Corn gluten meal is used as a source of protein in ruminants and particularly as a source of undegradable protein and metabolisable protein. However, the poor lysine content of corn gluten meal can be limiting and the replacement of traditional, lysine-rich protein sources such as soybean meal with corn gluten meal should be implemented with care.
Digestibility and energy content
The OM digestibility of corn gluten meal is generally higher than 90%, resulting in high ME values (16.4 and 16.8 MJ/kg DM) (Sauvant et al., 2004; Volden, 2011). Other sources have reported a True Digestible Nutrient value of 84% (NRC, 2001; Rocha Junior et al., 2003; Azevêdo et al., 2011b).
The protein of corn gluten meal is potentially highly degradable in the rumen, with potential degradability values about 90% (Sauvant et al., 2004) or higher (NRC, 2001; Volden, 2011). However, the fractional degradation rate is very low (1.6, 2.5 and 5.2 %/hour) (Volden, 2011; Sauvant et al., 2004; NRC, 2001) resulting in low effective degradation rates (around 30%): corn gluten meal is the plant feed resource which provides the highest quantity of rumen undegradable protein, ranging from 45 to 50% DM.
The intestinal digestibility of the by-pass protein of corn gluten meal is high (90-91%), which is higher than in rapeseed meal (about 80%) and cottonseed meal (less than 90%), but lower than in soybean meal (more than 95%) (Sauvant et al., 2004; Yue Qun et al., 2007). For that reason, corn gluten meal is the best plant source of metabolisable protein. However, due to is low lysine content, the proportion of lysine in the metabolisable protein is also low (less than 3.5%; Sauvant et al., 2004). Consequently, it is important to evaluate carefully the lysine content of metabolisable protein in diets including large quantities of corn gluten meal as the lower dietary limit of the lysine/metabolisable protein ratio (as a percentage) is considered to be about 6.1%.
Corn gluten meal has been extensively studied in dairy cows. In most trials, corn gluten feed alone or in combination with other protein sources gave similar or better results than the control diets. Combinations of extruded soybeans and corn gluten meal, as the protein supplement, gave results similar to those obtained with soybean meal alone for lactating cows (Annexstad et al., 1987). A mixture of corn gluten meal and blood meal produced a lactation response similar to that obtained with soybean meal in mid-lactation Holstein cows (De Gracia et al., 1989). In high-yielding Holstein cows, corn gluten meal supplementation used to raise crude protein by 1.1 to 1.5 percentage point in the diets had a slightly negative effect in early lactation and a generally positive one in late lactation, which suggests that lysine may have been a limiting factor in early lactation (Holter et al., 1992). In Brazil, in cows with restricted grazing on Italian ryegrass, supplementation with a 60:40 blend (22% crude protein) of ground maize and corn gluten meal significantly increased milk production (Ribeiro Filho et al., 2009). In Iran, cows receiving a supplement of corn gluten meal, thereby increasing their by-pass protein intake, increased their DM intake, milk yield, milk protein content and body condition score, while reducing body weight losses (Aboozar et al., 2012). However, feeding a combination of distillers dried grains and corn gluten meal depressed milk protein production when compared with a soybean-based control diet, probably due to the lower lysine content of the maize-based products compared to soybean (Voss et al., 1988).
In Brazil, corn gluten meal and cassava peels partly replaced energy concentrates, with no influence on DM intake, digestibility, microbial efficiency and nitrogen retention in heifers (Azevêdo et al., 2011a).
In sheep fed according a programme of protein supplementation on alternate days, corn gluten meal was an effective substitute for soybean meal (Collins et al., 1992). The replacement of a blend of soybean meal and wheat bran with a blend of corn gluten meal and corn gluten feed had no negative effect on apparent digestibility of nutrients, or on N and energy balance in sheep (Milis et al., 2005).
Two trials with dairy goats have resulted in contradictory results. In Brazil, the replacement of up to 50% of soybean meal protein with corn gluten protein linearly decreased milk fat production and slightly depressed milk yield in dairy goats (Macedo et al., 2003). In Italy, a trial compared a highly degradable protein diet based on pelleted total mixed rations containing soybean meal, sunflower meal and urea, with a low-degradable protein diet including corn gluten meal in the pellets. The dairy goats fed the corn gluten meal diet had the highest milk fat, protein and casein concentrations, with no significant effects on other milk components and renneting properties. It was concluded that a decrease of rumen degradable protein did not negatively influence nutrient utilization, and milk production and composition in dairy goats (Laudadio et al., 2010).