-National Research Initiative-funded project is to develop a forage sorghum and legume intercropping system to increase productivity, water use efficiency and nutritional quality.

Intercropping, which was an important technology in maintaining yield stability in subsistence agriculture, is fast being recognized as a technology to increase resource use efficiency and increase productivity per unit of land in high- input agriculture. Preliminary studies in the region have indicated potential to maintain yield and improve forage quality by mixing cereal and legume species in the same row.

However, these studies did not focus on maximizing resource use efficiency. The benefit of an intercropping system reaches maximum when both species complement each other and use the resources efficiently. The study led by Angadi, however, attempts to understand the resource use and efficiency of sorghum and legume intercropping systems better.

Intercropping is a system of growing two diverse species of crops on a piece of land at the same time with the assumption that they improve the efficiency of using both above-ground and below-ground resources compared to growing them separately.

As a result, often the total productivity of an intercropping system is more than growing any of the component crops alone. Since mechanical harvesting of intercropping systems for grain production is not possible, it is mostly used in developing countries.

However, in a forage production system, both crops can be harvested together. Thus, an opportunity exists for improving forage productivity in the region with appropriate intercropping systems. Identifying legumes that are well adapted to the region and fit well in the system will be an important step in designing intercropping scenarios.

Competition in intercropping can be for sunlight, nutrients and water. An intercrop that can tolerate lower light intensity during later growth periods will be of great benefit. A legume intercrop that climbs on the main sorghum crop to receive its share of radiation may also fit well.

With tap root systems, legumes differ significantly from lateral root systems of sorghum. In addition, sorghum roots take a long time to grow into the inter-row area and as a result water is lost through evaporation and nutrients can be lost through volatilization and leaching.

By having a legume in the inter-row space, these limited resources can go toward crop productivity and perhaps be used more efficiently. Spatial arrangement of sorghum and legume crops affects resource availability to both component crops and resource use efficiency.

The study compares mixed cropping, where two crops are mixed and planted in the same row, to a system that uses narrow strips of sorghum and legumes planted side-by-side. The developed intercropping system should be sustainable under declining irrigation levels, which will be studied in another trial.

Overall, we want to understand all possible interactions taking place and use that information to recommend suitable intercropping options for the High Plains. Another related study is looking at the ensiling potential of the intercropped mixture.

Different levels of legume are mixed with the forage sorghum to determine what blend (proportion of legume to sorghum) ensiles the best and gives the most nutritious feed. This is important to know because even if the legume-sorghum mixture is productive and resource-use efficient in the field, it won’t be widely utilized if it doesn’t feed well.

These are just a couple of many research projects ongoing at the NMSU Agricultural Science Center at Clovis that investigate improving sustainability and economics of forage systems in the region.

For more information on sorghum-legume intercropping, contact Mark Marsalis or Sangu Angadi at (575) 985-2292FG

Mark Marsalis
Extension Agronomist
New Mexico State

Sangu Angadi
Crop Stress Physiologist
New Mexico State