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Nutritive Value of Seven Tropical Weed Species During the Dry Season¹

Proliferatiion of weeds is a major constraint in pasture management, especially in the tropics, where fertilizers are rarely used and where poor grazing management is common. Nonetheless, grassland weed control should not be initiated indiscriminately without an evaluation of the palatability and nutritive value of the species present. One objective of a grazing trial conducted at Pichilingue, Ecuador, was to compare the palatability, in vitro dry matter digestibility, crude protein (CB) content, cell wall components, mineral and nitrate contents of seven weed species and four cultivated forages during the dry season. The weed species were Alternanthera gullensis (Mart.) Criseb (chaff flower), Amaranthus spinosus L. (spiny amaranth), Digitaria sanguinalis (L.) Scop. (large crabgrass), Eleusine indica (L.) Gaerth. (goosegrass), Momordica charanria L. (balsam apple), Sida acuta Burm. f. (southern sida), and Sida rhombifolia L. (arrowleaf sida).

Weeds were more digestible than cultivated forages, higher in CP and acid-detergent lignin, and lower in neutral-detergent fiber, acid-detergent fiber, and silica. The Ca:P ratios in cultivated forages were close to the recommended 1:l to 21 ratio. The only weed with a favorable Ca:P ratio was D. sanguinalis. The remaining weeds had Ca:P ratios of 3 to 21. During the dry season cattle were reluctant to graze the increasingly fibrous and protein-deficient cultivated forages, so they consumed A. gullensis and A. spinosus, despite the very spiny texture of the latter. With the exception of D. sanguinalis for which palatability observations were not possible, cattle did not consume the other weeds. There were no symptoms of mineral imbalance or nitrate toxicity in cattle from grazing weeds. The practical implication of this study is that some weed species may be useful as feeds for grazing livestock during periods of limited feed supply, and should not be removed indiscriminately. Based on this study, A. gullensis and A. spinosus should not be removed from pastures during the dry season.

Key Words: Tropical weeds • Dry season • In vitro dry matter digestibility • Crude protein • Cell wall components • Minerals • Nitrate • Alternanthera gullensis (Mart.) Griseb • Amaranthus spinosus L. • Digiraria sanguinalis (L.) Scop. • Eleusine indica (L.) Gaerth. • Momordica charanria L. • Sida acuta Burm. f. • Sida rhombifolia L.

² Formerly research associate; associate professor, Dep. of Animal Sciences and Industry; professor, Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506; head of analytical laboratories at Pichilingue and Santa Catalina, respectively, c/o INIAP, Apartado 343, Quito, Ecuador.

Received for publication August 30, 1982.