Influence of Regrowth Interval on Chemical Composition , Herbage Yield , Digestibility and Digestion Kinetics of Setaria sphacelata and Cenchrus ciliaris in Buffaloes

This study examined the influence of re-growth periods on chemical composition, biomass production, nutritive value and digestion kinetics of Setaria sphacelata (SS) and Cenchrus ciliaris (CC) in ruminally cannulated buffalo bulls. Two re-growth intervals i.e. clipping every month (CI1) and clipping after every two months (CI2) were compared with the control (clipped after 4 months). Mean values of leaf to stem ratio in SS and CC grasses were decreased (p<0.05) with increasing re-growth interval. The lowest leaf to stem ratio was recorded in control plots of both grasses. In both grasses, increasing growth period increased the concentrations of dry matter (DM), neutral detergent fiber (NDF) and organic matter (OM) and decreased crude protein (CP). Mean dry herbage, OM and CP yields of SS and CC were increased (p<0.05) with increasing re-growth interval. Ruminal DM and NDF digestibilities of SS and CC were decreased (p<0.05) with increasing interval. Ruminal rate of DM and NDF disappearance was higher while the ruminal lag time of these nutrients was lower with monthly than with bi-monthly clipping interval. The results from present study imply that SS and CC clipped after every two months is more beneficial than when clipped every month or every four months in terms of optimal biomass with adequate nutritional value for buffaloes. (Asian-Aust. J. Anim. Sci. 2006. Vol 19, No. 3 : 381-385)


INTRODUCTION
Efficient defoliation regimens for range grasses are prerequisite for the sustainability of grass lands and animal agriculture (Sarwar et al., 2006).Setaria sphacelata (SS) is native to northern Rhodesia (Zambia) and has been developed for grazing and hay production in tropical regions.Plants of this grass species attain a height of 1 m at flowering.It is palatable and drought-resistant grass species and is better suited to the more shallow soils and lower rainfall situations.Cenchrus ciliaris (CC) is another aggressive perennial grass of arid tropical habitats around the globe.It is preferred in tropical range lands because of its better forage nutritive value for ruminants (Duke, 1983;Evitayan et al., 2004) and its ability to recover from grazing due to rapid formation of dense monocultures.
Grass clipped at optimum intervals had shown to increase herbage yields of grass species (Masters et al., 2005 andSvejcar andRittenhouse, 1982).The concentrations of neutral detergent fibre (NDF) and acid detergent fibre (ADF) were increased in CC with increasing maturity (Gupta and Sagar, 1987;Sarwar et al., 2006).
Frequent clipping had provided ruminants with higher levels of digestible nutrients (Duke, 1983).However, it produced a cumulative destructive effect on forage yield because of rapid exhaustion of carbohydrate reserves in the stubbles and roots.Frequent clipping of crested wheatgrass and of rough fescue (McLean and Wikeem, 1985;Sarwar et al., 2006) resulted in low nutrient yields and had reduced the plant survival during summer season.Too frequent defoliation of range grasses resulted in weak plants with more susceptibility to drought, heat, cold, injury and prevalent diseases (Svejcar and Rittenhouse, 1982).This necessitates determination of an appropriate clipping regimen of range grasses.
Setaria sphacelata and CC grasses are preferred in tropical grass lands because of their better survivability and higher nutritive value for various livestock species (Elizalde et al., 1999).However, scientific evidence regarding the influence of re-growth periods on chemical composition, herbage yield, digestibility and digestion kinetics of SS and CC is limited.Thus, this study was conducted to determine the influence of re-growth periods on chemical composition, biomass production, nutritive value and digestion kinetics of SS and CC in ruminally cannulated buffalo bulls.
2004.Nursery of SS and CC grasses were separately raised through planting tuft splits in 1×3 m plots at 0.3×0.3m spacing to maintain optimum plant density of 5 to 10 plants/m2 (Butt and Ahmad, 1994).The soil was sandy loam and no fertilizer was applied during the experiment.
Two re-growth intervals i.e. clipping every month (CI 1 ) and clipping after every two months (CI 2 ) were compared with control (plots that were harvested after 4 months).Grass biomass was manually clipped with sickle at 5 cm stubble height.Leaf to stem ratio, dry matter (DM) and fresh biomass yield were determined at each defoliation date.On each clipping date, leaf to stem ratio were recorded.To determine leaf to stem ratio at each harvest, a sample (about 500 g) was removed from the innermost two rows of each sub-plot cut at a height of 5 cm.Tillers from this nonweed sample were divided into leaf blades and stem plus sheath fractions immediately after removal from the plot.The leaf and stem fractions were dried separately at 55°C to a constant weight.Leaf to stem ratio was calculated from the dry weights (Baron et al., 2000).Grass samples were chopped in a locally manufactured chopper and then were ground through a Wiley mill (2 mm screen) and preserved in plastic bags for chemical analysis.The samples of both grasses were analyzed for DM, crude protein (CP) and total ash by using AOAC (1990) method, NDF by method of Van Soest et al. (1991).The chemical composition and nutrient yield values of cutting interval after one and two months were averaged for 4 and 2 time-harvesting, respectively.

In situ digestion kinetics
Nylon bags measuring 10×23 cm, with an average pore size of 50 μm were used to determine ruminal digestibility, rate of disappearance, extent of digestion and lag time of DM and NDF.The in situ bags each containing 10 g sample DM (Setaria sphacelata or Cenchrus ciliaris) were incubated in the rumen for 1,3,6,12,24,36,48 and 96 h in triplicate (Sarwar et al., 1995).The bags were closed and tied with nylon fishing line.After removal from the rumen, bags were washed in running tap water until the rinse was clear.The bags were then dried in a forced air oven at 55°C for 48 h.After equilibration, the bags were weighed back and the residues were transferred to 100-ml beakers for analysis as described previously.Digestibility of DM and NDF were calculated at 48 h of ruminal incubation.Rate of disappearance, lag time and extent of digestion of DM and NDF were determined by the methods described by Sarwar et al. (2004).The digestibility and digestion kinetics parameters of cutting interval after one and two months were averaged for 4 and 2 time-harvesting, respectively.

Statistical analysis
The data collected on different parameters were statistically analyzed by SAS (1988)  Duncan's multiple range test (Steel and Torrie, 1981).

RESULTS
Mean daily minimum temperature ranged from 15 to 31°C while corresponding maximum temperature was 32 to 48°C.Soil was sandy loam to loam.
Chemical composition, yield and leaf to stem ratio of SS and CC grasses are given in Table 1.Mean values of leaf to stem ratio in SS and CC grasses were decreased (p<0.05) with increasing re-growth interval.The lowest leaf to stem ratio was recorded in control plots (clipped at 4 month) of both grasses.
In both grasses, concentrations of DM, NDF and OM increased and CP decreased with increasing re-growth period.The DM, NDF and OM contents were the maximum in control plots.Mean dry herbage and CP yields of SS and CC were increased (p<0.05) with increasing re-growth interval.
Ruminal DM and NDF digestibilities of SS (Table 2) and CC (Table 3) were decreased (p<0.05) with increasing interval.Ruminal extent of DM and NDF digestion (calculated at 96 h ruminal incubation) were reduced in both SS and CC grasses with increasing re-growth interval.Ruminal rate of DM and NDF disappearance in both SS and CC grasses were higher while the lag time was lower with monthly than with bi-monthly clipping interval (Tables 2  and 3).

DISCUSSION
Decreased leaf to stem ratio in SS and CC with increasing re-growth interval was because of increased plant age, advanced synthesis (Cuomo et al., 1996;Nguyen1 et al., 2004) and thus higher cell wall contents.Increased tiller density with increasing plant maturity of switch grass (Madakadze et al., 1999) had reduced the leaf to stem ratio.Crowder and Chheda (1982) reported that a short vegetative period of 4 to 6 weeks in Cenchrus ciliaris, followed by stem elongation and floral parts, resulted in decreased leaf to stem ratio of this grass with maturity.Similarly, Chaparro and Sollenberger (1997) reported decreased leaf to stem ratio with frequent defoliations in Elephant grass.
Higher DM and OM of SS and CC grasses with increasing re-growth period may be because of reduced leaf to stem ratio with increasing clipping interval of Elephant grass (Chaparro and Sollenberger, 1997).Crowder and Chheda (1982) reported that with increasing clipping interval, older leaves contained higher cell wall constituents and thus, reduced intercellular spaces as well as condensed cellular inclusions.The midribs and leaf sheaths of grasses attained a greater percentage of fibre and lignin with increasing plant age (Chaparro and Sollenberger, 1997;Islam et al., 2004), older leaves senesced and lost water (Mero and Uden, 1998), stems elongated and were less succulent, resulting into increased DM contents (Fraser et al., 2001).Similar to present study, decreased CP concentrations with increasing maturity of CC (Mero and Uden, 1998) and Brassica oleracea (Fraser et al., 2001) were attributed to decrease leaf to stem ratio and accumulation of more cell wall constituents in mature grasses.Though in the present study, CP concentration decreased with increasing re-growth period, higher biomass yields with increasing re-growth interval of both grasses resulted in increased total protein yields.
Increased herbage and CP yields in SS and CC grasses with higher re-growth periods may be attributed to the increased basal area (Motazedian and Sharrow, 1986), number of tillers per plant (Adjei et al., 1989; Sarwar et al.,  2006), more leaf elongation and stem development (Pitman and Holt, 1983;Nisa et al., 2005) with increasing grass  maturity.Pitman and Holt (1983) observed increased DM production in kleingrass (Panicum coloratum L.), green sprangletop (Leptochloa dubia) and plains bristle grass (Setaria macrostachya) with 8-week clipping frequency than with 2 and 4-week harvest frequencies.In present study, low herbage yield at shorter re-growth interval may also be attributed to the reduction in photosynthetic area to the extent that photosynthate was not adequately available for re-growth after clipping (Geber, 2002;Sarwar et al., 2005).Similarly, Mero and Uden, (1998) reported that grass DM and OM yields were more sensitive to the interval between defoliations than to the severity of defoliation.Decreased ruminal DM and NDF digestibilities of SS and CC grasses with increasing re-growth interval may be because of increased fibre concentration in plant tissues, higher lignification and reduced leaf to stem ratio.Shorter ruminal lag time and faster rate of disappearance of DM and NDF in both grasses with monthly than with bi-monthly clipping might have resulted in higher digestibilities with more frequent clipping.Increased ruminal lag time with increasing clipping interval was because of more lignification (Mero and Uden, 1998) and less cell soluble material of the grasses at advanced maturity (Terrill et al., 2003;Sarwar et al., 2006).

CONCLUSIONS
The results from present study imply that SS and CC clipped after every two 2 month is beneficial than those clipped at every month and at every four month to get optimum biomass with adequate nutritional value for buffaloes and to sustained grass vigor.

Table 1 .
Influence of clipping intervals 1 on leaf to stem ratio, plant height, nutrient composition and yields of Cenchrus ciliaris and Means within a row bearing different superscripts differ significantly (p<0.05).

Table 2 .
Influence of clipping interval 1 on nutrient digestibility and digestion kinetics in Setaria sphacelata

Table 3 .
Influence of clipping interval 1 on nutrient digestibility and digestion kinetics of Cenchrus ciliaris