Cafeteria trials to determine relative preference of six desert trees and shrubs by sheep and goats

Abstract

Six fodder plants consisting of two leguminous trees, Acacia salicina and Acacia saligna, a leguminous shrub, Cassia sturtii and three halophytic shrubs, Atriplex canescens, Atriplex halimus and Atriplex nummularia were offered ad libitum in cafeteria trials to four fat-tailed Awassi sheep (41.1 ± 3.2 kg) and four local Negev goats (36.5 ± kg). Leguminous plants are characterized by high tannin contents and halophytes by high ash contents. The animals, aged 1.5–2.0 years, were randomly picked from a large free-grazing Bedouin flock of sheep and goats that had access to the fodder plants while grazing. They were penned for three weeks prior to measurements during which time they were provided with a maintenance diet. After a 12 h fast, the animals were offered the six feeds over a 15 min period each day for 5 consecutive days to determine (1) fodder selectivity by these small ruminants and (2) whether the ranking and proportionate feed intakes were similar between sheep and goats. Time-series analyses showed that the intakes and ranking of intakes did not change over the five days. Total DMIs per 15 min were similar in sheep (95.6 ± 10.1 g) and goats (92.6 ± 17.8 g) and feed selection in goats tended to be positively correlated with that of sheep (Mantel test: n = 15; r² = 0.48; P = 0.071). A. saligna was the most preferred feed in both small ruminants (by Jacobs' selectivity index) and the two Acacia species comprised more than 86% and 70% of dietary intakes in goats and sheep, respectively.

Introduction

Large tracts of desert and semi-desert lands of African and Middle Eastern countries cannot be cultivated but can be used to raise livestock, mainly sheep, goats and camels. Even in favourable rainfall years, animal performance can be poor due to a lack of adequate dietary protein and energy intake in the dry season. Furthermore, the herbaceous forage is fibrous and of low quality, and, in the event of drought, livestock mortality can be high (Le Houérou, 1980). In contrast, trees and shrubs remain green all year, even during droughts. (Topps, 1992). This has led to the examination of a large variety of trees and shrubs, in particular legumes and halophytes, as potential sources of fodder (Bhattacharya, 1989). Leaves/phylodes of leguminous trees and shrubs are rich in crude protein (CP) and can be particularly important in the dry, summer season when only poor quality roughage or forage and mature herbage are available. Halophytic shrubs, such as Atriplex spp., also can exist under extreme desert conditions, thriving on poor, saline soils (Pasiecznik et al., 1996). They have high CP concentrations and have been used as fodder under desert conditions (Kessler, 1990, Van Niekerk et al., 2004).

However, in spite of these positive attributes, trees and shrubs are usually not readily consumed by livestock, although relatively high intakes have been reported for Acacia saligna (Dumancic and Le Houérou, 1980) and Atriplex halimus (Valderrabano et al., 1996). In addition to high fibrous material, low intakes are primarily due to plant defense mechanisms, notably tannins (Robbins et al., 1987), which limit consumption by the grazing animal. Tannins form precipitates with proteins resulting in the formation of indigestible complexes (Murdiati et al., 1992). Halophytic shrubs contain high mineral concentrations, in particular sodium chloride, which limit their intakes (Alicacta et al., 2002) and Atriplex spp. also contain high concentrations of oxalate which can be toxic to livestock (Abu-Zanat et al., 2003). Consequently, it is generally accepted that foliage from trees or shrubs cannot serve as the sole source of feed for livestock but can serve as a valuable supplementary fodder, in particular during droughts (Dynes and Schlink, 2002, Shetaewi et al., 2001, Ventura et al., 2004).

In this study, we offered six tree and shrub fodders to sheep and goats in a cafeteria trial to determine (1) fodder selectivity by these small ruminants and (2) whether the ranking and proportionate feed intakes are similar between sheep and goats.