Introduction
Two-third of the world’s poor live in Asia below nationally defined poverty line and 65% of them are poor livestock keepers who derive a large part of their household from domesticated animals. Ruminant animals play a significant role in conversion of low quality plant materials into high quality protein rich food besides playing greater role in conserving fertility of soil, through organic manure. The rapidly changing patterns of demand for livestock and livestock products point to livestock production being an increasing component of the agricultural economies of Asia. The extent to which the rural poor will benefit from these changes depends on how livestock can be integrated into developing markets and whether cheaper livestock products benefit the rural poor as consumers as well as producers.

The grazing animals differ in their feeding status due to differentiation in the availability of grazing land. Grazing animals are most susceptible to gastrointestinal nematode (GIN) infection (55.48%) than stall fed animals (19.44%) due to exposure to faecal contamination of grazing land and also due to pressure on grazing land. There is lack of grasses available during different season for grazing livestock. Even under such circumstances farmers do not supplement their animals with protein, salt or mineral mixture. The under feeding such situation might be accompanied by poor quality of feed leading to deficiency diseases. The poor quality roughages (wheat /rice straw) provide bulk to the ruminants, which are deficient in protein, energy, minerals and vitamins and adversely affects the nutritional status of the animals (Pathak, 2011). The poorly nourished animals so maintained become the main contributing factors to high prevalence and frequent outbreaks of parasitic diseases (Pathak and Pal, 2008). The GIN infections have been found widely in tropics, and may cause serious clinical disease, welfare problems and loss in production in farm animals. As livestock production has become more intensive the threat of parasitic diseases has increased. The GINs infect sheep, goats, deer and other small ruminants and have been a significant cause of economic loss to small ruminants (Lange et al., 2006). GIN infections are characteristic of pastoral grazing systems and many GIN species have developed resistance to anthelmintic drugs (Kaplan, 2004).

Infestation with GINs causes significant production losses, ranging from 13% to 33% in grazing ruminants, which typically have reduced immunity to GINs (Kaplan, 2004; Stuedemann et al., 2005). Sub-clinical GIN infections occasionally depress feed intake and animal production (production losses, mortality and weight loss), increase the costs of prophylaxis and treatment (Pathak and Tiwari, 2013) and can impair tissue deposition and skeletal growth (Parkins and Holmes, 1989). Gastrointestinal nematode (GIN) infection is arguably the most serious constraint affecting small ruminant production worldwide. The GIN of particular concern is Haemonchus contortus, which can cause severe blood loss resulting in anemia, anorexia, depression, loss of condition, and eventual death (Pathak and Tiwari, 2012a).

The customary mode of control of these GINs has been based on the repeated use of chemical anthelmintics. However, drug resistance has become an important issue in small ruminant husbandry, when anthelmintics are applied at high levels and increasing frequency and inappropriate doses (Pandey et al., 2001). The development of GIN resistance against anthelmintics and the increasing concern about anthelmintic residues in animal products is of great concern (Prichard, 1994; Kaplan, 2004). Alternative parasite management strategies are required for sustainable livestock production. Feeding of condensed tannins (CT) containing forages are shown to have increased the proportion of dietary protein reaching to intestine (Waghorn et al., 1994b; Dey et al., 2008) and high protein intakes have been associated with increased immuno-competence in lambs and kids (Coop and Holmes, 1996; Pathak and Tiwari, 2012b). Moreover, growing interest in global organic food production in recent years primarily due to adverse impact of intensive farming on environment, animal health and as well as consumers concern for food safety restricts use of chemicals in animal feeds (IFOM, 2006). Thus, alternative environmental friendly sustainable novel strategies are required, which could reduce the exclusive reliance on anthelmintics treatment. Use of phyto-chemicals is becoming preferable and may offer better control than anthelmintics to treat GINs (Chandrawathani et al., 2000). Condensed tannins (CT) may enhance resistance to GINs through increases in tissue protein supply, which are prioritized for repair and immune response (Barry et al., 2001; Neizen et al., 2002). The CT could complex with nutrients and inhibit nutrient availability for larval growth or decrease GINs metabolism directly through inhibition of oxidative phosphorylation (Scalbert, 1991), causing larval death (Athanasiadou et al., 2001).

The CT, or Proanthocyanidins, are a diverse group of polymeric flavanoids that readily complex with carbohydrates and proteins (Hagerman, 1992). Consequently, the tannin-protein reaction has been widely used to improve protein metabolism in ruminants (Aerts et al., 1999). Supplementation of CT from potential tropical tanniferous tree leaves may be a possible alternative approach to control gastrointestinal nematodes (GIN) without any residual effect in animal products. This raises the possibility that feeding locally available tropical tanniferous tree leaves containing CT may be an alternative method for controlling GIN infections, especially in areas such as the tropics and subtropics. Therefore, potential source and optimum level of CT to be used in the diets to reduce GIN load in small ruminants’ warrants investigation. This may provide three-way advantage to conquer the limitations of small ruminants by reducing GIN infections and improve small ruminants’ performance, and strategic use of tanniferous tree leaves, which otherwise remain under-utilized as animal feed. Keeping this in view, the review article was prepared to explore the effect of CT on nutrient metabolism, growth, immunity, productive performance and GIN load in small ruminants.

Tannins
Tannins are naturally occurring plant polyphenolic compound of high molecular weight (500 to 4000 Dalton) containing sufficient phenolic hydroxyl groups to permit the formation of stable cross-link with proteins (Despande et al., 1986). They are generally devided in two groups: (1) the hydrolysable tannins (HT), consist of a carbohydrate moiety in which the hydroxyl groups are esterified with gallic acid or ellagic acid, and (2) condensed tannins (CT), also known as Proanthocyanidins (PA), consist of oligomers of the flavon-3-ols and related flavanol residues linked by carbon-carbon bonds and produce anthocyanidins on acids degradation (Mangan, 1988). The molecular weight of CT can range from 6000 to 12000 Dalton (Morris and Robbins, 1997).

Occurrence of Tannins in Tropical Tree Leaves
Tannins are widely distributed in plant kingdom. The levels in plants vary greatly between species, within species, stage of development, from location and from year to year (Mehansho et al., 1987). They are found in wood, bark, leaves and fruits of many species but