Soybean meal is an important part of the diets of ruminants due to its high amount of rumen-degradable protein (more than 60%), good amino acid balance and high cell-wall digestibility (INRA, 1988). It is also very palatable to ruminants. Inclusion levels in ruminant and pre-ruminant diets are about 35% in dairy cows and beef, 30% in ewes and 20% in calves and lambs (Ewing, 1997).
Soybean meal is a staple of the diets of high-producing dairy and beef cattle in developed countries. In dairy cows, it has positive effects on feed intake, milk yield and milk protein content (Rego et al., 2008; McDonald et al., 1998; Polan et al., 1997; Baldwin, 1986). In steers, supplementation of soybean meal on prairie diets resulted in higher forage intake and nutrient digestibility (Krysl et al., 1989; Guthrie et al., 1988). In calves younger than 3 months, methionine, lysine and tryptophan are the 3 first limiting amino acids of soybean meal, but this deficiency disappears after 3 months (Abe et al., 1999; Abe et al., 1998).
While soybean meal is well degraded in the rumen and provides ammonia, amino acids and peptides for rumen microbial protein synthesis, it may not provide enough undegraded intake protein to meet the demands of highly productive animals. Therefore, an important line of research has consisted in developing techniques aiming at improving the rumen by-pass quality of the soybean meal protein. Many methods have been tested over the years:
Thermal and mechanical processes: heating, extruding, extruding-expelling, various combinations of heat and pressure, etc.
Chemical treatments: alcohol, formaldehyde, NaOH, NaCl, xylose, tannins, heated Ca salts, bentonite, acids, alkalis, encapsulation with blood, zein or fat (Castro et al., 2008; Colmenero et al., 2006; Chen KuenJaw et al., 2002; Wacyk et al., 2000; Atwal et al., 1995; Smith, 1986).
Replacing part of the soybean meal by non-protein sources of nitrogen such as urea has also been extensively studied. For recent examples of this type of research, see Hadjipanayiotou, 1998; Melo et al., 2003; Paengkoum et al., 2009; Pires et al., 2004.
Due to the importance of soybean meal and to its high cost, particularly in countries that have to import it, there have been innumerable attempts at replacing it by other protein sources such as protein oil meals (cottonseed meal, sunflower meal, rapeseed meal, groundnut meal…), legume seeds (peas, faba beans, lupins…), starch and distillery by-products, leaf meals (alfalfa…), land animal proteins (meat and bone meals, poultry by-products and other slaughterhouse by-products) and fish meals. For recent examples of this type of research where soybean meal was compared to an alternative protein source, see Abu-Ghazaleh et al., 2001; McDonald et al., 1998; Brzoska, 2008; Wanapat et al., 2007; Froidmont et al., 2004; Seoane et al., 1990; Veira et al., 1990; DelCurto et al., 1990; Claypool et al., 1985; Ravichandiran et al., 2008; Tripathi et al., 2001.
The success (or failure) of replacing soybean meal by an alternative protein (or non-protein nitrogen) source, or of using a technical process to improve its feed value, is actually measured by a cost-benefit analysis: an alternative protein source that is nutritionally inferior to soybean meal may have a price and availability that makes it economically more interesting. Conversely, a relatively expensive process may result in such higher animal performance that its cost is easily absorbed by the additional gains. However, concerns about the potential health and safety issues associated with alternatives to regular soybean meal should be taken into consideration.
As with cattle, there have been numerous attempts at replacing soybean meal in sheep diets with locally available and less expensive protein sources. In recent years, products as varied as sunflower meal (Irshaid et al., 2003), linseed (Giannico et al., 2009), bitter vetch (Haddad, 2006), pongam cake (Soren et al., 2009), banana trunks (Mathius et al., 2001) and fish meal (Aimone et al., 1996; Urbaniak, 1995) have been successfully tested from an economic perspective. Soybean meal, compared with energy sources such as maize or barley grains in late pregnancy or early lactation ewes, gave the same performance in animals fed low quality hay as a basal diet (Hill et al., 1995).
In goats, adding 1.6% urea to a soybean meal-based diet allowed a reduction of soybean meal by 12% (from 25% to 13% inclusion level) resulting in lower feed costs (Costa et al., 2009). In countries where such a practice is allowed, soybean meal can also be replaced by meat offals or poultry meal without altering animal performances (Oyeyemi et al., 2006; Sanchez Estrada et al., 2002).