INTRODUCTION

In India, high cost of cereals and quality forages has raised
the interest in using fibrous feed in the
diet of ruminants (Rojo et al.
2015; Salem et al. 2015). Roughages are the main sources of feed for
ruminants (Kholif et al. 2014).
These feeds are characterized by high
lignocellulose content, low crude protein content, poor palatability and low nutrient digestibility (Togtokhbayar et al. 2015).
High fiber content of these feeds and plant
epidermal surface with a high concentration of silica prevents the access of
ruminal enzymes to the plant cell wall and reduce nutrient digestibility (Khattab et al. 2013, Kholif et al. 2014; Abdel-Aziz et al. 2015, Elghandour
et al. 2015a, Togtokhbayaret al. 2015). The efficacy by which
ruminants obtain energy from these feeds and, in turn, produce high quality
milk protein and meat is increasingly important if the demands of increasing
human population is to be met (Meale
et al., 2014). Hence, there is a need to
develop feeding strategies that improve the nutritive value of such fibrous
feeds. Improving the utilization of these forages would enhance
production efficiency and reduces cost. Cellulose and hemicellulose are major
structural components, quantitatively the most abundant carbohydrates in animal
diets. Degradation of cellulose and hemicellulose requires a large number of
enzymes produced by cellulolytic bacteria, protozoa and fungi that act
synergistically to hydrolyze forage cell wall in the rumen (Bhat and Hazlewood, 2001).
However, the amounts of ruminal endogenous enzymes might be insufficient due to
the complex structural carbohydrates and lignin from fibrous feeds. So, their
digestibility would be enhanced by supplementation of exogenous fibrolytic
enzymes (EFE) (Salem et al.,
2011; Bhasker et al., 2013). Various physical (heat, steam, pressure),
chemical (acid, alkali, ammonia) and biological agents (white rot fungi) have
been attempted to improve the quality of forage for ruminant livestock (Adesogan et al., 2014), but, none
of these methods is widely used to improve the forage quality and animal
performance. The supplementation of exogenous
fibrolytic enzymes (EFE) in feeds has been identified as a prom­ising
biological treatment to improve carbohydrate and cell wall degradation of low
quality feeds (Alsersy et al.
2015, Salem et al.
2015c, Valdes et al.
2015; Morsy et al. 2016)
and thus, reduces feed cost, improves feed utilization, energy avail­ability and
animal performance (Rojo et
al. 2015; Morsy et al. 2016). During the past two decades
different enzymes (protease, amylase and cellulases) have been tested for their
potential ability to improve the digestibility of nutrients and
metabolizability of energy (Eun
and Beauchemin, 2005; Klingerman et al., 2009; Beauchemin et al., 2004a;
Beauchemin and Holthausen,
2010).

 

Feeding of exogenous enzymes increases the palatability due to release of
sugars by pre-ingestive fiber hydrolysis, which is often accompanied by increased
feed intake. However post-ingestive enzyme effects, such as increase in
digestion rate and/or extent of digestion (Gado et al., 2011; Salem et al., 2011) may improve access of rumen
microorganisms to the cell wall matrix (Nsereko et al., 2002).  increase
hydrolytic activity in the rumen to reduce gut fill and thus enhance feed
intake. Within the rumen, EFE hydrolyse certain components of the cell wall, alter
the fiber cell wall structures (Giraldo et al. 2004; Salem et al., 2015)
or thinning of fiber cell wall (Vyver
et al., 2013) and produce
substrates that favour selected ruminal microorganisms and/or enhancement of
microbial enzyme activity in the rumen (McAllister et al., 2001), even with low-forage diets
(Bedford and Cowieson 2012).
In the ruminal environment, they can affect microbial populations, bacterial attachment
and colonization (Nsereko
et al. 2002; De Souza et al. 2008; Wang et al. 2012) by
increasing the numbers of ruminal fibrolytic microbes (Morgavi et al., 2000), affecting rumen
microbial protein synthesis (Gado
et al., 2009); forestomach digestibility; in vitro (Ranilla et al. 2008; Bandla et al.
2008), in situ (Tirado-Estrada
et al. 2015) and in vitro NDF digestibility (Adesogan et al., 2014; Elghandour et al., 2014; Salem
et al., 2014). They work synergistically with rumen microbial enzymes (Khattab et al. 2011) to increase the digestion and nutritive value of
fibrous feeds (Morgavi et al.,
2000) by creating a stable complexes between enzymes and feeds (Kung et al. 2000) . A synergism between exogenous enzymes and endogenous
enzymes in the ruminal fluid results in increased numbers of fibrolytic and non-fibrolytic
bacteria causing increased feed digestibility and utilization (Eun et al., 2006).

 

The effectiveness of enzymes depends upon substrate,
enzyme specificity, and enzyme dose (Elghandour et al. 2015). However, there is inconsistency in
its supplementation due to variations in experimental conditions, diet
compositions, enzyme activity, application rate, ruminal activity, differences in enzyme
activity, application rate and composition, stage of lactation of dairy cows, ruminal
microbial activity, EFE-feed specificity and the portion of the diet to which
EFE are applied (Beauchemin
et al., 2003; Beauchemin et al., 2004; Adesogan, 2005; Dean et al., 2014). Additional factors includes
using experiments with insufficient statistical power, inappropriate
experimental designs or durations, inappropriate enzyme choices, use of
inappropriate measures of enzyme activity and misleading enzyme designations (Adesogan et al., 2014).

 

Importantly,
positive effects were reported with addition of exogenous enzymes to the diets of
dairy cows, feedlot cattle (Gado
et al., 2009; Álvarez et al., 2009) and other species. Earlier, various
studies have reported positive effects of using EFE as supplements for
ruminants on dry matter and neutral detergent fibre in vivo digestibility (DMD
and NDFD) (Knowlton et al.
2007; Arriola et al. 2011; Gómez-Vázquez et al. 2011; Lunagariya et al., 2017), total volatile fatty acids
(VFA), propionic acid proportion, acetate: propionate ratio (A:P ratio) (Gurbuz 2009; Arriola et al. 2011;
Gado et al. 2011; Tirado-Estrada et al. 2015), dry matter intake (DMI) (Chung et al. 2012), milk
production (Mohamed et al.
2013; Kholif and Aziz 2014; Valdes et al., 2015; Romero et al., 2016), average daily gain (ADG) (Tirado- Estrada et al. 2011) and
reduces methane production (Mocherla et al., 2017) . However, in some studies,
EFE did not positively influence DMD, NDFD, VFA, fermentation patterns (Elwakeel et al. 2007; Holtshausen et
al. 2011) or animal performance (Arriola et al. 2011). This occurs due to difference
in the interaction within the ruminal ecosystem, the cell walls of plants and
the type of EFE (Meale et al.
2014).