Dry faba bean

Protein solubility is not a reliable indicator of rumen degradability

Proteins in less commonly used grain legumes, such as in pea and lupin, are highly soluble and so the in sacco (nylon bag) technique over-estimates protein degradability because protein washes out of bags irrespective of whether it is degraded. Soluble protein from lupin seeds can escape rumen degradation. Recent work with rapeseed proteins showed that soluble proteins can be adsorbed to microbial cells or taken up directly into microbial cells. Both pathways result in more under-graded protein passing from the rumen than would be predicted from protein solubility.

Solubility methods produce widely divergent values for grain legumes

It has long been known that factors such as extraction time, pH, ionic strength, and temperature affect protein solubilisation and this seems to be particularly evident for grain legumes. De Jonge et al. (2009) showed that there were large effects of pH on N solubility (Figure 1), with much lower solubility at lower pH levels (5.0–5.6) that are quite common in high producing ruminants.

Given these effects, it is not surprising that there are no consistent relationships between measurements of N solubility and estimates of N degradation based on in sacco or in vitro measurements. Results from Kandylis and Nikokyris (1997; Figure 2), de Jonge et al. (2009; Figure 3) and our own results of analysis of N solubility using pH 6.8 buffer, water, or a 16-hour in vitro incubation with buffered rumen fluid (Figure 4) all confirm that N solubility methods are not an appropriate method for evaluating the nutritional value of pea, faba bean or lupin – nor for comparison with soybean meal (for which laboratory methods are more secure).

Key practice points

• The nylon bag technique under-estimates undegradable dietary protein (UDP) supply from grain legumes. Estimates of protein (N) degradability should not be based on in sacco (nylon bag) techniques for such highly soluble feeds.
• Significant proportions of soluble protein can pass from the rumen undegraded. This means that promising grain legumes, such as pea, bean and lupin, may have been under-valued relative to other protein sources, including soybean meal.
• Solvent characteristics, particularly pH, have a very large effect on protein (N) solubility estimates for grain legumes. Low pH (acid condition) leads to lower values for degradable protein.
• This latter effect will also occur in the rumen so that protein degradability values for grain legumes will be much less when included in diets leading to lower rumen pH (5.6 and below). This is potentially a very useful phenomenon because requirements for undegraded dietary protein are often highest in high performing ruminants that are offered higher levels of high concentrate diets, resulting in lower rumen pH. Thus, the under-estimation of protein value of grain legumes may be most pronounced when feeding the most productive ruminants.