Discussion

The findings of the present study suggest that domestic animals may act as potentially effective distributors of seeds of the parasitic weed buda (Striga hermonthica). There is evidence that these seeds may be transported through animal dung.

The general contention is that storm water and wind are the chief means of spreading the seeds, but Farquahar (1937) stated that cattle are the chief means of spreading. He noticed that witchweed (Striga spp.) spread more rapidly in Rhodesia than in Natal; the reason attributed was that in Natal the pest was killed off early by heavy frosts. In an experiment, Farquahar also noticed that in every field that had been shoocked, there was an increase in the pest the following season. Lands were cleaned up faster when cattle were not allowed in cultivated lands. In another experiment, he compared the dung from cattle fed solely on grasses and from cattle fed a small quantity of the weed. The dung was applied to soil containing maize seeds. Witchweed appeared profusely in boxes which received dung from cattle that had been fed on witchweed plants and none appeared in the other.

The present findings provide additional evidence suggesting that the effects of digestive juices of ruminant animals may affect buda seeds either adversely or favourably and that this in turn depends on the seed strain and also on the incubation period of the seed in rumen liquor. The adverse effects may range from reduction of germination capacity to complete loss of seed viability.

Over all, the adverse effects of digestive juices of ruminants in both in viva and in vitro experiments is indicated by the significant reduction of germination capacity of buda seeds, with approximately 14 and 20 per cent below the control at one or two days' incubation period in vivo and approximately 8, 37, or 76 per cent below the control at one, two, or four days' incubation period in vitro. However, not all striga strains responded adversely to an increased incubation period in rumen liquor, the exception being the Medani strain, which had an increase of germination response of approximately 11 per cent and 33 per cent over the control at one or two days' incubation period in rumen liquor in vivo.

TABLE 12. Germination response of buda as affected by different incubation periods in rumen liquor in vitro

C = control (not incubated; 1, 2, 4, 8 = days of incubation
LSD at P = 0.05 for:

TABLE 13. Chemical composition of S. hermonthica on dry-matter basis (percentages)

TABLE 14 Mean digestibility coefficients for the components of S. hermonthica

TABLE 15. Nutritive value of S. hermonthica

TABLE 16. Change in the concentration of VFA (mequivalent/100 ml rumen liquor) and ruminal ammonia (mg N/100 ml rumen liquor)

It is interesting to note that incubation of buda seeds in rumen liquor for more than two days in viva may lead to complete loss of seed viability. Thus the time taken for the seeds to pass through the digestive tract of ruminants appears to be a critical factor in the survival and distribution of the seeds. It is known that the rate of excretion of animal faeces depends to a large extent on the fluid content of the digestible matter. Consequently it may be assumed that the percentage of moisture in the feed stuff or the quantity of drinking water available or the drinking interval may influence the process of digestion and the time taken for digestible material to pass through the digestive tract of animals. Therefore, it may be assumed that in field situations under irrigation domestic animals fed on buda. infested forage may excrete dung within 8 to 24 hours. This may be conducive to relatively less damage to germination capacity than in field situations under rain-fed conditions where the intervals between drinking are prolonged because of the distances between and limited availability of water sites. However, there are villages in rain-fed areas whose domestic animals go out early in the morning to graze and return later in the evening to drink. These animals of course do not experience the long intervals between drinking and therefore constitute a major hazard leading to buda dispersal, especially in the vicinity of the villages.

The high percentage of crude protein in buda obtained in this study is of particular importance in a tropical country like Sudan, where most of the roughagesare deficient in this important nutrient. This relative abundance of crude protein coupled with the low crude fibre content brings the buda parasite nearer to the class of concentrates than roughages. One of the striking features of the composition of buda was its high ash content (21.3 per cent), which was appreciably higher than that found in normal feedstuffs. The richness of this plant material in ash could not be attributed to roil contamination as all the different samples analysed indicated more than 20 per cent ash content.

The proximate analysis that appears in table 13 provides information about the potential value of the material as a feed, but the actual value of a feed to the animal can be determined only after feeding it to the animal and making allowance for the inevitable losses that occur during the utilization of the feed. The most important of these losses is that represented by the part that is not absorbed and is excreted in the faeces. The apparent digestibility measurement is therefore the most important factor in determining the value of the feed.

The apparent digestibility of the buda dry matter was found to be 47 per cent, which is significantly lower than the in vitro digestibility value of 58 per cent we reported in an earlier study (Bebawi et al. 1980). The discrepancy between the two digestibility values might be mainly attributed to the difference in the chemical composition of the two samples of buda evaluated. It is known that the crude fibre percentage increases with age and maturity and as the crude fibre increases, digestibility decreases. The material used in the in vivo study might have been more mature than that used in the in vitro study. It was noticed that the apparent digestibility of the buda dry matter was lower than that obtained for the most common foodstuff available in Sudan, bummera (desert grasses), (El-Hag 1976). However the TDN value, starch equivalent, and digestible crude protein were higher for the buda than for hummra (41.4 vs. 31.4, 29.0 vs. 21.1, and 6.1 vs. 1.6 for TDN, SE, and pop for buda and hummra respectively).

The production of VFA, the ultimate metabolic products of feed degradation in the rumen, is a good indicator of the activity of the rumen micro-organisms. The 3.9 mequivalent/100 ml rumen liquor change in the concentration of VFA compared favourably to that obtained with most foodstuffs found in Sudan. This confirmed that the activity of the micro-organisms was not affected as a result of feeding buda.

The high concentration of rumen ammonia three hours after feeding indicated that a great proportion of buda nitrogen was not utilized for useful purposes by the animal as it is well established that there is a close negative correlation between ammonia concentration in the rumen and the usefulness of dietary protein to the animal (Annism et al. 1954; Chalmers and Synge 1954). This low utilization of protein might be attributed to a shortage of soluble carbohydrates in buda, as without this nutrient the ammonia produced in the rumen cannot be trapped by the microorganisms to synthesize microbial protein. An implication of this finding is that buda should be fed together with other foodstuffs rich in soluble carbohydrates in order to improve nitrogen utilization.

Measurements of nitrogen balance are considered more useful than ammonia concentration in assessing the value of protein to ruminant animals, but the amount of work usually involved is far greater. The average amount of nitrogen retained by the animals was 4.43 9, a figure that reflected an efficient utilization of the crude protein, especially if we know that the nitrogen retained by sheep fed on Medicago stiva was only 3.4 9 (El-Hag 1976). The results of the nitrogen balance measurement seemed not to be in line with the high ammonia concentration found in the rumen. However, there is always difficulty in measuring the N-balance since it is affected by the pre-experimental history of the animal and the duration of the experimental period.

Buda was readily accepted by the sheep and produced no adverse influence on rumen fermentation. It was evident that it contained no toxic material at the level taken by the animals as none of them showed any ill effects from its consumption.

It may be concluded from the findings of the present study that domestic animals may be one of the chief distributors of S. hermonthica (buda) seed in areas both of irrigated and of rain-fed agriculture in Sudan. Howver, it is expected that relatively more efficient distribution of the seed may be achieved by animals in areas of irrigated agriculture or by animals belonging to settled villagers in rain-fed areas than by those owned by pastoralists in rain-fed areas.

It could also be concluded that buda is a potentially useful feedstuff and could safely be considered as a mediumquality roughage. Since it grows in association with aura, which is usually grown in summer in Sudan, the weed will be available at a time of great feed scarcity, and this adds to its potential importance as a feed. The question of using buda as a feed is therefore worth further investigation. Two points should be noticed, however: (a) buda not only leads to the reduction of the aura crop yield but also stunts the stem of the aura, and it is not yet clear whether the gain in buda as a fodder would compensate for the reduction of stubble which is also used as feed for the animals; and (b) this method of using the buda plant can only really be effective if quick-maturing varieties of aura can prove themselves popular and thus allow farmers to resort to trap-cropping, so that the products can be used as fodder and at the same time help reduce the level of buda infestation.

In summary, the existence of Striga hermonthica (buda) in the arid and semi-arid region of Sudan has acted as a deterrent to the adoption of improved husbandry methods. Methods of control so far developed are too complicated for the illiterate farmer to comprehend and too costly for the poorer peasants. At the same time severe questions about the efficacy of many control measures makes many of the better-off farmers in the modern sector doubt the economic feasibility of such methods. It has been demonstrated that buda can be successfully used as a feed in association with other feeding stuffs, but the price to be paid seems likely to be an increase in buda infestation, especially in irrigated areas and rain-fed areas with a regular daily watering pattern. Nomadic animals may be less of a hazard than previously suggested because of the loss of buda seed viability with increased length of immersion in rumen liquor. Clearly more research and a coherent programme of agricultural extension among farmers on buda is required before this obstacle to the adoption of better husbandry in the semi-arid and arid lands of Sudan can be overcome.

H. B. J. Davies