Growth rates of fat-tailed sheep tethered or free on range compared to free in a Leucaena leucocephala pasture

Damião Nguluve and James P Muir

Instituto de Produção Animal, C.P. 1410, Maputo, Mozambique
j-muir@tamu.edu

Abstract

Dry season average daily liveweight gains of castrated, growing Landim fat-tailed sheep tethered on native grass (TR) or free-ranged in a cultivated pasture with a Leucaena leucocephala over-story (FL) were compared to sheep free-ranged on native grass (FR). During the year with 591 mm rainfall, growth rates (g/day) were 15.2, 37.8 and 25.8 for the respective treatments. During the year with 780 mm rainfall, growth rates were 17.0, 89.8 and 39.5 for the respective treatments. High rainfall versus low rainfall year differences in growth rate were not as large for TR animals (12% greater for high rainfall) compared to FR (53% greater) or FL (138% greater). The trial indicated that tethering fat-tailed sheep, thus limiting grazing selectivity, decreased growth rate compared to untethered animals. It also indicated that sheep with both greater herbaceous biomass and high crude protein leaves (from L. leucocephala) to select from had higher growth rate compared to animals in mixed cattle/sheep herds on native pasture.

Keywords: silvopasture, rainfall, grazing management, small ruminants

Introduction

Intake and digestibility of tropical dry season grasses by sheep tend to be low due to high fibre and low crude protein concentrations in those forages (Lascano and Palacios 1993). Although the inclusion of a herbaceous legume component in a diet does not ensure an increase in intake and digestibility, it does generally increase digestible N available to the animal (Lascano and Palacios 1993). In one study, in which Leucaena leucocephala leaves substituted up to 40% of a grass diet,  it was observed that growth rate increased in sheep despite decreased intake (Adejumo and Ademosun 1991). Tomkins et al (1991) found that inclusion of L. leucocephala at up to 30% of the diet increased digestibility and intake of the total diet when sheep were fed tropical hays with over 50% in vitro digestibility.

There appear to be no studies to indicate the effects of L. leucocephala intake on the growth of Landim fat-tailed sheep in southern Africa. Studies with Landim goats in this region have indicated that L. leucocephala does increase growth rate in that species during the dry season with no visible deleterious effects of mimosine toxicity (Muir et al 1994; Muir and Massaete 1996). With the spread of plantings of this multi-purpose legume tree in Mozambique, the need has arisen for additional information focused on sheep. In addition, increased competition between small ruminants and dry season vegetable (irrigated) growing has resulted in an increase in the system of tethering. The objectives of this study were to study growth rate of Landim fat-tailed sheep, during the dry season, as affected by:

• tethering (limited forage quantity and selectivity) on native range and
• access to L. leucecephala silvo-pasture.
  

Castrated, 4-6 month old Landim fat-tailed sheep were assigned to 3 groups of 6 animals balanced for age and initial body weight. Each group was either tethered on a 3 m rope on native range (TR), allowed free movement on native range (FR) or allowed free movement in a Leucaena leucocephala pasture (FL). The trials took place during the dry season and were terminated each year at the onset of rains. During 1996 the sheep were studied for 208 days (20 May to 10 December), while during 1997 they were studied for only 112 days (1 April to 14 August) due to early rains.

The experiment took place in southern Mozambique (33^°E and 26^°S) at 40 m altitude on sandy, well drained soils with an average pH of 6.3. In 1996, average monthly rainfall from January up to the initiation of the experiment was 83 mm/month and 11 mm/month during the trial. In 1997, average monthly rainfall from January up to the trial initiation was 121 mm/month and 23 mm/month during the trial (see Figure 1 for monthly rainfall pattern).

The range was dominated by Cynodon dactylon, Urochloa mosambicensis, Cenchrus ciliaris and Eragrostis spp. Cattle also used the range during both the rainy and dry seasons. In May, the beginning of the dry season, herbaceous above-ground dry matter in the range was estimated at 1.7 tonnes/ha in 1996 and 2.3 tonnes/ha in 1997 (Table 1). This estimate was based on twenty clipped samples cut at a 5 cm height in an area of 0.25 m^² on a diagonal across the 3 ha paddock. Crude protein concentration and in vitro digestibility (IVD) on a total, above-ground plant basis was estimated at 5.2% and 41.4%, respectively, averaged for both years.

Table 1: Estimated forage accumulation and quality indicators  (whole plant) in May of 1996 and 1997
Pasture Type	1996	1997
Range
DM, tonnes/ha	1.7	2.3
Average, CP % in DM	5.2
Average, IVD %	41.4
Leucaena herbaceous layer
DM, tonnes/ha	6.2	7.2
Average CP, % in DM	4.8
Average, IVD%	36.6
Leucaena leaves
DM, kg/tree	1.96	2.40
Average, CP % in DM	24.1
Average, IVD %	53.8
(DM dry matter; CP crude protein; IVD In vitro DM digestibility)

The cultivated (leucaena) pasture consisted of six paddocks of approximately 880 m^²grazed rotationally by sheep alone during the trial and rested during the growing season. The rotation period depended on a subjective, visual forage availability estimate and averaged 10 days grazing and 50 days resting during both years. The herbaceous layer consisted primarily of Cenchrus ciliaris, Cynodon dactylon, Panicum maximum and Urochloa mosambicensis. In May 1996, the above-ground dry matter was estimated at 6.2 tonnes/ha and in May 1997, dry matter was estimated at 7.2 tonnes/ha. This estimate was based on 20 hand-clipped samples cut at a 5cm height on an area of 0.25 m^² along a diagonal transect through the 6 paddocks (Table 1). Averaged for both years in May, herbaceous whole plant crude protein concentration was estimated at 4.8% while IVD was estimated at 36.6%. Double rows of 7 year old L. leucocephala, planted at 1 m x 1 m spacing every 6 m contributed leaves with an estimated average 24.1% crude protein   and average 53.8% IVD over both years. Each plant produced an estimated 1.96 kg dry matter leaves/plant in 1996 and 2.40 kg dry matter leaves/plant in 1997. Branches of 6 trees were lopped off at a 1.2 m height each day whenever the sheep had consumed all leaves within reach.

Sheep were allowed 7 hours of grazing time per day and tethered animals were moved, at most, once per day during the late dry season. Animals were provided with water in a stall at night. They were weighed individually once every 28 days during the trial after 12 hours without water or forage.

There were measurable differences in growth rate among treatments for both seasons (Table 2). Sheep in the TR treatment gained the least weight, 16.1 g/day on average for both years. There was little difference in growth rate of TR sheep between years despite increased rainfall the second year. This may have been a result of heavy grazing by the cattle which shared the paddock. These animals kept the pasture close-cropped and limited forage availability in both years.

The FR animals on range gained weight at twice the rate of the tethered animals when averaged over the two years (Figure 2). In the second year, especially, better soil moisture conditions allowed for greater grass regrowth which the sheep, despite close cropping by cattle, were able to exploit. As a result, FR sheep had 53% greater growth rate the second year when compared to the first year (Table 2).

Table 2: Average daily liveweight gains of fat-tailed sheep tethered on range, free on range or free on a Leucaena leucocephala pasture during the dry season. Grazing System 1996 1997 Liveweight gain, g/day Free on L. leucocephala 37.8c    89.8c Free on range      25.8b 39.5b Tethered on range     15.2a 17.0a Probability         0.002     0.003 CV, %   22.8 15.6     abc Values in the same column followed by different letters differ (P<0.05) according to Duncan’s Multiple Range Test.

The FL sheep ate the legume readily, both on hanging as well as lopped branches. They showed no sign of mimosine toxicity despite being allowed to browse leaves ad libitum. These animals had nearly four times the growth rate compared to the TR animals (Table 2; Figure 2). They likewise had nearly twice the growth rate compared to FR sheep. This can be attributed, at least in part, to two factors: 

• 6.7 tonnes forage dry matter/ha in the cultivated pasture herbaceous layer on average was much greater than the 2.0 tonnes forage dry matter/ha for the range herbaceous layer. Since the L. leucocephala pastures were not grazed by cattle during the rains, more forage accumulated. Lower quality indicators (by 0.4% units of crude protein  and 4.8% units of  IVD) in the "improved" pasture  can be discounted, since these were measured on whole, mature plants which were not likely to be   representative of selective grazing by sheep.
• The legume contributed considerably to the overall crude protein concentration in the silvo-pasture, despite the fact that IVD was rather low for the L. leucocephala leaves. These leaves had accumulated over the rainy season and were very mature. The additional protein in the diet would also be expected to increase intake of the grazed grasses.
  

Conclusions

The following conclusions may be drawn:

• In a  (dry season) pasture with limited accumulated forage of low nutritive quality, the restriction of selectivity by tethering was detrimental to weight gains in fat-tailed sheep.
• Tethered sheep in competition with grazing cattle did not fully benefit from limited grass regrowth from dry season rains since quantity was still limited.
• Fat-tailed sheep grazing grasses (high-quantity, low-quality) in combination with L. leucocephala had higher growth rates compared to animals grazing low-quantity, low-quality paddocks.
  

Adejumo J O and Ademosun A A 1991 Utilization of leucaena as supplement for growing dwarf sheep and goats in the humid zone of West Africa. Small Ruminant Research 5:75-82.

Lascano C E and Palacios E 1993 Intake and digestibility by sheep of mature grass alone and in combination with two tropical legumes. Tropical Agriculture (Trinidad) 70:356-358.

Muir J P, Jordao C and Massaete E S 1994 Comparative growth characteristics of goats tethered on pasture and free-ranged on cultivated pasture. Small Ruminant Research 17:111-116.

Muir J P and Massaete E S 1996 Effect of physical restriction and supplementation with Leucaena leucocephala on goat growth. Small Ruminant Research 23:103-108.

Semali A and Mathius I W 1984 Effects of Leucaena leucocephala supplementation on feed intake and digestions for sheep. Proceedings: Sheep and Goats in Indonesia, Bogor, Nov. 22-23. pp 8-11.

Tompkins N W, McMeniman N P and Daniel R C W 1991 Voluntary feed intake and digestibility by red deer (Cervus elaphus) and sheep (Ovis ovis) of pangola grass (Digitaria decumbens) with or without a supplement of leucaena (Leucaena leucocephala). Small Ruminant Research 5:337-345.

Received 15 March 1999

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