In Vivo Digestibility, In Vitro Gas Fermentation and Enteric Methane Values of Sunn Hemp’s  (Crotalaria juncea L.) as Affected by Cutting Stages

Ruminant animals, such as sheep and cattle, possess a unique digestive system that relies on a bustling community of microbes living inside their stomachs to break down tough plant material. These microbes ferment the food, releasing energy for the animal and producing gases as a byproduct. One of these gases, methane, is a potent contributor to climate change, and its production is closely tied to the type of plant the animal eats. Young, leafy plants are generally easier for microbes to digest and produce less methane, while older, tougher plants with more fibrous stems require more effort to break down, often leading to higher methane emissions. As the global demand for sustainable livestock production grows, finding forage crops that offer high nutrition while minimizing environmental impact has become a critical goal for farmers and scientists alike.

In a recent study conducted in the Mediterranean climate of Izmir, Turkey, researchers investigated a tropical legume known as Sunn Hemp to determine the best time to harvest it for animal feed. The plant is known for its rapid growth and ability to enrich the soil, but its value as food changes dramatically as it matures. The scientists grew the crop and harvested it at four distinct stages of development: when the plant first showed buds, when half the flowers were open, when all flowers were fully bloomed, and when the plant began to set seeds. They then fed samples from each of these four harvests to a small group of sheep to measure how well the animals could digest the food and how much gas the plant produced during fermentation.

The results revealed a clear and significant shift in the plant's nutritional profile as it aged. In the earliest harvest, the plant was rich in protein, containing about 16 percent crude protein, which is a vital nutrient for animal growth. However, as the plant moved toward the seed-setting stage, this protein content dropped to roughly 13 percent. At the same time, the plant became much tougher. The amount of fibrous material, which acts like a structural skeleton for the plant, increased from about 64 percent in the first cut to over 84 percent in the final cut. This structural change made the plant significantly harder for the sheep to digest. The animals were able to absorb about 71 percent of the dry matter from the first, youngest harvest, but this digestibility fell to roughly 50 to 55 percent for the older, more mature plants. Consequently, the energy the animals could extract from the feed also declined sharply, dropping from a high of 10.67 megajoules per kilogram in the first cut to just 7.07 megajoules in the last.

Beyond digestion, the study looked at what happened inside the animal's stomach during the fermentation process. When the researchers simulated the rumen environment in a laboratory, they found that the youngest plants were broken down much faster by microbes in the first few hours compared to the older, fibrous plants. More importantly, the type of gases produced changed with the plant's age. The fermentation of the older, mature plants led to a higher production of a specific acid called acetic acid, which is known to trigger the creation of methane. As a result, the calculated methane emissions rose from about 10 moles per mole of volatile fatty acids in the first cut to nearly 14 moles in the fourth cut. This indicates that feeding older Sunn Hemp not only provides less energy to the animal but also results in a greater release of greenhouse gases.

The study concludes that Sunn Hemp is an excellent, high-quality feed source, but only if harvested early. The first and second cuts offer superior protein and energy levels while keeping methane emissions low, making them ideal for productive livestock. In contrast, waiting until the plant is fully mature or setting seeds transforms it into a lower-quality feed that is harder to digest and more environmentally taxing. While the mature plant still has value as a roughage filler for less demanding animals, the data strongly suggests that for maximizing animal performance and minimizing environmental impact, farmers should prioritize harvesting this crop during its early vegetative stages.