Para grass (Brachiaria mutica), ensiled or supplemented with sugar palm syrup, improves growth and feed conversion in "Yellow" cattle fed rice straw

Keo Sath, Kem Khen, Kjell Holtenius* and Thomas Pauly*

Centre for Livestock and Agriculture Development (CelAgrid), Phnom Penh, Cambodia
keosth@gmail.com
* Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences (SLU),
Kungsängen Research Centre, SE 753 23, Uppsala, Sweden

Abstract

Two experiments were conducted to determine: i) an appropriate additive level of sugar palm syrup for ensiling para grass, and ii) the growth response by local cattle when the syrup was applied either at ensiling or at feeding. In the ensiling experiment, 72 plastic bag silos each containing 1 kg fresh grass were allocated to 4 treatments (SP0, SP2, SP4 and SP6 of 0, 2, 4 and 6% syrup on fresh matter basis) with 3 replicates per treatment and 6 sampling times.

The pH in SP0 decreased from 7.1 to 6.1 after 5 days and stayed at that level until day 180. The pH of silage made with sugar palm syrup (SP2, SP4 and SP6) decreased from 7.1 to below 5.0 at day 5. After day 14, the pH of all syrup-treated silages increased by 0.5 units until day 180, possibly due to air entry. The NH₃-N content increased with storage time and was highest in SP0 (about 220 g/kg total N) after 90-180 days. Applying at least 2%  sugar palm syrup to para grass at ensiling reduced NH₃-N and pH values and improved the quality of the resulting silage. 

In the feeding trial, 15 female cattle of the local "Yellow" breed with an average live weight of 110 kg were randomly allocated to one of three diets identified as PS0:  rice straw (RS) + a rumen supplement (urea, molasses, minerals), PSE: RS + para grass silage made with 3% sugar palm syrup,  PSF: RS + para grass silage made without sugar palm syrup but with 3% sugar palm syrup added at feeding.

Daily DM consumption was highest, and crude protein (CP) intake and apparent digestibility were lowest for diet PS0 compared with diets  PSE and PSF.  Live weight gain was lower and feed conversion better, and rumen pH lower,  with diets PSE and PSF than PS0.

Key words: feed conversion, growth, rumen supplement, silage

Introduction

As in many countries in South East Asia, ruminant nutrition in Cambodia relies mainly on rice straw, with the addition of some crop residues and/or grazing on small patches of marginal land. The feeds available are usually of low nutritive value, particularly in terms of digestibility and protein content, especially during the later stage of the dry season. During this period, shortage of feed resources and lack of appropriate feed preservation techniques are the most important constraints limiting livestock production in Cambodia (Sath et al 2008). 

Para grass (Brachiaria mutica) is a creeping perennial grass that is well adapted to a wide range of soils in the tropical and subtropical regions, especially in swampy and flooded areas. Para grass is a good source of the essential minerals sodium (Na) and phosphorus (P), has four-fold higher CP content than rice straw and it is highly palatable (Sath et al 2013). Silage making is an appropriate way of promoting animal production in tropical regions when fresh feed supplies are limited. Various categories of additives have been widely discussed in silage making with the aim of improving fermentation and aiding the preservation process (McDonald et al 1991; Umaña et al 1991; Henderson 1993; Bolsen et al 1996; An and Lindberg 2004; Nisa et al 2008). Most tropical grasses, including Brachiaria species, have water-soluble carbohydrate (WSC) contents ranging from 32 to 77 g/kg DM, which is rather low for producing well-fermented silages (Heinritz et al 2012). Therefore the addition of a suitable sugar source would help to produce more desirable fermentation characteristics and a lower pH in the silage (Yokota et al 1991; Tjandraatmadja et al 1994b; Van Niekerk et al 2007; Mendieta-Araica et al 2009; Qamar 2009). Tjandraatmadja et al (1994a) pointed out that the fermentation pattern of silages and their nutritional composition are influenced by forage species, growth stage at cutting and inclusion of molasses at ensiling. Sugar palm syrup is a valuable energy supplement for most farm animals, including cattle. Supplementing silage at feeding with a source of readily available energy increases the flow of microbial protein to the small intestine (Rooke et al 1987; Huhtanen 1988) and linearly increases DM intake (Broderick and Radloff 2004). However, negative effects of adding sugars, such as decreased fibre digestibility and reduced rumen pH, have also been reported (Khalili 1993; Petit and Veira 1994).

The aims of the present study were to identify an appropriate  level of sugar palm syrup for ensiling para grass and to evaluate the growth response by local cattle when the syrup was applied either at ensiling or at feeding.

Materials and methods

Para grass ensiling experiment

Para grass was collected from a monoculture growth area and chopped into pieces of 4-8 cm length. The chopped grass was spread out on a plastic sheet and wilted in the shade for 2h to reduce the moisture content. Four levels of sugar palm syrup were applied to the wilted grass just before ensiling: 0 (SP0), 2 (SP2), 4 (SP4) and 6% (SP6) of fresh forage. Portions of para grass were thoroughly mixed by hand with the corresponding portion of syrup on the plastic sheet on the ground. The  forages were placed in plastic bags, compacted by hand and sealed with rubber bands. In total, 72 plastic bag silos, each containing 1 kg, were made (4 syrup levels x 6 sampling times x 3 replicates). Triplicate bags were sampled 0, 2, 5, 14, 90 and 180 days after closure and the samples were analyzed for DM, pH and ammonia nitrogen (NH₃-N). All bags were stored under a roof from February to August 2010, at mean daily temperatures ranging from 26 to 33 ºC.

Performance study in cattle

Fifteen female cattle of the local "Yellow"  breed with an average LW of 110 ± 8.4 kg (18 months of age) were allocated in a completely randomised design to one of three experimental diets. The cattle were de-wormed using Ivermectin and vaccinated against foot-and-mouth disease prior to the experiment and were kept in individual pens (1 m x 1.8 m) on a concrete floor in an open-sided, roofed shed, with free access to the experimental diet and water. The experimental diets were fed for a period of 3 months from January to May 2011 (average daily temperature ranging from 24 to 35 ºC).

The three diets were:

PSO: Rice straw + rumen supplement

PSE: Rice straw + para grass silage made with 3% sugar palm syrup

PSF: Rice straw + para grass silage made without sugar palm syrup + 3% sugar palm syrup at time of feeding. 

Rice straw was fed ad libitum. Para grass silage was fed at a level of 1.3 % of LW (DM basis). PS0 animals received a rumen supplement (13% urea, 27 % molasses, 33.5% rice bran and 26.5% mineral mixture) at 0.25% of LW. For the PSF treatment,  sugar palm syrup was added prior to feeding at 3% of  the silage. Fresh quantities of rice straw and grass silage were offered at 08:00, 12:00 and 16:00. Rumen supplement was given to the PS0 cattle once daily at 08:00. The rice straw, grown during the rainy season, was collected after harvesting and stored under a roof. Para grass was collected from a site nearby and was chopped to a length of 4-8 cm. The chopped grass was sun-dried for 3-5h and ensiled immediately (PSF) or mixed thoroughly with diluted sugar palm syrup (PSE: 3% sugar palm syrup + 1% water to reduce its viscosity; % on fresh matter basis). The silages were compacted by treading in cylindrical containers lined with double plastic bags and each bag was sealed with rubber bands. Each container contained about 140 kg  grass silage. The containers were stored under a roof for 10 weeks.

Data collection and chemical analysis

Daily feed consumption was recorded and refusals collected from individual animals every morning.  The cattle were weighed in the morning before feeding every two weeks and on two consecutive days at the start and end of the trial. Means from the two consecutive weighing were used. During the last five days of the experiment, total fecal excretion was individually recorded to determine apparent digestibility.  Rumen fluid samples were taken using a stomach tube 2h after first morning feed on the last day of the experiment.  

The DM content of feed and fecal samples was determined by micro-wave radiation (Undersander et al 1993).  Ash, N and NH₃-N were determined according to AOAC (1990). Neutral detergent fibre (NDF) and acid detergent fibre (ADF) were determined using the procedure of Goering and Van Soest (1970). The pH value of the silage and rumen fluid was determined on fresh samples using an electronic pH meter fitted with a glass electrode. 

Statistical analysis

For the silage trial and the animal performance study, data were analysed  using the General Linear Model option in the ANOVA program of the MINITAB Software, version 16.1.1 (Minitab 2010). Treatment means which showed significant differences at the probability level of p<0.05 were compared using Tukey’s pair-wise comparison procedure.  

In the silage trial, the statistical model was:

             y_ijk= µ + α_i +ß_j + (αβ)_ij + e_ijk

where y_ijk = the dependent variable, µ = overall mean value, α_i = effect of treatment, ß_ij = effect of ensiling time, αβ_ij= interaction between treatment and ensiling time and e_ijk = effect of random error.

In the animal performance study, the statistical model was:

              y_ij = µ + α_i + e_ij

where y_ij = the dependent variable, µ = overall mean value, α_i = effect of treatment and e_ij = effect of random error. 

Results

Para grass ensiling experiment

The pH value in all syrup-treated silages decreased from day 2 onwards (Table 2). In the untreated silage pH dropped slightly to  6.1 at day 5 and remained on that level during the entire storage period. The lowest pH values in the syrup-treated silages occurred after 5-14 days. The values increased slightly thereafter towards day 180 (Figure 1). Among the syrup-treated silages, the pH values were lower between day 90 and 180 in silages made with the two highest syrup levels (SP4, SP6). The NH₃-N levels showed a rapid increase during the first five days, increased thereafter slowly up to day 90 and remained at a relatively high level until day 180. All three levels of sugar palm syrup decreased NH₃-N values from day 5 as compared with the untreated silage. The NH₃-N end point at day 180 was lowest in silages made with the two highest levels of syrup (SP4, SP6). 

Figure 1. pH changes in para grass ensiled with different levels of sugar palm syrup	Figure 2. NH3-N concentration in para grass ensiled with different levels of sugar palm syrup

Cattle performance

Total DM intake was highest on the PS0 diet and lowest on PSE (Table 3). There were no differences among diets in apparent digestibility for proximate constituents OM, ADF and NDF. Data for CP cannot be compared as most of this component was in the form of urea on diet PS0. Ruminal NH₃-N concentrations were high and did not differ among diets, but rumen pH was slightly lower with diets PSE and PSF compared with PS0. 

 Daily weight gain was increased and DM feed conversion improved when rice straw was fed together with ensiled para grass rather than with a urea-based supplement (Table 4). There were no differences between para grass ensiled with syrup or supplemented with syrup at the time of feeding.

Discussion

Ensiling Para grass

In the ensiling experiment, the DM content of para grass was increased from 196 to 256 g/kg by wilting. This may have contributed towards creating better conditions for an efficient fermentation process, because with temperate forages, an increased DM content normally results in silage with better fermentation characteristics (McDonald et al 1991). In the present study the pH of the syrup-treated silages dropped from approximately 7.0 to below 5.0 in five days. This can be compared with the control silage (SP0), where the pH remained at or above 6.1 within the same period. According to McDonald et al (2002), well-preserved tropical grass silages with a DM content below 30% should have pH around 4.2. The most likely reason for the less pronounced drop in our silages is lack of fermentable substrate, which inhibited the further formation of lactic and acetic acids.  After 14 days the pH rose slightly but steadily up to 180 days, particularly in the silages supplemented with palm syrup. This might have been caused by the ingress of small amounts of air during the storage period, which would stimulate yeast growth. Yeasts and moulds are able to metabolise lactic acid in the presence of oxygen, leading to increasing pH values (Lindgren et al 1985; Woolford 1990). 

In this study, the NH3-N values stabilised at a high level (125-221 g/kg N) around day 90. Because a large proportion of the NH3 originates from the deamination of amino acids, NH3-N levels should be low in well-fermented silage, preferably below 100-120 g/kg N (Kung and Muck 2000; McDonald et al 2002). Without the addition of syrup the para grass silage fermented very badly, resulting not only in a less pronounced drop in pH but also very high NH3-N levels (>210 g/kg N). Applying increasing amounts of sugar palm syrup to para grass (SP2-SP6), decreased the NH3-N levels. Tjandraatmadja et al (1994a) found that adding an increasing level of molasses to three types of forage reduced NH3-N concentrations and that NH3-N levels increased with increasing storage time.

Cattle performance

The lower DM intake on the silage diet PSE is difficult to explain as normally an improvement in the protein: energy ratio in the absorbed nutrients from  a forage diet (ie: by the feeding of grass silage rather than a urea supplement, the former having more digestible CP) should have led to an increased DM intake (Preston and Leng 2009).  

Surprisingly, the digestibility coefficients were not improved when ensiled para grass replaced half of the rice straw in diets PSE and PSF.  The improvements in growth and feed conversion must therefore have resulted from the greater intake of protein from the para grass compared with the urea-based rumen supplement. This would have increased the protein/energy ratio in the absorbed nutrients and hence would explain the better growth performance as predicted by  Preston and Leng (2009).

Cattle offered the rice straw-based diet with rumen supplement (SP0) had an average weight gain of 145 g per day, about two-fold higher than in an earlier study using a similar diet Mom et al (2001). This would probably be considered satisfactory by most Cambodian famers, who are accustomed to cattle losing weight during the dry season.

Conclusions

• The ensiling of para grass in small polyethylene bags plastic bags (capacity 1 kg) was not satisfactory as indicated by a final pH of 6.1, compared with a pH of 5.1 when 2-6 % sugar palm syrup was added. By contrast, when the para grass was ensiled in rigid PVC containers in larger quantities (about 140 kg) the silage quality appeared to be the same as between with and without addition of 3% sugar palm syrup. 

• Local "Yellow" cattle grew about 30% faster with a similar degree of improvement in feed conversion when half of the rice straw in their diet was replaced by para grass ensiled with 3% sugar palm syrup, or para grass ensiled alone and with the same quantity of syrup given as a feed supplement.

Acknowledgements

The authors are grateful to the Swedish International Development Agency (Sida), through the regional programme Mekong Basin Agriculture Research Network (MEKARN), for the financial support of this research.

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Received 17 June 2013; Accepted 23 June 2013; Published 1 July 2013

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