植物間的相生或相剋作用(Allelopathy)係一自然現象。植物在其代謝中經常會釋放一些調節植物生長的物質，如植物生長促進質即相生物質如吲哚乙酸(IAA)及植物相剋物質如酚酸、植物鹼、松烯類化合物等。近二十年相剋作用的研究漸成時尚。植物相剋物質被釋放至環境後便抑制自己及或其鄰近地區植物的生長、發育、開花及生產量，並扮演著植物消長，優勢植物及極峰植物形成之機制，因此植物相生相剋作用在植物生態上扮演相當重要的角色，近五年來，我們已成功地以相生相剋作用的原理運用於實際的\"林間植草"或\"林間放牧"制度上，而發現將優勢的克育草(Pennisetum clandestinum)種植於森林砍伐地上，在不干擾其表土及除草的情形下，在種植半年後，克育草快速地成為該地優勢植物並抑制其他雜草的生長。一年後，整個砍伐後的森林地幾乎被克育草所佔有。綠油油的克育草亦可提供牛羊為飼料，而克育草之生長並不影響森林植物之再生。因此從林間放牧與林間植草的觀點上，克育草扮演著相生及相剋的雙重角色。譬如，在杉木林(Cunninghamia lanceolata)及三種闊葉樹林如赤楊(Alnus formosana)，櫸木(Zelkova formosana)及樟樹(Cinnamomun camphora)的林地上，種以克育草而研究其相生相剋的實驗結果看來，克育草抑制林地雜草之生長，但對杉木林之再生及闊葉樹幼苗之生長並無顯著影響。此舉，不但可以節省造林初期可觀的除草費用及避免農藥的使用，以防止農地的污染。因此克育草或其他覆蓋作用在永續農業利用上扮演著重要角色，而其基本的學理則與植物相生相剋有密切的關係。

An allelopathic interaction of a cover grass, Pennisetum clandestinum called kikuyu grass, intercropped with Chinese fir I Cuuninghamia Ian ceo/ala ) was evaluated by experiments in field, greenhouse and laboratory assays. After deforestation of the Chinese fir, a split-plot design of four treatments ( litter removed, litter retained, litter removed and kikuyu grass planted, and litter retained and kikuyu grass planted) was imposed. Field experiments showed that fir litter left on the ground slightly suppresses the growth of weeds in the first four months after deforestation, while kikuyu grass significantly reduced the growth of weeds longer than four months. Neither fir litter nor kikuyu grass reduced growth of fir seedlings. Bioassay of aqueous extracts of fresh fir leaves, fir litter, and kikuyu grass showed that fresh leaves produced significant phytotoxicity while litter and kikuyu grass has less toxicity. Nine phytotoxic phenolics and many unidentified flavonoids were found in the plant materials. Additionally, a similar experiment of pasture-forest intercropping system was also evaluated by employing the kikuyu grass and three hardwood forest species, Alnus [ormosana, Cinnamomum camphora, and Zelko va [ormosana. Results showed that kikuyu grass retarded growth of weeds soon after it was established but had little effects on the seedling growth of these three hardwood plants. However, aqueous extracts of the three hardwood plants significantly retarded the root growth of kikuyu grass, indicating that the growth of kikuyu grass might be suppressed when the woody forest became reestablished. Of them, Zelko va [ormosana exhibited the highest amount of allelopathic compounds, which might be" responbsible for the reduction of weed growth beneath Z. [ormosana forest. The aforementioned findings show that a good correlation between the degree of phytotoxicity and phytotoxin levels was established, indicating that allelopathic mechanisms may play a significant role in the interaction of forest-pasture intercropping systems, and thus potentially reduce the need of herbicides and alleviate the labor cost of weed control. The findings concluded that an allelopathic potential cover grass, such as kikuyu grass, could widely be planted in areas of deforestation and hillsides in Taiwan .