Effect of regrowth age on intake and digestion
of
Digitaria decumbens
consumed by
Black-belly sheep
H. ArchimeÁde
a,*, M. Boval
a, G. Alexandre
a,
A. XandeÂ
a, G. Aumont
a, C. Poncet
baInstitut National de la Recherche Agronomique, Unite de Recherches Zootechniques,
Prise d'eau, 97170 Petit-bourg, Guadeloupe
bInstitut National de la Recherche Agronomique, Station de Recherches sur la Nutrition des Herbivores,
Centre de Clermont-Ferrand-Theix, Theix, 63122 Saint-Genes-Champanelle, France
Received 17 February 1999; received in revised form 9 November 1999; accepted 29 August 2000
Abstract
The intake and digestion of freshDigitaria decumbensgrass were studied following the stage of regrowth. Six rams (mean liveweight: 40:80:6 kg) received successively a 14, 28, 42 and 56-day old forage during four 4-week periods. The range of variation of crude protein and acid detergent ®bre content (g/kg) of the forages was 57±130 and 380±442, respectively. The DM intake (g/ kg W0.75) and the organic matter total tract digestibility decreased from 83 to 56 and from 0.728 to 0.628, respectively, between 14 and 56 days of regrowth. The fractional degradation rate (hÿ1) of dry matter in the rumen, estimated by the nylon bag method, decreased curvilinearly with a mean daily rate of 0.0010. A curvilinear relationship was recorded between the rumen turnover rate and forage regrowth stage. The mean daily decrease (per hour per day) was 0.0005. The total nitrogen duodenal ¯ow (g per day) decreased from 22.7 to 11.6 between 14 and 56 days. The mean ef®ciency of microbial protein synthesis was similar with the four diets (31.8, S.E. 2.2 g microbial nitrogen/kg organic matter apparently digested in the rumen). In conclusion, intake, digestibility and duodenal nitrogen ¯ow are high with the 14-dayD. decumbens. As a consequence, the nutritive value of the latter is similar to the one of a good temperate grass forage. Good nutritive value of a 14-day oldD. decumbensand fast maturation and ligni®cation in C4forage before the ®rst month of
regrowth suggest the need to investigate ruminant feeding strategies with forages younger (<28 days) than those classically used when the aim is the increase in animal individual performances in humid tropical area.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Intake; Digestion;Digitaria decumbens; Sheep 87 (2000) 153±162
*Corresponding author. Tel.:590-255933; fax:590-255936. E-mail address: [email protected] (H. ArchimeÁde).
1. Introduction
Fresh forage is often the sole diet of ruminants in the humid tropics. Digestible organic matter intake from tropical grass forage is the main factor of variation of their nutritive value (Aumont et al., 1995). Intake and digestibility of tropical grass (C4 grass) are
relatively low at early stage of growth (28 days) compared to temperate grass (C3)
(Minson, 1990) due to differences in grass physiology with an early maturity for C4grass
(Wilson, 1994). Consequently, compared to temperate area, low animal performance is observed when ruminants are fed relatively young (28 days) tropical forage grass (Humphreys, 1991). In humid tropical areas, an original ruminant feeding strategy is necessary to increase forage conversion in animal protein. Several questions are open. One of them concerns the pertinence of feeding ruminants with tropical forages younger (<28 days) than those classically used. The experiment was designed to evaluate variations in nutritive value of tropical forage grass with its stage of regrowth, within bounds larger than those classically studied.
2. Materials and methods
2.1. Location
The research was carried out in 1996 at the experimental animal station of the National Agronomic Research Institute (INRA) of the French West Indies (Guadeloupe, latitude 168160N, longitude 618300W). Temperatures ranged in average from 21±258C to 27± 318C. The mean rainfall on the experimental site is 3000 mm yrÿ1. The rainfalls were regular during the experiment.
2.2. Experimental design, animals, diets and feeding
The harvest of a perennialDigitaria decumbens(pangola) pasture, divided in plots and subplots, has been planned in order to have, during four successive 29-day periods, 14 days, 28 days, 42 days and 56 days pangola regrowth age. The plots P14, P28, P42 and P56 were used during the period 1, 2, 3 and 4, respectively. The plots P14, P28, P42 and P56 were subdivided in 15, 30, 30 and 30 subplots, respectively. The ®rst subplot of P14, P28, P42 and P56 has been cut 15, 29, 43 and 57 days, respectively, before the beginning of the experimental period 1, 2, 3 and 4. One subplot has been cut per day. One kg/ha/ regrowth age of mineral nitrogen fertiliser was applied on each subplot of the same day. Areas of the subplots were 600, 400, 300 and 300 m2 for P14, P28, P42 and P56, respectively. Each subplot of P14 has been cut two times during the period 1. At the beginning of period 1, 2, 3 and 4, the grass of the ®rst subplot of P14, P28, P42 and P56 has 14, 28, 42 and 56 days of regrowth, respectively. Inside of a plot, the grass on the subplotn have a days of regrowth less than the one on the subplotnÿ1. Consequently, during the experimental periods 1, 2, 3 and 4, the regrowth age of the grass harvested every day was exactly 14, 28, 42 and 56 days, respectively. All the experimental period
took place during the intermediary season, where no signi®cant variation has been registered for rain and temperature.
Six Black-belly rams (mean liveweight: 40:80:6 kg) were used in this experiment.
They were ®tted with rumen and duodenal cannulae and maintained in metabolism cages. The rams received each day, during four successive experimental periods of 29 days, a 14, 28, 42 and 56-day old fresh regrowth of D. Decumbens (pangola) fertilised (1 kg nitrogen/ha per day of regrowth). The 29-day experimental periods consisted of 14 days of adaptation to the diet, 5 days of intake and total tract digestibility measurements, 3 days of duodenal sampling, 7 days of rumen sampling and nylon bag incubation. Each experimental period ended with a rumen emptying. The pangola grass was cut daily early in the morning and chopped (5 cm length) before being offered. The amount of forage provided was 1.15 times greater than the animal voluntary intake estimated during periods of adaptation.
2.3. Measurements
2.4. Chemical analyses
DM content of fresh forage, refusal were determined every day by drying at constant weight at 608C in a forced-draught oven. DM content of faeces was determined in similar condition with a representative sub-sample. This last one came from a sample obtained by pooling for each animal 10% of the daily amount of faeces excreted. It was then stored at ÿ208C. Samples were then ground (1 mm) prior to chemical analysis. The organic matter (OM) content was measured after a 10 h ashing at 5508C. Neutral detergent ®bre (NDF), acid detergent ®bre (ADF) and acid detergent lignin (ADL) were estimated following the methods of Van Soest et al. (1991). Nitrogen concentration was determined on forage and fresh frozen faeces using the Kjeldahl method. OM, NDF, ADF, ADL and nitrogen content of forage, refusals, faeces, duodenal digesta and ruminal digesta were expressed on laboratory DM basis obtained by drying the corresponding samples during 24 h at 1038C. Microbial nitrogen was estimated from the purine bases to total nitrogen ratio in reconstituted duodenal digesta and rumen bacteria. Purine bases were analysed according to Zinn and Owens (1986). The ef®ciency of microbial synthesis was computed as follows : (daily duodenal microbial nitrogen ¯ow (g per day))/(amount of OM apparently digested in the rumen (kg per day)). The ammonia contents were estimated on the rumen liquor by distillation. The Cr content of faeces and duodenal samples (DG, PR) was determined by atomic absorption spectrometry after dry-ashing and extraction of marker according to Siddons et al. (1985) but with reagents (acid mixture and permanganate solution) diluted one-fourth (LalleÁs, 1988).
2.5. Statistical analyses
Data were analysed using the General Linear Model procedure of SAS (1987) including the forage (d.f. 3) and animal (d.f. 5) effects. Moreover, another model including forage, forageforage and forageforageforage was performed to analyse the response trends (linear or quadratic) and quantify the daily variations of the studied parameters.
3. Results
3.1. Diet composition
The composition ofD. decumbensis reported in Table 1. The main difference appeared in the crude protein content when the grass maturity increased. The decease of crude protein content between 14 and 28 days of regrowth represented 70% of the general decrease observed between 14 and 56 days.
3.2. Intake
DM intake decreased from 83.1 to 55.9 (S.E. 2.3) g/kg W0.75(Table 2) with the age of the forage regrowth at an average daily rate of 0.658 (S.E. 0.07) g/kg W0.75. The
digestible organic matter intake (DOMI) decreased from 53.1 to 30.9 (S.E. 1.5) g/ kg W0.75with a mean daily decrease of 0.524 (S.E. 0.061).
3.3. Total tract digestion
The OM, NDF, ADF and CP total tract digestibility of diets are reported in Table 2. The OM digestibility decreased curvilinearly with age. Seventy-one percent of the total decrease occurred between 14 and 28 days with the corresponding values for NDF and ADF of 75 and 69%.
3.4. Rumen digestion
The potential degradable DM (pdDM), the fractional degradation rate (hÿ1) and the effective degradation are reported in Table 3. The pdDM decreased curvilinearly with the regrowth age amounting the difference between 14 and 28 days to 49% of the total decrease between 14 and 56 days.
Table 1
Chemical composition (g/kg DM) of a 14, 24, 42 and 56-day old fertilisedD. decumbensgrass harvested during the intermediary season
Item Regrowth age (days)
14 28 42 56
Organic matter 840 887 896 879
Crude protein 130 79 72 57
Neutral detergent ®bre 740 777 790 790 Acid detergent ®bre 380 429 442 441 Acid detergent lignin 71 74 78 78
Table 2
Intake and total tract digestibility of a 14, 24, 42 and 56-day oldD. decumbensgrass given ad libitum to six Black-belly rams
Itema Age of regrowth (days)
14 28 42 56 S.E. Dry matter intake (g per day/LW0.75) 83.1 73.8 62.7 55.9 2.3 Digestible organic matter intake (g per day/LW0.75) 53.1 42.8 35.9 30.9 1.5 Total tract digestibility
Organic matter 0.728 0.657 0.645 0.628 0.009 Crude protein 0.670 0.442 0.469 0.324 0.012 Neutral detergent ®bre 0.731 0.642 0.655 0.613 0.012 Acid detergent fibre 0.787 0.662 0.659 0.605 0.012
The mean daily decrease of the fractional degradation rate (per hour per day) was 0.0010 (S.E. 0.0002). Nevertheless, 44% of the decrease was observed between 14 and 28 days. A relative stability was recorded for the DM effective degradability.
OM, NDF and ADF rumen digestibilities are reported in Table 4. The ADF rumen digestibility (ADFrd) decreased curvilinearly with the regrowth stage ofD. decumbens
grass. Fifty-four percent of the total decrease of the ADFrd between 14 and 56 days was recorded before 28 days of regrowth. Trend observed with NDF was similar to that of ADF.
The ruminal turnover rate (rtr, hÿ1) estimated as the lignin passage rate decreased from 0.044 to 0.023 (S.E. 002). Mean daily decrease of ruminal turnover rate (hÿ1) was 0.0005 (0.0001) hÿ1, whereas the values 0.0003, 0.0005 and 0.0007 were estimated between 14 and 28, 28 and 42 and 42 and 56 days, respectively. Nineteen, 33 and 48% of the decrease
Table 3
Degradation rate parameters and effective dry matter (DM) degradability of 14, 28, 42 and 56-day oldD. decumbensgrass incubated in nylon bags in the rumen of six rams fed a similar grass
Itema Age of regrowth (days)
14 28 42 56 S.E.
Rapidly degradable fraction (a) 0.009 0.155 0.209 0.217 0.008 Slowly degradable fraction (b) 0.774 0.575 0.482 0.438 0.006 Potential degradable fraction ab 0.783 0.730 0.691 0.655 0.006 Fractional degradation rate (c) 0.073 0.054 0.045 0.030 0.003 Effective DM degradabilityb 0.492 0.487 0.491 0.467 0.010
aThe modelyab 1ÿectof érskov and McDonald (1979) was used.
bRuminal turnover estimates in this experiment was used in this calculation.
Table 4
Rumen digestibility, rumen lignin passage rate, duodenal nitrogen ¯ow and microbial synthesis in the rumen of a 14, 28, 42 and 56-day oldD. decumbensgrass fed ad libitum to six Black-belly rams
Item Age of regrowth (days)
14 28 42 56 S.E.
Rumen digestibility
Organic matter 0.522 0.569 0.442 0.441 0.019 Neutral detergent ®bre 0.622 0.555 0.524 0.536 0.013 Acid detergent ®bre 0.711 0.618 0.534 0.539 0.011 Lignin passage rate (hÿ1) 0.044 0.040 0.033 0.023 0.002
Nitrogen intake (g per day) 27.9 15.4 11.9 7.8 0.46
Duodenal ¯ows(g per day)
Total nitrogen 22.7 18.5 15.3 11.6 1.15 Microbial nitrogen 18.1 15.7 12.7 10.6 0.81 Microbial synthesis (g nitrogen/kg OMADRa) 31.4 31.4 32.0 32.5 2.20
aOrganic matter apparently digested in the rumen.
of turnover rate of diet particles in the rumen were between 14 and 28, 28 and 42 and 42 and 56 days, respectively.
3.5. Nitrogen utilisation
The total nitrogen and microbial nitrogen duodenal ¯ows (Table 4) decreased by 0.26 (S.E. 0.04) and 0.18 (S.E. 0.03) g per day with the age of regrowth. In contrast, no difference was observed for the microbial nitrogen synthesis ef®ciency.
There were good correlations between total tract r0:78or rumen ADF r0:79
digestibility and DM intakes. Lower correlations r0:55linked the ruminal turnover
to intake.
3.6. Ruminal ammonia and pH
The rumen mean ammonia content (mg lÿ1) decreased from 180 to 17 (Table 5) with the regrowth age. The ammonia level was always lower than 30 mg lÿ1when the rams were fed the 56-day pangola grass, whereas the range of variation was 44±66 with the 42-day grass. There were low differences for the rumen acidity (Table 5) as illustrated by the mean pH values ranging from 6.4 to 6.3 (S.E. 0.05,P>0:05).
4. Discussion
4.1. Nutritive values of pangola grass
The decrease of CP content and the associated decline in intake and digestibility with the age of regrowth registered in this experiment are classical results already observed
Table 5
Ruminal ammonia and ruminal pH of six Black-belly rams fed ad libitum for 14, 28, 42 and 56-day oldD. decumbensgrass
Item Age of regrowth (days)
14 28 42 56 S.E.
Rumen pH
0 h 6.5 6.5 6.4 6.4 0.05
3 h 6.3 6.3 6.3 6.2 0.05
6 h 6.2 6.2 6.2 6.4 0.04
12 h 5.9 6.0 6.1 6.2 0.04
0±12 h 6.2 6.2 6.2 6.3 0.04
Rumen ammonia
0 h 155 101 66 28 8
3 h 193 92 65 16 8
6 h 215 89 45 14 6
12 h 159 98 44 12 6
with C4and C3grass forage (Demarquilly and Jarrige, 1981; Chenost, 1975). Moreover,
the low intake and digestibility of 42 and 56-day old pangola are already cited by Aumont et al. (1995). Nevertheless, in the literature there are no data related to intake and digestion of 14-day grass forage. Our results indicate that intake, ruminal and total tract digestibility of the 14-day pangola grass are similar to those reported for leafy temperate grass forage (Minson, 1990). However, the nutritive value of the 28-day pangola grass is lower than similar aged temperate grass forage. This suggests a faster evolution of tropical grass compared to temperate forage during the third and fourth week of regrowth. Little changes are observed in cell wall composition after the ®rst month of regrowth. The curvilinear decrease recorded in ruminal and total tract cellwall digestibility in the present study, would account for those signi®cant qualitative changes ofD. decumbens
before 1-month regrowth. This is con®rmed by a similar evolution of the potential degradable dry matter or its fractional degradation rate. These results illustrate a rapid maturation of the cell components of C4 compared to C3 grass, and consequently the
decrease of the ef®ciency of cellulolysis as suggested by Wilson (1994). The evolution in nitrogen content of the experimental pangola grass displays also the fast maturation in the forage during the ®rst month of regrowth. The evolution of nitrogen duodenal ¯ows and as a consequence the supply of intestinal digestible protein are in accordance with previous observations. Moreover, the nitrogen duodenal ¯ow recorded for the 14-day D. decumbens grass is similar to that of a high leafy Lolium perenne grass (INRA, 1980).
4.2. Intake, cellulolysis and diet particle turnover in the rumen
In this study, the fast decrease of daily intake is associated with a decrease of the cellulolysis and the turnover rate of diet particles in the rumen. Probably there is a cumulative effect of these two factors. We still have to identify the ®rst limiting factor because the ways to improve the intake of mature grass partly depend on it. Regarding the minimum level of rumen ammonia nitrogen (60 mg lÿ1) for microbial cellulolysis (INRA, 1980), de®ciencies appear before 42 days of regrowth. As a consequence, the microbial activity is the rumen was not maximised when rams were fed 42 and 56-day old forage. Nevertheless, it cannot be concluded that nitrogen was the ®rst limiting factor for digestion in view of the relative stability of effective degradability values recorded for the four studied stages of forage regrowth. Lower cellulolysis was compensated by a higher retention time of diet particles in the rumen. High retention time or low rumen turnover has been explained by the higher resistance of C4 grass cell walls to mastication as already indicated by
Wilson (1994). Mastication is the mean pathway for large particles breakdown (Mc Leod and Minson, 1988) which is fundamental to maintain high turnover of particles in the rumen and consequently a high intake as reported by Poppi et al. (1981a,b). Results from this experiment, suggest that the breakdown rate of large diet particles in small particle is the ®rst limiting factor of intake for 42 and 56-day old pangola grass. Low rate of breakdown could limit the total diet particle surface necessary to maximise the microbial cellulolytic activity. Speci®c studies are necessary to test this hypothesis.
4.3. Strategies of valorisation of tropical grass
General analysis of the results from this experiment displays for the high nutritive value of a highly fertilised 14-day tropical grass forage which was found to be comparable to a high leafy temperate forage. Although, these results need to be con®rmed, they suggest that grazing pasture at 14 days of regrowth would be an advisable practice to improve individual animal performances. Nevertheless, experiments on pasture with large and small ruminants are necessary because the physical structure (high, density) of the canopy is a limiting factor to maximise intake of grazing animals (Boval et al., 1996). In order to study the pertinence of this hypothesis, we suggest to develop in humid tropical areas and semi-intensive conditions, ruminant feeding strategies with forages younger (<28 days) than those classically used. We need more references about pasture productivity. In intensive or semi-intensive conditions (water, fertiliser), the mean yield of young (<28 days) tropical pasture is high (20 t DM/ha yrÿ1, Cruz et al., 1989). More than an exhaustive study of grass, a choice of model plants classi®ed on the basis of their physiology (leaves senescence, rate of stem and leaf development) could give rapid and general results about nutritive potential of a 14 days tropical pasture.
Acknowledgements
The authors would like to thank B. Calif, D. Feuillet, L. Philibert, E. Delval, P. Despois, F. PeÂricarpin, F. Nipeau, M. Arjounin and G. DeÂsireÂe for their technical assistance. This study has been supported by the ``Region Guadeloupe'' and the ``European Community'' (FEOGA).
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