Comparison of in vitro digestibility of feedstuffs using rumen inoculum from sheep or red deer

The Ankom fermentation system was used to investigate the effect of rumen fl uid donor (sheep vs red deer) on in vitro digestibility of twelve substrates (four forages, four cereal grains and four shrubs). Dry matter and neutral-detergent fi bre disappearances and in vitro true digestibility showed some signifi cant differences related to the species of the rumen fl uid donor, suggesting that although sheep could be used as a model to estimate in vitro digestibility of some conventional feeds (e.g., forages and cereal grains), important differences may be found between sheep and red deer when low-quality feeds (e.g., shrubs) are evaluated.


INTRODUCTION
Although it is known that the ultimate arbitrator of the nutritive value of any feed is the host animal, the sheep has been widely used as a model for studying the digestive functions of other ruminants.However, the development of the deer farming industry has created a demand for accurate rationing systems, especially for deer, that permit a more effi cient use of the available feed resources.
Most approaches to feed characterization are intended to meet the needs of rationing systems and, because digestibility is the principal cause of variation of the metabolizable energy content, the energy value of feeds is commonly predicted from in vitro estimates of digestibility.Over other in vitro techniques, the Ankom fermentation system presents the advantages of reducing labour costs and being able to analyse a high number of samples at once.
With the increased use of in vitro techniques to evaluate ruminant feedingstuffs, it is of great importance to identify whether the species from which the rumen fl uid inoculum is obtained has a signifi cant infl uence on the results obtained.This trial was carried out to determine the infl uence of rumen fl uid donor (sheep vs red deer) on in vitro digestibility of several substrates.

MATERIAL AND METHODS
Four forages (lucerne hay, barley straw, maize silage and beet pulp), four cereal grains (barley, maize, sorghum and wheat) and four shrubs (Calluna vulgaris, Erica australis, Cytisus cantabricus and Genista occidentalis) were selected as substrates to cover a wide range of chemical compositions (see Table 1).Four Merino sheep (48±2.1 kg; Ovis aries) fi tted with ruminal cannulae and four red deer (76±1.4kg; Cervus elaphus) belonging to the fl ocks of the CSIC (León, Spain) were used.Animals were fed lucerne hay (20 g kg LW -1 day -1 ) and barley grain (3 g kg LW -1 day -1 ), for a 15-day period before collecting the rumen fl uid for the in vitro incubations.
Incubations were carried out using the Ankom in vitro fermentation system Daisy II (Ankom Technology, Macedon, NY, USA).Two rumen inocula were obtained from each species (through ruminal cannula from sheep and immediately after slaughter from deer).Rumen fl uids were collected into pre-warmed thermo fl asks, taken immediately to the laboratory, stained through four layers of cheesecloth and diluted into the reduced buffer medium (Goering and Van Soest, 1970) in the proportion 1:4 (v/v).Ground feed samples (≈500 mg milled at 1 mm) were weighed into Ankom fi lter bags (F57, Ankom Technology, Macedon, NY, USA), heat sealed and placed in the incubation jars.Each jar was fi lled with 24 bags (2 bags per substrate) and 2 L of buffered rumen fl uid dispensed under CO 2 and then deposited in the rotating incubator at 39ºC.After 48 h, the bags were gently rinsed in cold water, dried and weighed to determinate dry matter disappearance (DMD).They were then treated with a boiling neutral detergent solution for 1 h using an Ankom 200 fi bre analyser (Ankom Technology, Macedon, NY, USA), washed with distilled water, dried and weighed to determine both neutral-detergent fi bre disappearance (NDFD) and in vitro true digestibility (ivTD).
Procedures described by AOAC (1995) were used to determine dry matter (DM), ash and Kjeldahl nitrogen.Neutral-detergent fi bre (NDF), acid-detergent fi bre (ADF) and acid-detergent lignin (ADL) were determined by the methods of Goering and Van Soest (1970) and Van Soest et al. (1991).
One-way analysis of variance using the general linear model (GLM) procedure of the SAS package (SAS, 1999) was utilized to compare the differences between animal species (sheep and red deer) for each substrate.

RESULTS AND DISCUSSION
Values of DMD, NDFD and ivTD (Table 2) showed not only the expected variation depending on the substrate incubated but also some signifi cant differences related to the species of the rumen inoculum donor (sheep vs deer).HERVAS G. ET AL.
DM disappearance was higher (P<0.05) for three forages (lucerne hay, barley straw and maize silage) and for Calluna vulgaris, and lower (P<0.05)for Cytisus cantabricus when incubated with rumen inoculum obtained from sheep.Statistical differences between ruminant species showed the same behaviour for values of NDFD and ivTD.Thus, these were greater (P<0.05) in sheep for the same three forages above mentioned and for Calluna vulgaris.Deer, however, seemed to be better able to digest barley and wheat grains and Erica australis and Cytisus cantabricus (P<0.05).
As proportionate increases, the highest differences were observed for the disappearance of NDF and for incubations of low-quality feedstuffs (shrub species).On the other hand, differences between sheep and deer for ivTD of forages and cereal grains were never higher than 4.2%.
The diffi culty of generalizing about the digestion capacity of sheep and deer imposed by the interaction between animal species and feed ingredients has been pointed out in the literature (Ru et al., 2002).Comparisons of digestion by sheep and deer have resulted in differences in favour of the sheep, in favour of the deer, or even no differences depending on the feedstuff studied (Milne et al., 1978;Domingue et al., 1991;Gordon et al., 2002).

CONCLUSIONS
These results suggest that sheep could be used as a model to estimate in vitro digestibility, using the Ankom fermentation system, of conventional feeds (e.g., forages and cereal grains) for rationing for deer.However, important differences may be found between sheep and red deer when non-conventional low-quality feeds (e.g., shrubs) are evaluated.

Table 2 .
Dry matter (DMD; g/kg) and neutral-detergent fi bre (NDFD; g/kg) disappearances and in vitro true digestibility (ivTD; g/kg) of different substrates incubated with rumen inoculum from sheep or red deer