Print version ISSN 0375-1589
S. Afr. j. anim. sci. vol.39 no.5 Pretoria 2009
Abubeker Hassen#; W.A. van Niekerk; F.M. Bechaz
Centre for Nutrition, Department of Animal and Wildlife Sciences, University of Pretoria, Pretoria 0002, South Africa
The aim of this study was to compare two tropical grass species, Panicum maximum and Digitaria eriantha, in terms of silage fermentation attributes and certain rumen fermentation characteristics of silage made either at the boot or full bloom stages of growth. A lower silage pH was recorded for the D. eriantha than for the P. maximum silage. Neither species nor maturity stage had a significant effect on silage ammonia nitrogen, or lactic, acetic and butyric acid concentrations. For P. maximum silage total N was higher at full bloom than at the boot stage. D. eriantha had a higher total nitrogen content than P. maximum silage at the boot stage. Rumen pH was lower in sheep fed D. eriantha than P. maximum silage. In P. maximum fed sheep, a higher rumen NH3-N concentration was recorded when silage from the full bloom stage was fed compared to silage from the boot stage. In sheep fed D. eriantha silage, higher concentrations of acetic, propionic, butyric and total volatile fatty acids in the rumen were recorded from full bloom than from the boot stage. Neither species nor stage of maturity had significantly influenced the acetic : propionic acid ratio. The results suggested no significant difference between the species, but the full bloom stage showed a higher nutritive value and better preservation compared to the boot stage silage in both D. eriantha and P. maximum silage.
Keywords: Boot stage, Digitaria eriantha, full bloom stage, Panicum maximum, rumen fermentation
Panicum maximum (Guinea grass) and Digitaria eriantha (Smuts finger) are high yielding subtropical perennial grasses grazed by cattle and sheep in southern Africa. Excess forage can be removed for hay or silage making. Ensiling tropical grasses such as D. eriantha and P. maximum showed sub-optimal (often poor) initial silage fermentation (Mühlbach, 2000) due to either a low dry matter (DM) content, high buffer capacities, low level of suitable lactic acid bacteria and a low water-soluble carbohydrate (WSC) content, or a combination of these factors (Mühlbach, 2000). The objective of this study was to compare silage fermentation attributes and certain rumen fermentation characteristics of P. maximum and D. eriantha silages made either at the boot or full bloom stages of growth.
Materials and Methods
Panicum maximum and D. eriantha pastures were harvested from an area of 0.4 ha at two different stages (boot and full bloom) of growth. The harvested forage for each species were separately wilted to a dry matter concentration of 30% and ensiled with an addition of 8 kg sugar/ton DM in airtight plastic bags (3.1 m x 2.75 m). The silos were opened at day 120 and representative samples were analysed to determine the fermentative and nutritive value characteristics. Silage nitrogen (N) content was analyzed according to AOAC (2000).
Four rumen cannulated sheep were randomly allocated to one of the four silage diets for the rumen fermentation study. It was a 2 x 2 factorial treatment consisting of two grass species (P. maximum or D. eriantha) and two growth stages (boot or full bloom stage of growth) at harvest. The sheep were housed in individual metabolism cages and were fed ad libitum three times daily. The animals had free access to a 50 : 50 dicalcium phosphate/salt lick and fresh water was available.
Sampling of rumen fluid was done over four consecutive days at 09:00 and 21:00 on day 1; at 12:00 and 24:00 on day 2; at 15:00 and 03:00 on day 3; and at 18:00 and 06:00 on day 4. The rumen pH was measured immediately after sampling and the rumen fluid was stored in a freezer for later analysis. These rumen fluid samples were thawed and then filtered. The supernatant was used for the determination of rumen ammonia nitrogen (NH3-N) and volatile fatty acid (VFA) concentrations after separate samples from each treatment were preserved with 4 mL 0.5 M sulphuric acid or 2 mL of a 10% sodium-hydroxide solution, respectively. Volatile fatty acid and lactic acid concentrations were measured according to the methods of Payne & McDonald (1966) and Pryce (1969), respectively.
All parameters measured in the experiments were analyzed using Proc GLM of SAS (2001). The influence of grass species and growth stage was investigated and where the F ratio showed significance differences (P <0.05), the means were tested using the Bonferroni's test according to Samuels (1989).
Results and Discussion
The silage DM content differed significantly between the grass species (Table 1). At boot stage, P. maximum had a higher DM content than D. eriantha silage, whereas at full bloom stage P. maximum had a lower DM concentration than D. eriantha silage. The DM concentration of the forages prior to ensiling and at feeding plays a key role in determining the silage fermentation characteristics and intake by the animal (Ingvartsen, 1992). In general, the organic matter (OM) concentration of D. eriantha silage was higher than that of P. maximum silage. At boot stage the N concentration of D. eriantha silage was higher than that of P. maximum, but when compared at full bloom stage the species did not differ in terms of their N concentrations. The NH3-N concentrations were also not significantly different between the two species. In general, NH3-N values were within an acceptable range (<100 g NH3-N/kg N) for well-fermented silages (Stark & Wilkinson, 1988), and lower than values reported by Meeske et al. (1999) for D. eriantha (50.3 g N/kg) silage ensiled for 44 days. In general, D. eriantha silage had lower pH values than the P. maximum silage. However, the pH values for grass silage were closer or less than 5, and according to McDonald et al. (1991), for wilted silages, a stable silage can be achieved at pH values of 5 and above due to the higher osmotic pressure associated with wilting that probably inhibits clostridial growth. In contrast, Marsh (1979) reported lower pH values for wilted silages. Accordingly, except for P. maximum silage at full bloom stage, the pH values of the four silages were within an acceptable range as recommended by the same authors. However, the best indicator of silage quality is the acid profile of the silage (Bethard, 2006). In the present study the silage from the two grass species did not differ (P >0.05) in terms of lactic, acetic and butyric acid concentrations.
In general, the rumen pH value of sheep fed D. eriantha silage was lower than that of sheep fed the P. maximum silage (Table 2). The rumen pH followed a similar pattern to that of silage pH. For all silage diets, however, the levels of rumen pH recorded were within the optimal pH recommendation ranges for high proteolytic (pH 6 to 7) and cellulolytic (pH 6.2 to 6.8) activities. The rumen ammonia, acetic acid, propionic acid and total volatile fatty acid concentrations did not differ (P >0.05) between the two grass species. However, sheep fed D. eriantha silage had a higher (P <0.05) rumen butyric acid concentration than P. maximum silage when compared at the full bloom stage. A similar pattern was recorded for silage pH, though the difference was not statistically significant. Higher butyric acid in silage is an indicator of undesirable fermentation and/or secondary fermentation (Schroeder, 2004). The rumen NH3-N values recorded for sheep fed on both tropical grass silages were sufficient to support maximum microbial activity in the rumen. According to Satter & Roffler (1975) values lower than 2.5 - 5 mg/100 mL rumen fluid may inhibit rumen activity, while Ørskov (1982) suggested a rumen NH3-N concentration of 20 - 24 mg per 100 g rumen fluid for maximum fermentation rate.
For D. eriantha silage, the DM concentration of the silage made at full bloom stage of harvest was higher than that made at boot stage (Table 1). In contrast, Van Niekerk et al. (2008) reported no difference in terms of DM% when ensiled in a 1 kg mini-silo. For D. eriantha, growth stage had no effect on the N concentration of the silage, while for P. maximum the N concentration of silages at full bloom stage was higher than for the boot stage silage. This was not expected and the reason for this is not clear, taking into consideration a decline in N concentration with advanced maturity (Relling et al., 2001). In general, growth stage had no significant effect on silage pH, and NH3-N, acetic, lactic and butyric acid concentrations of silage.
Growth stage had also no significant effect on the rumen pH of sheep fed on the two grass silages (Table 2). However, harvesting at full bloom stage resulted in a higher NH3-N concentration of sheep fed P. maximum silage compared to silages made from the same species at boot stage. The NH3-N concentration recorded for boot stage silage does not support maximal fermentation rate (Ørskov, 1982). In P. maximum silage, the observed higher N concentration recorded at full bloom stage and the corresponding higher rumen NH3-N concentration of sheep on this diet confirmed earlier results (Meissner et al., 1993; Relling et al., 2001) that showed a positive correlation between N intake and rumen NH3-N concentration. In D. eriantha silage, however, harvesting at full bloom stage resulted in higher rumen acetic, propionic, butyric and total volatile fatty acid concentrations than silages made at the boot stage. This is in accordance with Van Niekerk et al. (2008). Except for D. eriantha at boot stage, the values recorded for total VFA concentration are within the range reported for mature P. maximum pasture (Relling et al., 2001). Growth stage had no effect on the acetic acid to propionic acid ratio.
There were no significant differences between the important qualitative silage characteristics of P. maximum and D. eriantha silages. The higher nutritive value of the full bloom stage compared to the boot stage of D. eriantha silage was due to an improved VFA production in the rumen. Similarly, a higher rumen NH3-N concentration was recorded for P. maximum harvested at the full bloom stage than at the boot stage.
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