Stocking rate of sheep and survival of helminth infective larvae on Brachiaria pastures

K.J.N. Ndamukong, S.E. Mbomi and S. Killanga

Institute of Animal Research, Mankon Station
P.O. Box 125, Bamenda, Cameroon

Abstract
Introduction
Materials and methods
Results
Discussion
Conclusions
Acknowledgements
References

Abstract

A study was undertaken at Mankon, Cameroon, to determine a pasture grazing system for sheep that would produce optimum animal performance and herbage yield as well as limit the hazards of helminthiasis. The optimum stocking rate for Cameroon Grassland Dwarf ewes was determined by comparing animal performance, herbage yield, pasture contamination and infectivity of the animals. The animals were grazed on predominantly Brachiaria pastures at three stocking rates: 29, 39 and 47 animals/ha. The results indicated that helminth infestation rate and pasture contamination rose as the stocking rate increased. Pasture contamination at 29 animals/ha remained insignificant till after 12 weeks of grazing. Live weight was little affected by stocking rate. The results of the regression of herbage yield over time differed (P<0.01) with the stocking rate. Helminth larvae survived on Brachiaria pastures for about 11 weeks during the wet season. This study indicates that optimum animal performance and herbage yield can be achieved at a stocking rate of 29 animals/ha, provided animals are moved after three months to allow for the previously grazed pasture to be decontaminated.

Taux de charge ovine et survie des larves infestantes d'helminthes sur pâturages à Brachiaria

Résumé

Une étude a été entreprise à Mankon (Cameroun) afin d'établir un système d'alimentation en vert destiné à des ovins et qui permette d'obtenir des performances zootechniques et une production de matières vertes optimales, tout en limitant les risques d'helminthiase. Pour déterminer le taux de charge optimum pour des brebis Djallonké du Cameroun, on a procédé à une comparaison des performances zootechniques, de la production de matières vertes, de la contamination des pâturages et de l'infectiosité des animaux. Ceux-ci étaient conduits sur des pâturages à dominante Brachiaria à raison de 29, 39 et 47 animaux par hectare. Les résultats font apparaître un accroissement du taux d'helminthiase et de la contamination des pâturages avec l'augmentation du taux de charge. La contamination des pâturages qui portaient 29 animaux à l'hectare demeurait insignifiante jusqu'après la 12^e semaine de pâture. Le taux de charge n'avait presqu'aucun effet sur le poids vif Les résultats de la régression de la production de matières vertes sur le temps (P<0,01) variaient en fonction du taux de charge. Les larves d'helminthes survivaient sur les pâturages à Brachiaria pendant 11 semaines environ au cours de la saison des pluies. Il ressort de l'étude que les meilleures performances zootechniques et la production optimale de matières vertes s'obtiennent avec un taux de charge de 29 animaux par hectare, à condition de déplacer les animaux après trois mois afin de permettre la décontamination du pâturage.

Introduction

Stocking rate is the most important management factor influencing the output of animal products on pasture, the stability and persistence of the pasture components and also the financial returns which the farmer receives (Humphreys, 1978).

Overstocking reduces the opportunity for selective grazing. This may result in overgrazing, poor nutrition of the animals and an increased risk of helminthiasis, with consequent reduction in the condition of the animals. There is little information in Cameroon on the relation between flock management practices, the level of parasitism and the production of herbage. This study was therefore undertaken to determine a pasture grazing system for sheep that would produce optimum animal performance and pasture yield as well as limit the hazards of helminthiasis.

Materials and methods

Experiment 1. Studies on stocking rate

Recently bred Cameroon Grassland Dwarf ewes were maintained at three stocking rates (29, 39, 47 ewes/ha) on paddocks of predominantly Brachiaria pastures for 14 weeks during the rainy season (July to September). The rates chosen were based on previous experience of grazing sheep and goats at the research station and were similar to those used by Cameron and Gibbs (1966). The animals were dewormed with Ivermectin at 200 g/kg of body weight prior to the start of the study. Supplementary feeding was not practised. However, water and salt were provided ad libitum.

Data collected included measurements of live weight, faecal strongyle egg counts, pasture larval counts, feed availability measured at ground level (forage yield/ha) and grazing pressure (leaf-stem ratio).

Experiment 2. Survival of strongylid infective larvae on pasture

The method used was similar to that described by Okon and Enyenihi (1977). The study was carried out on a 1/8 hectare fenced area of level pasture land consisting predominantly of Brachiaria ruziziensis. Twelve experimental plots were used, each of 2 m² and with boundaries marked by 20-cm wide shallow furrows. The grass on each plot was maintained at a uniform height of 6 cm by periodic clipping, the clippings being allowed to remain on the plot. During the first three days of each month throughout the calendar year, fresh goat faeces containing a known number of strongyle eggs were spread uniformly on one of the 12 plots. A sample of the faeces spread each month was cultured for two weeks at ambient temperature to determine the proportion of eggs of each species in the sample. One week after the last faeces were spread on the pasture, a 100 g grass sample was collected at random from the plot and infective larvae were extracted and counted using a modification of the technique of Lancaster (1970). The collection of grass samples was continued weekly until no larvae could be extracted for two successive weeks. The procedure was repeated for each plot until all 12 plots had been examined. Daily records of minimum/maximum temperature and rainfall were kept throughout the study period.

Statistical analyses were carried out according to the methods described by Steel and Torrie (1960). Differences in faecal egg counts between animals on the three stocking rates were determined by the Kruskal-Wallis test (Neter and Wasserman, 1974). Changes in liveweight with time within stocking rates were subjected to the paired l-test. The regression slopes for forage yield were compared using multiple regression (least squares regression). The differences in survival period on pasture for infective larvae of Haemonchus contortus and Trichostrongylus were assessed by Mann Whitney u test (Snedecor and Cochran, 1967). The correlation between rainfall and relative transmission potential was assessed by Spearman's rank correlation coefficient (Snedecor and Cochran, 1967).

Results

Liveweight changes

The data on liveweight changes under the three stocking rates are shown in Table 1. The weights adjusted for lambing were not significantly affected by stocking rate (P>0.05). The regressions had similar slopes and did not differ in magnitude. Parturition and lactation, on the other hand, generally resulted in a progressive decrease in the live weight of the ewe for at least 10 weeks, the decrease already being significant (P<0.001) by two weeks post parturition.

Parasitology

The parasitological data are presented in Tables 2 and 3. The infection rate generally rose as the stocking rate increased from 29 animals/ha to 39 animals/ha, although the difference in egg count was not significant. The pasture stocked at 29 animals/ha remained free of contamination for 12 weeks from the start of grazing (Table 2). The animals grazing on this pasture maintained a median egg count of less than 500 eggs per g of faeces during this period. By contrast, the median egg count increased to more than 600 eggs per g of faeces between the 6 and 12 weeks in the ewes stocked at 39 and 47 animals/ha. Egg counts and pasture contamination were generally lower at 47 animals/ha than at 39 animals/ha (Table 2). Ewes stocked at 29 and 39 animals/ha had lambing rates of 67 and 71% respectively. The post-parturient rise in faecal egg counts was more pronounced at 39 animals/ha. At 47 animals/ha only two ewes lambed, the first at 10 weeks and the second at one week before the end of the experiment. The post-parturient rise in egg count was evident in the ewe that lambed earlier, where the egg count rose from 100 eggs to 3250 eggs per g of faeces within 2 weeks of lambing. Haemonchus contortus was the predominant helminth species in the animals of the three groups, accounting for 91% of the strongyle eggs in the faeces. Trichostrongylus sp and Oesophagostomum eggs together comprised only 9% (Table 3).

Table 1. Mean initial liveweight and mean weight changes in Cameroon Grassland Dwarf ewes at three stocking rates on predominantly Brachiaria pastures.

Stocking rate (animals/ha)	Class of animal	No. of animals	Initial weight (kg)	Liveweight change (kg)
29	Ewes that lambed	7	32.3	-2.4
	Ewes that did not lamb	2	30.5	4.5
39	Ewes that lambed	5	32.6	-3.4
	Ewes that did not lamb	2	32.0	4.5
47	Ewes that lambed	2	35.5	2.0
	Ewes that did not lamb	4	31.8	4.2

Table 2. Larval contamination of pasture grazed by Cameroon Grassland Dwarf ewes at three stocking rates.

Stocking rate(animals/ha)	Infective larval counts per kg dry + matter (DM) of pasture (weeks)*
	2	4	6	8	10	12	14
29	0	0	0	0	0	0	146
39	0	0	0	0	278	250	268
47	0	0	0	20	105	86	124

* Period after the start of study.

Production of herbage

The leaf-stem ratios measured from the eighth week of the experiment are shown in Table 4. The leaf-stem ratio was lower at 39 and 47 animals/ha than at 29 animals/ha, but the difference was not statistically tested. The regressions of herbage yield on time differed according to the stocking rate (P<0.01). The regression slopes were -0.14,-0.17 and -0.21 at 29, 39 and 47 animals/ha, respectively, but these were not significantly different from each other.

Survival of infective larvae on pasture

The duration of survival of infective larvae on pasture in relation to the month of deposition of the faeces containing the eggs is shown in Table 5. Infective larvae developed from eggs deposited on pasture between April and October (rainy season). None developed from eggs deposited between November and March (dry season) except in December when there were unseasonal rains. The shortest survival time during this period was for larvae hatched from eggs deposited at the beginning of April. Infective larvae that hatched during the rainy season survived on pasture for an average of 10 weeks, with a range of 6 to 13 weeks. Larvae of H. contortus survived longer than those of Trichostrongylus sp (11 and 9 weeks, respectively), but the difference was not significant.

Table 3. Proportion of each strongyle egg type in faeces from sheep grazing at three stocking rates at Mankon, Cameroon.

Stocking rate (animals/ha)	Proportion of egg type in faeces (%)*
	Haemonchus	Trichostrongylus	Oesophagostomum
29	90.5	7.5	2
39	93.5	1.5	S
47	90.0	4.0	6

* Mean values at the 12th and 14th weeks from the start of the study.

Table 4. Leaf-stem ratio for pastures grazed by Cameroon Grassland Dwarf ewes at three stocking rates.

Weeks from the start of grazing	Leaf-stem ratio
	29 animals/ha	39 animals/ha	47 animals/ha
8	1.0	0.5	0.3
10	1.0	0.4	0.3
12	0.9	0.3	0.3
14	0.6	0.2	0.2

Table 5. Survival of infective larvae of H. contortus and Trichostrongylus on pasture at Mankon.

Month of deposition of eggs on pasture 1988-89	Duration of survival of infective larvae on pasture (weeks)		Number of infective larvae recovered		Relative transmission potential (No. of days x % eggs)
	HC	TC	HC	TC	HC	TC
April	7	6	430	2884	0.98	38.64
May	13	11	6570	17455	16.38	154.77
June	10	10	4087	4214	32.20	51.10
July	11	9	4411	3564	25.41	15.12
August	10	9	1747	9057	45.50	103.95
September	12	12	27664	15561	10.44	39.48
October	13	9	7968	9504	29.12	97.65
November	0	0	0	0
December	6	6	338	56	0.84	0.42
January	0	0	0	0	0	0
February	0	0	0	0	0	0
March	0	0	0	0	0	0

Table 6. Seasonal variation in population of H. contortus (HC) and Trichostrongylus (TC) on pasture at Mankon.

Period (month in 1988-89)	No. of infective larvae on pasture plots			% of infective larvae of each species on pasture
	HC	TC	Total	HC	TC
April	376	2704	3084	12.2	87.8
May	5248	16308	21556	24.3	75.7
June	4487	5167	9654	46.5	53.7
July	3068	2966	6034	50.8	49.2
August	3073	8548	11621	26.4	73.6
September	14496	12092	26528	54.6	45.4
October	18196	12816	31012	58.7	41.3
November	3178	1662	4840	65.7	34.3
December	981	62	1043	94.1	5.9
January	112	26	138	81.2	18.8
February	0	0	0	0	0
March	0	0	0	0	0

Table 5 also shows the number of infective larvae recovered from each experimental plot as a percentage of the number of eggs spread and gives the relative transmission potential of these larvae. A significantly (P<0.05) greater proportion of eggs of Trichostrongylus sp successfully developed into infective larvae during the rainy season than did eggs of H. contortus. The relative transmission potential was highest in August for H. contortus and in May for Trichostrongylus sp. The correlation between rainfall and transmission potential was significant (P<0.01) for both species (H. contortus: r = 0.8; Trichostrongylus sp: r = 0.73).

Table 6 gives data on the proportion of each larval type present on the plots during each month of the year. The data revealed that the proportion of infective larvae of H. contortus on pasture steadily increased as the rainy season progressed. The proportion of Trichostrongylus sp larvae on the pasture was highest at the beginning of the rains and decreased progressively towards the end of the rainy season.

Discussion

The results of this study show that the best performance of Cameroon Grassland Dwarf ewes grazing on predominantly Brachiaria pastures can be obtained at a stocking rate of 29 animals/ha. The changes in body weight adjusted for parturition were not different between the three stocking rates. However, there was an indication that grazing for longer than 12 weeks would depress body weight at stocking rates higher than 29 animals/ha.

More obvious was the effect of stocking rate on helminth infestation. The greater mass of herbage at the beginning of the study plus a washing effect by the rain probably tended to reduce the concentration of larvae on the herbage. The larval count of zero during the early period of grazing meant that larvae were either absent or were so few that they could not be recovered by the extraction method used. The change in level of pasture contamination and infestation rate with stocking rate was clearly evident as the stocking rate rose from 29 to 39 animals/ha. There was no evidence of an increase in pasture contamination or nematode burden with the increase in stocking rate from 39 to 47 animals/ha.

Cameron and Gibbs (1966) obtained lower counts of H. contortus, Ostertagia circumcincta and Trichostrongylus sp in lambs stocked at 48 animals/ha than in those stocked at 41 or 35 animals/ha. Michel (1964) observed that at a heavier stocking rate the grass is grazed low resulting in a smaller daily intake of herbage per head. While this would result in the ingestion of fewer larvae, a lower nutrition status resulting from low herbage intake may reduce the animal's resistance to worm infection and to its harmful effects.

This probably explains why a stocking rate of 39 animals/ha resulted in lower faecal egg counts and lower pasture larval counts than a stocking rate of 47 animals/ha. However, it is also probable that the higher lambing rate of the animals grazing at 39 animals/ha (71 %) could have resulted in a more rapid rise in faecal egg counts (post-parturient rise) and pasture contamination than occurred in the group stocked at 47 animals/ha. In this group a 33% lambing rate was recorded only during the last four weeks of the experiment.

The stocking rate of 29 animals/ha resulted in patch grazing with uneven decline in pasture yield since there was an unlimited opportunity for selective grazing. At 39 and 47 animals/ha, the grazing pressure was heavy and the pastures were grazed more evenly. Overgrazing was clearly evident at 47 animals/ha. The high stocking rate resulted in a rapid decline in herbage production and less feed being available to the animals.

Haemonchus contortus and Trichostrongylys sp are the predominant helminth species affecting sheep and goats in the high- and mid-altitude zone of Cameroon (Ndamukong et al, 1986). One of the most interesting features of the trichostrongylids is their dependence on climate for development and survival (Dunn, 1978). Rainfall appears to be the basic requirement for development and survival of eggs and larvae of H. contortus and Trichostrongylus sp on pasture. Shorb (1944) noted that a lowering of the moisture content of the faeces raised the death rate of the eggs. Temperature, on the other hand, did not appear to be a limiting factor for larval development and survival. The mean minimum temperature (14.8 ± 1.8°C) recorded during the study was well above the minimum (10°C) at which infective larvae of H. contortus and T. colubriformis can develop (Wang, 1967; Ndamukong, 1987).

The results presented here reveal that a significantly greater proportion of eggs of Trichostrongylus than of H. contortus developed into infective larvae during the rainy season (Table 5). However, the high fecundity constant of H. contortus (Crofton, 1957) enabled the latter to leave more surviving infective larvae on pasture at the end of the rains. Although eggs placed on pasture during the dry season did not develop, larvae that had developed during the last month of the rains survived until January. This is in agreement with the finding of Andersen et al (1966) that infective larvae are more resistant to the influence of external conditions than are eggs and pre-infective larvae.

Conclusions

These studies indicate that infective larvae of Haemonchus and Trichostrongylus on Brachiaria pasture die after about 12 weeks on the pasture. The great majority of eggs or larvae probably die within a week after arrival on pasture, but enough survive for 9-11 weeks to produce severe infestations in susceptible sheep and goats. Under a rotational system of grazing management, optimum animal performance and herbage yield can be achieved at a stocking rate of 29 animals/ha. The risk of parasitic gastro-enteritis at this stocking level is minimal. However, animals should not be allowed to remain on the same pasture for more than three months, as pasture larval contamination starts escalating after this time. Safe pastures can be produced from contaminated pastures by resting them for at least three months. Since pastures are virtually free of contamination during the last two months of the dry season (mid January-mid March), the duration of grazing on a pasture during this period could depend on herbage availability rather than on parasitic considerations. These findings can be used in grazing pasture management to limit the intake of infective larvae from pasture and ensure adequate nutrition for the animals.

Acknowledgements

We appreciate the technical assistance of Oscar Ndi and Rene Degrace Tiako. We are grateful to the Cameroon Government for financing this study.

References

Andersen F L, Wang G T and Levine N D. 1966. Effect of temperature on survival of the free-living stages of T. colubriformis. Journal of Parasitology 52:713-721.

Cameron C D T and Gibbs H C. 1966. Effects of stocking rate and flock management on internal parasitism in lambs. Canadian Journal of Animal Science 46:121-124.

Crofton H D. 1957. Nematode parasite populations in sheep on lowland farms. III. The seasonal incidence of species. Parasitology 47:304-318.

Dunn A M. 1978. Veterinary helminthology. 2nd edition. William Heinemann Medical Books Limited, London, UK. 323 pp.

Humphreys L R. 1978. Tropical pastures and fodder crops. Intermediate Tropical Agricultural Series. Longman Group Ltd. London, UK. 135 pp.

Lancaster M B. 1970. The recovery of infective nematode samples. Journal of Helminthology 44(2):219-230.

Michel I F. 1964. Helminthiasis and grazing management. Agriculture London 71 (2):80-83.

Ndamukong K J N. 1987. Sheep and goat production in the North West Province of Cameroon with special reference to parasitic gastro-enteritis. PhD thesis. The University of Edinburgh, Edinburgh, UK. 405 pp.

Ndamukong K J N. Sewell M M H and Asanji M F. 1986. Parasitic gastro-enteritis in sheep and goats at Bamenda, N.W. Province of Cameroon. 1. Productivity epidemiology. Revue Science et Technique. Serie Sciences Zootechniques 2(4):101-117.

Netter J and Wasserman W. 1974. Applied linear statistical models R. Irwin Inc., Homewood, Illinois, USA. 842 pp.

Okon E D and Enyenihi U K. 1977. Development and survival of Haemonchus contortus larvae on pastures in Ibadan. Tropical Animal Health and Production 9:7-10.

Shorb D A. 1944. Survival on grass plots of eggs and larvae of stomach worm H. contortus. Journal of Agricultural Research 68:317-324.

Snedecor G W and Cochran W G. 1967. Statistical methods. The Iowa State University Press, Ames, Iowa, USA. 593 pp.

Steel R G D and Torrie J H. 1960. Principles and procedure of statistics. McGraw-Hill, London, UK. 481 pp.

Wang G T. 1967. Effect of temperature and culture methods on the development of the free-living stages of Trichostrongylus colubriformis. American Journal of Veterinary Research 28:1085-1090.