White lupin (Lupinus albus) is a valuable lupin species for ruminants due to its high protein and oil contents and it can be used as an alternative to soybean meal. Sweet lupin seeds can be used whole or dehulled, by feed manufacturers and for on-farm feeding (Emile et al., 1991). However, its high ruminal degradability may result in a low metabolisable protein (MP) value. The protein value of lupin can be improved through technological treatments (Poncet et al., 2003).
Use as an alternative to soybean meal
White lupin seeds can be included in diets up to 20-25% (DM) in dairy cows (Benchaar et al., 1991; Emile et al., 1991; Brunschwig et al., 2002) and growing steers (Brunschwig et al., 2002). A higher level (32%) could be used for double-muscled Belgian Blue bulls (Froidmont et al., 2008). Up to 29% (DM) could be used in growing lambs (Kung et al., 1991), 15-20% in dairy ewes and 20-30% in dairy goats (Brunschwig et al., 2002).
White lupin seeds should be ground before feeding to high-producing dairy cows, though they can be eaten unground by fattening cattle (Emile et al., 1991). In dairy cows and fattening cattle, lupin seeds resulted in performance similar to that obtained with a cereal-soybean meal concentrate. However, they tended to decrease milk protein content and had mixed effects on milk fat content (Emile et al., 1991; White et al., 2007).
Intake of dry matter and milk protein content tend to be lower for lupin-supplemented dairy cows than for soybean meal fed cows (Robinson et al., 1993). Milk fat had higher concentrations of long-chain fatty acids (Singh et al., 1995). A shift of the fatty acid profile of milk (increase in C18:1, decrease in C12:0-C16:0) towards guidelines for improved cardiovascular health in human populations has been observed (White et al., 2007).
Grinding and flattening
N solubility (Moss et al., 2001) and in situ protein degradability of lupin (Jahn et al., 1999) both increase with the fineness of grinding, and the protein value can be controlled through substrate particle size by grinding. The effective degradability of N is lower with coarse grinding (5-mm screen) than with fine grinding (through a 1.0 or 1.5-mm screen) (63 vs. 93-95%). This increases the amount of protein digested in the intestine, and thus the MP value, although the whole tract apparent digestibility of CP is unchanged (Kibelolaud et al., 1991). In vivo measurements of digestive flows in fistulated bulls show that lupin seed should be coarsely ground or flattened to obtain a mean particle size between 2.0 and 4.2 mm for cattle, which provides the highest level of digestible protein in the small intestine. With finer particles, proteins are highly degraded, and with insufficient grinding level (6 mm), a higher ruminal degradation of lupin protein is also observed, probably due to more intense rumination (Froidmont et al., 2008).
Thermal treatments
Dry roasting
Roasting increases the undegraded protein fraction of lupin seed (+70 g/kg DM) without increasing the ADF-N fraction, which indicates a minimal protein damage due to roasting (Singh et al., 1995). However, an increase in ADF-N could be observed with roasting at 300°C during 1 to 4 min (Zaman et al., 1995). In situ N disappearance of whole lupin or dehulled lupin decreases as roasting temperature increases from 130 to 175°C (Kung et al., 1991). In situ ruminal incubation followed by in vitro enzyme digestibility trials suggests that dry roasting whole lupin seeds can shift the digestion of protein from the rumen to the lower gastrointestinal tract without depressing the digestion of rumen-undegraded protein. Among the tested conditions (110, 130 or 150°C for 15, 30 or 45 min), dry roasting at 150°C for 45 min gave the best results (Yu et al., 1999c). A similar study conducted under different dry roasting conditions (100, 118 and 136°C for 3, 7, 15 and 30 min) lead to similar conclusions: rumen undegraded protein increased (+108 g/kg DM between raw and roasted at 136°C/15 min), due to decreased soluble fraction and degradation rate. Estimated microbial protein based on available energy in the rumen is only slightly reduced, and true digestibility of dietary protein is unchanged (88% with the mobile bags technique), so that the value of true digested protein in the small intestine is increased by dry roasting from 119 to 197 g/kg DM (Yu et al., 1999a). However, the potential damaging effects of processing on individual amino acids, especially on the first limiting amino acids, remains unclear (Yu et al., 1999a).
In dairy cows, roasting, while decreasing lupin protein degradability, does not always have an effect on milk production. Cows fed roasted lupins produced more milk (with higher milk fat, protein and lactose yields) than those fed raw lupins (Singh et al., 1995), but roasting had no effect on DM intake or milk protein content (Robinson et al., 1993). Roasting increased protection of lupin oil from ruminal hydrogenation, as evidenced by increased concentrations of long-chain fatty acids in milk from cows supplemented with roasted lupins (Robinson et al., 1993).
In lambs, roasting decreased ruminal in situ protein disappearance but had no effect on growth rate and feed efficiency, which were similar to that obtained with a soybean meal-based diet, suggesting that whole or dehulled lupins can replace soybean meal as the sole protein supplement for growing lambs (Kung et al., 1991).
Extrusion
Extrusion decreases ruminal N in situ degradability by up to 20-30 percentage points (Cros et al., 1991; Benchaar et al., 1992; Kibelolaud et al., 1993; Aufrère et al., 2001; Rémond et al., 2003; Solanas et al., 2005; Barchiesi-Ferrari et al., 2011). This effect depends on extrusion temperature (110°C to 195°C) or moisture (dry to 20% humidity). It is linked to a reduction in the soluble fraction (Kibelolaud et al., 1993), and to an increase in the slowly degradable fraction (Barchiesi-Ferrari et al., 2011).
Extrusion can also increase the intestinal digestibility of by-pass protein (Solanas et al., 2005). Both effects contribute to a positive impact of extrusion on the amount of truly digestible protein in the intestine (Benchaar et al., 1992; Cros et al., 1991; Kibelolaud et al., 1993). The inclusion of a source of carbohydrates before extrusion at 140°C increases this response (Solanas et al., 2005).
Extrusion at 195°C did not alter the amino acid (AA) profile of whole lupin seeds, but did affect markedly the AA profile of the lupin protein that escapes ruminal digestion (Cros et al., 1991; Benchaar et al., 1993). The increase in intestinal disappearance varies substantially among AA (Benchaar et al., 1993). Based on estimated AA chemical score (test-to-milk ratio), the absorbed protein shows a higher protein quality in extruded than in raw lupin seed (Cros et al., 1991).
Extrusion was shown to affect the MP supply in dairy cows (Benchaar et al., 1991; Benchaar et al., 1994) and in bulls (Froidmont et al., 2008). It resulted in a decrease in protein (and to a lesser extent OM) ruminal true digestibility, in an increase in duodenal non-ammonia N and dietary N flows, and in an increase in AA true digestibility and subsequently in the amount of AA absorbed in the small intestine (up to 700 g/d in dairy cows). The effect on ruminal NH3-N and VFA concentrations and on the efficiency of microbial synthesis seems less clear. In sheep (whethers), the increase in dietary protein at the duodenum was balanced by a decrease in microbial flow, so that the total non-ammonia N flow and the profile of apparently absorbed AA did not differ between raw and extruded lupin (Rémond et al., 2003).
In growing/fattening bulls, extrusion shifted lupin protein digestion from rumen to small intestine, but was less efficient than grinding (4.2 mm) for improving the nutritional value of lupin seeds, and grinding was also simpler and cheaper (Froidmont et al., 2008).
Other processes
Autoclaving at 120°C for 30 min reduced both the soluble fraction and the fractional rate of protein degradation of the slowly degraded fraction, and subsequently the effective degradability of protein (Aguilera et al., 1992). NaOH addition did not affect protein degradability (Jahn et al., 1999).
Limits of the in situ approach to assess the protein value of lupin
It is recognized that the in situ technique underestimates the protein value of lupin seed due to its high soluble protein fraction. Although this fraction is assumed to be fully degraded in the effective degradability calculation, a non negligible part may actually escape ruminal degradation as proteins, peptides or amino-acids and contribute to dietary non-ammonia N flow at duodenum as quantified in vivo on whethers (Aufrère et al., 2001; Rémond et al., 2003).