Evaluation of grass strips performance in soil and water conservation

Abstract

Although grass strips for soil and water conservation offer several advantages such as ease of establishment, low cost, effective means of soil erosion control, and as a source of fodder, they are not widely used. The study aimed at assessing the potential and actual performance of eight grass species in terms of formation of terraces, moisture conservation, protection of terrace banks, source of fodder, competition for water, and harbouring pests and diseases. The study had two components: technical evaluation of grass strips performance in soil and water conservation and an evaluation of farmers' perception on the use of grass strips. Technical evaluation of grass strips performance in soil and water conservation was undertaken on grass strips established at Kabete Steepland Research Site in 1989. It involved measurement of ground slope changes, soil moisture gradient, effect of grass establishment on adjacent crop yields, fodder potential of different grass species and bank stability. A field survey was carried out in Gatundu to assess the farmers' perception on the use of grass strips as a soil and water conservation measure. In addition to farmer interviews, the survey included measurements of width and spacing of grass strips, grass strip barrier continuity and fodder yield, and the resulting embankment height. In terms of terrace formation, the results from the Steepland Research Site, showed that grass strips planted for a period of 5 years since 1989, resulted in slope reductions of 30%, 8.3 %, 38%,36%,25%,20% and 20% for tall signal, makarikari, donkey, creeping signal, bana, guatemala and tall guinea grass respectively. This was as a result of sediment deposition on the upper side of the strips leading to the formation of forward sloping bench terraces. Bana and guinea grasses increased soil moisture storage by trapping runoff during rainy periods but reduced the same during dry spells due to high water use. Some significant soil moisture depletion by these grasses was observed within 0.5 m from the strip edge. It was also observed that competition between bana grass and the bean crop was higher in the first row than the third row. The bean yields were 0.33, 0.5, and 0.8 kg for the first, second and third row respectively. However, the competition effect of these grasses was compensated by their higher fodder yields. Fodder yields were 3.81 kg/m2, 2.63 kg/rrr", 2.28 kg/m2, 2.24 kg/m2, 1.49 kg/rrr', and 1.07 kg/rrr', for bana, guatemala, guinea, donkey, creeping signal and tall signal grasses respectively. Tall signal, creeping signal and donkey grasses provided barrier continuity above 90% while bana and guinea grasses had 84.5 and 86.5 % respectively. Their banks were well stabilised with minimum rills on gaps caused by their tufted growth. Due to their overgrowth, these grasses were found to harbour rodents like rats and squirrels. Makarikari and guatemala grasses gave poor ground barrier continuity of below 60% throughout the period. This was as a result of pests attack and tufted growth respectively. Farmers in Gatundu division perceived grass strips as a better alternative to mechanical structures due to their less labour requirement in establishment and their potential as a source of fodder to livestock. The use ~f grass strips was found on 54.4 % of the farms surveyed in the division. Napier grass tPennisetum purpureum) was the most common grass found to be grown on 98 % of the farms with grass strips due to its fodder potential. Land size, percentage level of other soil conservation measures and land slope were the main factors affecting grass strip adoption in Gatundu division. However, 14.6% of the farmers cited competition for moisture and nutrients by grass strips established from napier and its related species. In terms of design and maintenance, it was observed that many grass strips were poorly aligned and spaced thus leading to more erosion on the downhill side.