Feeding Acacia saligna to sheep and goats with or without the addition of urea or polyethylene glycol.(Report)

ABSTRACT : The objective of the research was to investigate the effect of polyethylene glycol (PEG) or urea supplementation in sheep and goats fed a basal diet of Acacia saligna and wheat straw. The 3 dietary treatments were: (1) Control: ad libitum A. saligna+ 400 g/d wheat straw (95% DM) (basal diet); (2) Basal diet+50 g/d PEG 4000; and (3) Basal diet+1% (on a DM basis) urea sprayed onto the straw and A. saligna 30 min prior to feeding. All animals maintained live weight, regardless of the dietary treatment. All sheep readily consumed the A. saligna in preference to straw. In sheep both DMD and OMD were higher (p<0.05) where PEG was included in the diet compared to the other two treatments. Contrary to findings by other researchers there was no significant difference in DMI, DMD or OMD between sheep and goats in corresponding treatment groups. All animals were in positive N balance. For both sheep and goats, rumen ammonia concentrations were increased with the use of either urea or PEG. In these groups the maximum ammonia concentrations exceeded 50 mg/L, considered the minimum required to maximise microbial growth. This threshold, however, was exceeded only for a period of 8-11 h. Of those measured, rumen ammonia levels were generally the highest at 4 h post feeding. None of the measurements of rumen ammonia for the control group approached 50 mg/L. It is unclear how and why feed intake and live weight were maintained when rumen ammonia levels were often sub-optimal. (Key Words : Acacia saligna, Sheep, Goats, Urea, PEG)

INTRODUCTION

As part of the battle against land degradation/ desertification there is an urgent need to develop sustainable grazing systems. Acacia saligna, a native to Western Australia has been widely acknowledged as a useful species for land conservation. More recently, there has been a focus on A. saligna as a potential source of fodder for ruminants, particularly in semi-arid/arid regions. It is during the traditional summer/autumn feed deficit that occurs in Western Australia that A. saligna grows best, thus providing a source of green feed to grazing animals.

The common conclusion drawn by researchers, however, is that A. saligna is inadequate as the ruminant's sole source of nutrients. This is largely attributed to its condensed tannin (CT) content that has been shown to have an inverse relationship with voluntary intake, digestibility and nitrogen (N) retention in ruminants. Previous studies undertaken by the researchers have shown that feeding A. saligna without supplementation has detrimental effects on rumen metabolism in sheep. The aim of this study was to determine whether the addition of either PEG or supplemental N in the form of urea could improve the nutritive value of A. saligna to maintain a minimum of animal maintenance. Goats were also included in this trial for the purpose of comparing their responses with those of sheep. Some researchers (Degen et al., 1995; Silanikove et al., 1997) have indicated that goats are better able than sheep to tolerate high tannin feeds.

MATERIALS AND METHODS

Experimental design

The experiment was based on a Latin square design, involving 3 dietary treatments and 2 animal species (sheep and goats). Six merino wethers and 6 mature Boer cross wether goats, each fitted with a permanent rumen cannula, were used. Within each species the animals were randomly allocated to 1 of 3 dietary treatments. Each experimental period was of 28 d duration, made up of 20 d for diet adaptation followed by 1 d of sampling of ruminal fluid and then 7 d of recording of feed intake and faecal and urinary output. Live weight of the animals was recorded at the start and end of each experimental period. On day 1 of each sampling period, approximately 1 kg (fresh weight) of leaves was randomly collected from each 20 kg batch for chemical analyses.

Diets

The A. saligna was sourced north east of Perth, Western Australia. The climate of the area is described as Mediterranean with an average annual rainfall of 622 mm. The soil in which the A. saligna was growing may be described as sandy gravel (Moore, 2001). Branches were cut from mature trees (5-6 year old) and then fed through a mechanical leaf stripper (McMeniman, 1975). The A. saligna offered to the sheep and goats consisted of leaves (mostly whole) and small twigs. After harvesting, material was stored at -18[degrees]C pending feeding.

The 3 dietary treatments were: (1) Control: ad libitum A. saligna+400 g/d wheat straw (95% DM) (basal diet); (2) Basal diet+50 g/d PEG 4000; and (3) Basal diet+1% (on a DM basis) urea sprayed onto the straw and A. saligna 30 min prior to feeding. Wheat straw was included in the basal diet to reflect a paddock situation in which plantations of A. saligna are interspersed with dry pasture during the summer/autumn period of Western Australia.

The quantity of A. saligna to be treated with urea was weighed and then spread out on a clean tarpaulin on the floor. The urea was dissolved in water (1:20 w/v) and then …