Published: 1996-07-19


One of the most productive green vegetables was the subject of a recent question at ECHO. Many of you wrote for Moringa seed in response to the article on a leaf-based diet for pigs (EDN 51-1). Some asked, “Are there other plants which can be used in the same way?”

We immediately thought of chaya (Cnidoscolus chayamansa, EDN 18-2), an attractive perennial shrub native to Mexico which produces abundant quantities of large, dark green leaves. Chaya thrives on a wide range of soils in both hot, rainy climates and areas with occasional drought. It grows very quickly, especially at higher temperatures, and resprouts well after harvesting. Young leaves and the thick, tender stem tips are cut and boiled as a spinach. It is a tasty vegetable, and is exceptionally high in protein, calcium, iron, and vitamin A. Chaya is virtually pest-free and has low weed potential, as it is propagated only by cuttings and does not generally produce seed. As a year-round source of high-quality food in a wide range of conditions, it is one of the most important plants at ECHO. [Important: many chaya varieties have stinging hairs which are very irritating during harvest. ECHO now distributes only a non-stinging variety, which we obtained from Belize about 10 years ago. If only the stinging ones are present in your area, you might wish to introduce this superior variety.]

 

If you request chaya from ECHO, we will send several stalks wrapped in paper; cuttings can survive for several weeks in the mail. Trim any blackened areas on the ends. Be sure to put the correct end in the ground, so the leaf scars look like smiles, not frowns. Chaya (especially a new cutting) prefers good drainage, but can survive some waterlogging. Since leaves are larger and more tender when grown in partial shade, many people like to plant chaya near a tree that provides light shade. Avoid contact with the irritating white, sticky sap when harvesting. Harvest often enough so the new shoots stay within arm’s reach, or coppice the whole plant and allow for regrowth. You will quickly have sufficient stems to share.

For its great nutritional value, exceptional drought tolerance, and productivity, chaya would seem a logical choice for animal feed, in addition to its primary use as a vegetable for people. However, uncooked chaya contains hydrocyanic glucosides, which release hydrogen cyanide when eaten. The toxicity is eliminated after a few minutes of boiling. We wondered about its effect on animals, which would eat uncooked leaves. We asked Dr. Ricardo Bressani, retired head of the Institute of Nutrition for Central America and Panama, about using dehydrated chaya leaves as a feed ingredient. “As far as I know, studies with pigs have not been conducted. Amaranth and many other leaves are used in pig feeding in many countries. Usually, leaves which are edible are consumed by pigs, but I have not seen pigs consume chaya which is often found as part of fences in rural areas, probably due to the toxins it contains. On the other hand, if it is processed and dried, it may be part of the feed, since antiphysiological factors are partially or totally destroyed. On the basis of the nutrient content of chaya leaves, it would be worth the effort to conduct more feeding trials with pigs.”

Dr. Bressani directed us to the article “Chemical composition of chaya leaf meal (CLM) and availability of its amino acids to chicks” (Anim. Feed Sci. Tech., 30: 155162). Chaya leaves were cut, air dried, and ground to produce CLM, which can be stored in air-tight containers. The study found that CLM was high in calcium, iron, and had a moderately high availability of amino acids (which make up proteins), comparable to many tropical legume leaves. Overall availability was 84%, but CLM was low in the sulfur-bearing amino acids cystine (66.7%) and methionine (69.9%).

No anti-tryptic activity was detected in the CLM. (Trypsin is an enzyme which splits proteins in the process of digestion. Many uncooked plants contain substances that inactivate this enzyme.) The samples did contain hydrocyanic glucosides and oxalates. The former may account for the low availability of sulfur-containing amino acids, because the body uses sulfur in the process of detoxifying cyanide.

Perhaps we can gain a perspective by considering another plant that has cyanide-producing substances. Cassava leaves, which also contain these glucosides, are commonly dried and stored in Brazil. Cory Thede reported on their convenient use in soups (EDN 49-6). We asked David Kennedy with Leaf for Life for his perspective on using dried cassava leaves as a food, since cassava contains substances that produce hydrocyanic acid (HCN) when fresh leaves are eaten or pulverized. “HCN is a fairly common toxin in food. Cassava, lima beans, and sprouted sorghum have caused HCN poisonings. Acute [severe, sudden onset] HCN poisoning is quite rare. The minimum lethal dose is estimated at 0.5-3.5 mg per kg of body weight. So a child weighing 20 kg would need to consume between 10 and 70 mg of HCN. Ten grams of a low-HCN variety of dried cassava leaf would contain something like 0.08 mg. Chronic toxicity (also quite rare) has been reported mainly where there is a great dependence on cassava and a very low protein intake. Damage to the nervous system and especially the optic nerve can be caused by chronic exposure to HCN. Low consumption of proteins, especially sulfur bearing amino acids, cigarette smoking, and air pollution all intensify the body’s negative reaction to HCN.

"One would be tempted to steer clear of cassava leaves altogether to avoid any toxicity problems, except that the plant has several important attributes as a leaf crop, yielding large quantities of leaf that is high in dry matter, protein, and micronutrients…throughout the year in most 53-3 locations. …People are currently eating cassava leaves as a vegetable in much of Africa, and parts of Asia, and Latin America. I think the question is not whether to eat cassava leaves, but rather how to. Encouraging the use of low-HCN varieties is critical to this effort. A grinding technique that ruptures cell walls will dramatically increase the rate and total amount of HCN that disperses into the air. It is important that the leaves be ground when fresh, and quite well pulped, not just shredded. The loss of HCN is very dramatic then during drying.”

David Kennedy sent us a Ministry of Agriculture publication from Brazil which showed the following HCN content for one variety (Cigana) of cassava: fresh–737 ppm; flour from a leaf dried whole–123.89 ppm; flour from a shredded leaf–75.58 ppm; and 33.60 ppm when dried after thorough pulping (a 95% reduction). This report showed a lot of variation in HCN content based on variety and drying method (in the shade or an oven), but all varieties showed greater drops in HCN content with increased pulping. Leaves of the six varieties tested ranged from 48-123 ppm dried whole, 20-78 ppm when shredded, and 7-36 ppm when pulped.

We cannot say for certain that this data for cassava can be applied to chaya, but it may help you decide how these leaves may be used in animal feeds. We have not been able to find data on the HCN content of raw chaya [please let us know if any of the scientists in our network have tested this!]. Drying the leaves to any extent would almost certainly reduce HCN content, and cutting them first would be even better. If you decide to use significant amounts of chaya, you can be totally safe by boiling the leaves for a few minutes before feeding it to the pigs. Chaya definitely has potential as a source of cheap protein in poultry diets, although no optimal levels of inclusion are available at this point. (As a guideline, poultry specialist Dr. John Bishop recommends that a chicken’s diet consist of no more than 5-10% leaf meal on a dry weight basis, since leaves are high in fiber with low digestibility.) Start with small amounts, and let ECHO know the results if you use CLM in your feed mixes.