Estimation of ruminal digestibility and fermentation characteristics of coffee pulp: A meta-analysis of in vitro experiments

Integrating various studies with statistical methods can be used by applying a meta-analysis method. The objective of this meta-analysis was to evaluate the effect of coffee pulp as ruminant feed on in vitro digestibility and rumen fermentation characteristics. The database was developed from six related articles containing 25 studies, collected from Scopus and Google Scholar. The parameters integrated were in vitro dry matter digestibility (IVDMD), in vitro organic matter digestibility (IVOMD), pH value, total volatile fatty acids (VFA), NH3 and total protozoa. The data were analyzed using a mixed model methodology. The level of coffee pulp was considered as a fixed factor and the different studies as a random effect. The results revealed that coffee pulp addition had a significant quadratic effect to enhance IVDMD (P<0.01) and a significant linear effect to decrease total VFA (P<0.05). However, coffee pulp addition did not significantly affect (P>0.05) on IVOMD, pH, NH3 and total protozoa. In conclusion, coffee pulp can be used as an alternative feedstuff due to its good digestibility in the rumen. Nevertheless, it should be applied in the right amount to avoid negative effects on animal performance.


Introduction
The availability of diet and feeding efficiency are the basis of livestock production.Feed costs affect profitability in the production process by more than 60% of the total production costs; thus, it is important to use an alternative feedstuff by utilizing local resources [1].Agro-industrial by-products are considered inexpensive local resources that can be used in ruminant diets [2], [3].Coffee is a plant that produces tremendous waste in the processing process in the form of coffee pulp produced as much as 50-60% of the harvest [2].Some studies have shown that coffee pulp can be used in animal diets.Coffee pulp contains 10,36% crude protein, 39,43% crude fiber, and anti-nutritional factors such as caffeine, lignin, and tannins, which can reduce palatability and affect livestock health [4], [5], [6].Coffee pulp can be considered as a corn substitution diet in sheep with a percentage of more than 25% and as a supplement to cow diet as much as 30-40% [7].
Feed digestibility is an indicator of feed quality.The degree of rumen diet digestibility is reflected by energy formed, especially for volatile fatty acids (VFA) production.A high concentration of NH3 is required to achieve a maximum diet fermentation rate in the rumen [8].A previous study showed that 0-40% fermented coffee pulp on diet has no significant effect on ruminal pH and ruminal protozoa (P>0.05).Nevertheless, it significantly decreased the total VFA concentration and IVDMD (P<0.05)[9].In contrast, using 0-30% coffee pulp showed no significant effect on IVDMD (P > 0, 05) but had a significant effect (P<0,05) on IVOMD [10].In another study, the ruminal pH, NH3, and total VFA were not significantly affected by using 0-6% coffee pulp in the diet.In addition, it significantly decreased IVDMD and IVOMD (p<0.05)[11].The use of coffee pulp at higher levels (58-58,8% as complete feed) significantly affected IVDMD and IVOMD (P<0,01) [12].
The multiplicity of studies that have been conducted can be analyzed into general conclusions related to in vitro digestibility of coffee pulp in ruminants.Generalization of results from various studies can be performed using a meta-analysis approach.By combining data from multiple studies, meta-analyses can provide more precise estimates than results derived from individual studies [13], it is highly recommended to integrate all previous research results to produce general conclusions [14], [15].

Materials and methods
This study was conducted by searching for articles related to the use of coffee pulp in ruminants.The articles were collected from Scopus and Google Scholar.The keywords and boolean operators used in the literature search were "coffee husk" or "coffee pulp" or "coffee skin" and "ruminant" or "cow" or "cattle" or "goat" or "sheep" and "feed" "kulit kopi" and "ruminansia" or "sapi" or "kambing" or "domba" and "pakan."Based on these keywords, 1022 articles were found.The next step was to eliminate duplication, screening the title, abstract, and the entire collected articles by sorting out research conducted on in vitro studies on ruminants.A total of six relevant articles were included in this study [9]- [12], [16], [17].After collecting the data, to allow direct analysis, different measurement units of the parameters were transformed into similar units.The included articles and statistical summary of the database are summarized in Tables 1 and 2, respectively.The parameters compiled in the database were in vitro dry matter digestibility (IVDMD), in vitro organic matter digestibility (IVOMD), pH value, total volatile fatty acids (VFA), NH3 and total protozoa.The collected data were analyzed using a mixed model methodology [14], [18].The different studies were taken as random effects, whereas the level of coffee pulp was analyzed as a fixed effect.The quadratic mixed-model method was employed to determine the effect of coffee pulp on the experimental parameters.Eventually, the linear mixed model was used when the quadratic mixed model was not significant (P>0.05).

Results and discussion
The descriptive statistics of the database obtained were 25 studies from six articles that reported the use of coffee pulp on in vitro rumen fermentability with various levels ranging from 2% to 100% on a dry matter basis.The responses of all the parameter results are presented in Table 3.The results revealed that coffee pulp addition had a significant quadratic effect to enhance IVDMD (P<0.01) and a linearly significant effect to decrease total VFA (P<0.05).However, coffee pulp addition did not significantly affect (P>0.05) on pH, IVOMD, NH3 and total protozoa.During the fermentation process, microbes utilize the cell content and crude protein of the diet as an energy source for their growth [10].Coffee pulp contains 10,36% crude protein and 39,43% crude fiber [4].The crude fiber contains 19,51% celluloses and 7,99% hemicelluloses [4].Although coffee pulp contains a high crude fiber content, it can significantly increase IVDMD around 40.80-70.48%.It is characterized by stable pH conditions so that the rumen fermentation process and dry matter degradation run optimally because microbial activity is supported by appropriate environmental conditions.The pH value is an indicator describing the degree of rumen fermentability.Under normal conditions, pH can support the activity of ruminal microbes to optimally degrade the diet [19].The results of this study showed that pH values around 6.77-7.06 were still below the normal range to support the degradation process of fiber.The rumen pH is around 6,0-7,0 is considered as normal conditions to support fiber degradation [20].This result indicates that the use of coffee pulp can defend against ruminal pH.
Although the value of IVDMD has a significant effect, it has no significant effect on IVOMD.However, the results of obtaining IVOMD are not much different from IVDMD, ranging between 41.50-70.34%.The high and low values of IVOMD are in line with the IVDMD value.This is because the organic matter component is part of the dry matter.The IVDMD pattern always follows the IVDMD pattern because both are influenced by the same factors [21], [22], [23].The value of IVOMD is strongly influenced by the crude fiber content of diet [24].It is known that coffee pulp contains very high crude fiber which is 39.43% [4].
Despite a significant decrease in total VFA production, the results obtained show that VFA concentrations are still in the normal concentration of 107.50-173.76mM.The optimal value of total VFA is 80-160 mM which indicates that the diet is well degraded at the time of fermentation by rumen microbes [25].A low pH value indicates the high levels of VFA as a result of fermentation, and vice versa pH conditions that are close to neutral indicate low levels of VFA formed [26].Based on the results of the study, the pH value of the rumen is in the range of normal to neutral.The high content of lignin can inhibit VFA formation, causing a significant decrease in VFA due to the addition of coffee pulp with a lignin content of 65.42%.Crude fiber is one of the factors that affect the production of VFA because cellulose and hemicellulose degradation produces VFA.However, coffee pulp as agroindustrial by-product contains lignin which forms lignocellulose and lignohemicellulose bonds, so the fraction is difficult to digest.In addition, the low protein content also cannot contribute more to the formation of VFA through the deamination pathway.Crude protein is degraded into amino acids and deaminated to form NH3 and α-keto acid, which are further converted into VFA [22].The low protein content of coffee pulp is thought to be one of the factors that reduces VFA production, along with an increase in the percentage used.This was also observed in the absence of significant NH3 concentrations.
NH3 concentration ranged between 11.84-13.41mM is still on the ideal range.The NH3 concentration in the rumen for optimum protein synthesis by microbes' is 5-17.65 mM [27].This is probably due to the tannin content that can inhibit the degradation process of coffee pulp in producing NH3 because it can obstruct the protein fermentation process by binding to protein compounds in coffee pulp.Tannins can reduce protein availability and inhibit digestive enzymes [5].The NH3 production depend on protein diet.Dietary proteins are fermented by bacteria and protozoa.As proteolytic microbes, bacteria, and protozoa produce protease, peptidase, and deaminase enzymes that degrade proteins into amino acids, peptides, and ammonia [28].Even though no statistically significant difference was observed in ruminal protozoa in this study, the average protozoan population in this study was 5.43 log 10 or equivalent to 2.7 × 10 5 cell ml -1 , which is still an ideal population.The normal limit of protozoa populations in the rumen ranged between 1.51 to 5.3 x 10 5 [29].This indicated that the coffee pulp in diets does not endanger the ruminal microbes.

Conclusions
Although it has no significant effect on pH, NH3, IVOMD and total protozoa, the results are still on the ideal values.It has been shown that coffee pulp has good digestibility and does not interfere with the rumen fermentation process; therefore, it can be utilized as an alternative feedstuff for ruminants.However, it should be applied in the right amount in order not to get negative effects on animal performance because the higher the levels of coffee addition, the lower the organic acid formation produced in the rumen.

Table 1 .
The relevant articles include in the meta-analysis.
a Number of experiments b Not available c Non treatment

Table 2 .
Descriptive statistics of the database were used to evaluate the effects of coffee pulp on in vitro digestibility and fermentation characteristic in the rumen a Number of studies b Standard deviation c Minimum d Maximum

Table 3 .
Effects of coffee pulp on in vitro digestibility and fermentation characteristics in the rumen.