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On irrigated pastures, buffel grass, rhodes grass, bambatsi panicgrass, and green panicgrass were generally more productive than para grass, blue panic, and switchgrass. Clipping at,4and 6-week intervals during the summer resulted in greater total annual yield than clipping at 2-week intervals. However, percent crude protein in grasses clipped at ,2-week intervals was double that in grasses clipped at .6-week intervals. Swtichgrass, para grass, and blue panic appeared least able to withstand clipping over the ,2-year period of the study. The results suggested that buffel grass, green panic, bambatsi panic, and rhodes grass, harvested at ,4-week intervals would be the best choice for production of nutritious forage on irrigated pastures in the semiarid region of the Sudan. Sudan is a vast country of nearly one million square miles that lies between latitude 2 1” 55’ N. and 3” 53’ N. and longitude 2 1” 54’E. and 38” 30’E. Ecological zones within this area range from the deserts and semideserts in the north to the tropical rainforests and swamps in the south. The semidesert region, with less than 300 mm annual rainfall, supports natural vegetation consisting mainly of annual grasses intermingled with short woody plants mostly of the genera Acacia (Harrison 1955). .-___ The author is lecturer, Faculty of Agriculture, Shambat, Sudan. Manuscript received February 2, 1978. Sheep, goats, camels, and a few cattle utilize this zone. The region as such does not support large numbers of livestock at present (FAO 1968). This is probably why there are fewer cases of the epidemic viral, bacterial, and protozoa1 diseases found in the other wetter ecological regions of the country. A large area of this region was, therefore, designated to serve as a diseasefree zone. At various times of the year, large numbers of livestock coming from different parts of the country enter and stay for some time in this zone for inspection under quarantine measures before they are exported. Consequently, animal fodder is in great demand. During most of the year, animals in this region depend upon the remains of crops or forage produced under irrigation. The only forages grown especially for animals are ‘lubia’ (Dolichos lablab L.), dura forage (Sorghum bicolor var. ‘Abu sabain’), lucerne (Medicago saliva L.), and ‘dukhn’ (Pennisetum typhoides (Bum.) Stapf and Hubbarad) produced on a limited scale under flood irrigation. Usually lubia and luceme are cut and utilized as green forage, while the dukhn and sorghum are cut as mature straw and stored in stacks to be utilized later during winter and early summer. Mature straw of both species is highly deficient in crude protein (CP). Values as low as 2.5% for sorghum straw and 1.6% for dukhn straw were reported by Snow ( 1952). Moreover, the acreage of land occupied by forage 182 JOURNAL OF RANGE MANAGEMENT 32(3). May 1979 crops is continuously decreasing to provide room for more important cash crops. Irrigated pastures have not contributed appreciably to the animal feed. Species such as Cynodon ductylon Pers. (‘nagil’); Echinochloa colona link. (‘difra’) and Sorghum sudanense Stapf. (‘garawia‘) are treated as agricultural weeds which may be used by livestock after harvesting the main crop. The desire to establish dairy farms in this region to supply dairy products for the cities and towns is increasing. Irrigated pastures could be a reliable source of good quality forage, and grasses may be treated as an irrigated crop rather than a weed. The objective of this study was to compare the persistence and productivity of two native and five introduced warm-season grasses under three frequencies of clipping when irrigation was adequate. Methods and Procedures The experiment was conducted at the farm of the University of Khartoum, Shambat, Sudan. The soil is a heavy alkaline clay soil with pH 7.5 8 .O and clay content 50 60%. This semidesert region has an average annual rainfall of 160 mm coming mostly during July and August. The minimum and maximum temperatures range between 13” and 27°C in January and between 25” and 4 1°C in May. May is the hottest month of the year, and temperature drops gradually by 4” to 5°C during the rainy season in July and August. The grasses tested were native species: switchgrass (Panicurn virgatum L.) and buffel grass (Cenchrus cilaris L.) and introduced species: para grass (Bruchiuriu muticu (Forsk.) Stapf), green panic (Panicurn maximum Jacq. var. trichoglume), blue panic (Punicum antidotale Retz.), bambatsi panic (Punicum colorutum L.), and rhodes grass (Chloris gayana Kunth). The switchgrass was an ecotype obtained from a river bank near Kassala, Kassala province, while the buffel grass was collected from Singa area, Blue Nile province (rainfall >500 mm). The introduced grasses came from Australia and were chosen following preliminary adaptability trials on a large number of species carried out by the Range Department administration, Khartoum, Sudan. All grasses were seeded on 2 X 5-m plots in April, 1976, and irrigated every week. All plots were cut to a 14-cm height on July 10, 1976; three cutting frequencies were then imposed. These frequencies were at 2-, 4-, and 6-week intervals to the same 14-cm height. A split-plot experimental design was used with the seven species on the main plots and the cutting frequencies on sub-plots. There were four replications. Cutting was discontinued in November, 1976, and resumed in March, 1977, after an initial cut for all the plots. The treatments were continued until November, 1977. The plots of 2-, 4-, and 6-week frequencies were cut 7,4, and 3 times respectively during the first year and 18,9, and 6 times during the second year. Harvested material was oven dried to a constant weight at 70°C and weighed. Percent crude protein (CP) in samples representing all treatments and replications from the first-year material was estimated from the determination of N by Kjeldahl technique. Results and Discussion The 2-week clipping treatment produced the lowest yield in all the species (Table 1). During the first season, the mean value for this treatment was only 68 and 50% of the yield obtained at 4and 6-week clipping intervals, respectively, and 59 and 69% of the same values during the second season. Similar yield reduction as a result of frequent defoliation of grasses was reported by many authors (Singh and Mall 1976; Perry and Chapman 1975; Bekele et al. 1974; Owensby et al. 1974). Apparently the 2-week cutting frequency reduced leaf area and consequently reduced the amount of food material translocated to the roots which supports new growth. Other workers have found that carbohydrate reserves may play a more important role than remaining leaf area in regrowth immediately after defoliation (Davidson and Milthorpe 1966; Ward and Blaser 1961). Some species were not as severely affected by the 2-week cutting intervals during the first season as they were the second season. For example yields were not significantly different between the 2or 4-week intervals the first year for buffel grass, rhodes grass, and bambatsi panicgrass. Those same species were also the highest producers among the seven species under the 2 -week frequency during this period. The total yield of switchgrass and blue panic on the other hand was greatly depressed by the 2-week clipping. Dry matter yield of the switchgrass was three times greater when clipped at 6-weeks instead of 2-weeks. Similarly, the yield of blue panic was two and three times greater when clipped at 4 and 6 weeks, respectively, instead of 2 weeks. Green panic produced twice as much dry matter when harvested at 4instead of 2-week intervals. Para grass was not only the lowest producer of all the species but recovered slowly from defoliation as well. This species is more adapted to the humid tropical and subtropical areas (FAO 1959). Switchgrass, para grass, and blue panic that were clipped at ‘-week frequency began to show signs of deterioration after the fourth cut during the first season. Switchgrass was the most affected of all the species by the 2-week cuttings; yields were reduced by 70, 85, and 86% of the fourth cut in the fifth, sixth, and seventh cut, respectively. Yield reduction in other species did not reach such magnitude except for para grass and bambatsi panic in their final two clippings. Species more tolerant to the 2-week frequency of clipping during the first season also produced the highest average yields during the extended period of harvesting in the second season. The average yields of these species increased considerably over the first season yields. This was probably due to the greater number of harvest cycles imposed during the second season. Table 1. Total forage production (kg/ha, oven dry) of seven grass species under three cutting frequencies.’

Productivity of Irrigated Tropical Grasses under Different Clipping Frequencies in the Semidesert Region of the Sudan