Abstract
Introduction
Materials and methods
Results and discussion
Conclusions
Acknowledgements
References
B.H. Dzowela
Department of Agricultural Research, Ministry of Agriculture, Chitedze Research Station, P.O. Box
158, Lilongwe, Malawi
Two pasture systems for fattening beef cattle in summer were studied: (1) a pure sward of Rhodes grass (Chloris gayana) plus 40 kg N/ha and (2) a mixture of Rhodes grass and Silverleaf desmodium (D. uncinatum). The forage yields of the two pastures were similar. The average daily liveweight gain of heifers on the grass-legume pasture was higher than that of heifers on the Rhodes-grass pasture, due to the higher crude protein content of the mixed sward.
Nitrogen is one of the major plant nutrients on which plant growth depends (Russel, 19663. In Malawi the cost of fertilizer nitrogen has increased considerably in the past decade. Between 1972/73 and 1983/84 the price of ammonium sulphate (21% N) and calcium ammonium nitrate (26% N) in Malawi increased by 92 and 100%, respectively. Between 1983/84 and 1984/85 the price of these fertilizers increased further by 29 and 19%, respectively, and prices are expected to continue to increase.
In smallholder farming systems, the use of fertilizer N is largely restricted to cereal crops, such as maize, that are used for human food. Thus there is a strong case for introducing tropical forage legumes into pastures in smallholder farming systems in Malawi.
Forage legumes provide an alternative source of N for pasture growth through their ability to fix atmospheric N, as well as increasing the nutritive value of the forage produced. The dry-matter yield of a pasture containing Stylosanthes guianensis, Desmodium intortum and Chloris gayana was reported to be equal to that of a C. gayana pasture fertilized with 448 kg of ammonium sulphate per hectare. Thus the legumes fixed the equivalent of at least 94 kg N/ha annually (Department of Agriculture, 1961; Thomas, 1973).
It is difficult to establish and maintain legumes in association with vigorous stoloniferous grasses such as C. gayana and Cynodon sp. However, Thomas (1976) and Dzowela (unpublished) found that Desmodium uncinatum, Macroptilium atropurpureum and Centrosema pubescens could be successfully established and could maintain a stand beyond the second year in association with these grasses.
Addy and Thomas (1977) demonstrated substantial levels of beef production of 369, 525 and 539 kg/ha at 2.5, 5.0 and 7.5 livestock units per hectare, respectively, from improved, fertilized pastures. The only comparable data for a legume-based pasture system are those reported by Dzowela (1984), who found that steers on a Stylosanthes-sp.-based natural grassland lost less weight than steers on a pasture that did not contain legume.
This study was undertaken to investigate the beef production potential of a Rhodes-grass pasture fertilized with inorganic N and of Rhodes grass grown in association with D. uncinatum.
Two treatments were investigated: Rhodes grass in pure stand fertilized with 40 kg N/ha, and a mixed sward of Rhodes grass and Silverleaf desmodium (D. uncinatum). The swards were established in the 1981/82 growing season on 1.5 ha paddocks, with 3 replicates. Each paddock received 16.6 kg P/ha annually.
The 'put-and-take' grazing system (Most and Lucas, 1952) was used to maintain optimum grazing pressure throughout the season. Eight 2-year-old Malawi Zebu 'tester' heifers were kept in each paddock throughout the growing season in 1982/83, 1983/84 and 1984/85. The animals were kept on the pasture from the beginning of the growing season in November/ December until they stopped gaining weight, which coincided with the beginning of the dry season in May/June. Additional animals, 'grazers', were put into the paddocks if there was an excess of forage.
Each paddock was divided into four parts to facilitate rotational grazing. The animals spent one week in each quarter, and the 'testers' were weighed every 28 days. The forage was sampled before the paddocks were grazed, and the samples were separated by hand into grass and legume components before being dried in the oven at 65 C for dry-matter determinations. Oven-dry samples were ground to pass through a 1 mm screen, and were analysed for crude protein (CP) content and in vitro dry-matter digestibility.
The forage dry-matter yields of the two pastures were similar over the three seasons (Table 1). The N status of the soil before and after the trial was also similar (Table 2), which indicates that all the growth was produced on the fertilizer N applied or on the N fixed by the legume.
The heifers on the grass-legume sward had significantly higher average daily liveweight gains in 1982/83 and 1984/85 than those on the pure Rhodes grass sward (P < 0.05), but there was no significant difference in 1983/84 (Table 3). There were no significant differences in the total liveweight gain or number of grazing days in any of the three seasons. However, the total liveweight gain per hectare tended to be higher in the grass-legume treatment than in the pure grass treatment.
Table 1. Forage dry-matter yields of Rhodes grass and Rhodes grass-Silverleaf desmodium pastures, 1982/83 to 1984/85.
|
Pasture |
DM yield (t/ha) |
||
|
1982/83 |
1983/84 |
1984/85 |
|
|
Pure Rhodes grass |
11.33 |
14.61 |
11.76 |
|
Rhodes grass plus |
|
|
|
|
Silverleaf desmodium |
11.17 |
14.71 |
12.49 |
Table 2. Soil nitrogen status before and after three seasons of Rhodes grass and Rhodes grass-Silverleaf desmodium pastures.
|
Pasture |
Prior to treatments |
After treatments |
|
November 1982 |
May 1985 |
|
|
Pure Rhodes grass |
0.32 |
0.31 |
|
Rhodes grass plus |
|
|
|
Silverleaf desmodium |
0.29 |
0.30 |
While the digestibility of the forage from both treatments was the same (57%), the grass-legume mixture contained significantly more CP (12.57%, compared with 9.43% for the Rhodes grass alone). A small proportion of legume in the pasture both increases the amount of N available to the associated grass and increases the level of protein in the diet (Most, 1981). The addition of as little as 10% of legume (DM basis) in the diet or forage system may lead to large increases in forage intake and animal performance (Minson, 1980). In this study the proportion of legume in the mixture averaged about 20% throughout the period of the trial.
Table 3. Average daily Liveweight gains, total gains/ha and number of grazing days/season on Rhodes grass and Rhodes grass-Silverleaf desmodium pastures, 1982/83 to 1984/85.
|
|
1982/83 |
1983/84 |
1984/85 |
||||||
|
Average daily gain |
Total gain |
Grazing days |
Average daily gain |
Total gain |
Grazing days |
Average daily gain |
Total gain |
Grazing days |
|
|
(kg) |
(kg/ha) |
(kg) |
(kg/ha) |
(kg) |
(kg/ha) |
||||
|
Rhodes grass |
0.49a |
662 |
1340 |
0.70 |
527 |
1177 |
0.62a |
521 |
1218 |
|
Rhodes grass plus |
|
|
|
|
|
|
|
|
|
|
Silverleaf desmodium |
0.62b |
669 |
1184 |
0.70 |
526 |
1165 |
0.70b |
560 |
1195 |
Figures within a column followed by different [esters differ significantly (P<0.05).
It would appear that the better performance of the animals on the grass-legume pasture was related to the higher CP content of the forage. There were strong positive correlations between daily liveweight gain from January to May and the CP content of the forages in both the pure grass pasture (r2 = 0.80) and the grass-legume mixture (r2 = 0.89).
The unfertilized grass-legume pasture gave similar forage dry-matter yields to the pure stand of Rhodes grass, which received 40 kg N/ha. In addition, the CP content of the mixed forage was higher than that of the grass alone, which resulted in significantly higher average daily liveweight gains from the grass-legume pasture in two of the three seasons of the trial. Thus, it would appear that for smallholder cattle fattening systems in Malawi, the inclusion of a legume component in pastures could eliminate the need for expensive N fertilizers without reducing animal performance.
The work reported in this paper was undertaken with logistical support from the Ministry of Agriculture. I would like to thank the Chief Agricultural Research Officer of the Malawi Ministry of Agriculture for permission to publish these results.
Addy B H and Thomas D. 1977. Intensive fattening of beef cattle on Rhodes grass pastures on the Lilongwe Plain, Malawi. Trop. Anim. Health Prod. 9: 99-106.
Dzowela B H. 1984. Highlights of pasture research in Malawi during the late 60s and early 80s. Paper presented at the IDRC/SADCC Workshop for Pastures in Eastern and Southern Africa, Harrare, Zimbabwe, 17-21 September 1984.
Minson D J. 1980. Nutritional differences between tropical and temperate pastures. In: Morley F W H (ed.) Grazing animals. Elsevier, Amsterdam. pp. 143-157.
Mott G O. 1981. Potential productivity of temperate and tropical grassland systems. Proceedings of the XIV International Grasslands Congress. pp. 39-42.
Mott G O and Lucas H L. 1952. The design, conduct and interpretation of grazing trials on cultivated and improved pastures. Proceedings of the VI International Grasslands Congress. pp. 1380-1385.
Russel E W. 1966. Soils and soil fertility. In: Tropical pastures. Faber and Faber, London.
Thomas D. 1973. Nitrogen from tropical pasture legumes on the African continent. Herb. Abst. 43(2):33-39.
Thomas D. 1976. Effects of close grazing on the productivity and persistence of tropical legumes with Rhodes grass in Malawi. Trop. Agric. (Trinidad) 53(4):321-327.
