Soybeans for Dairy: Heat Treatment and Protein Fractions

Ruminal protein degradation of feed ingredients can be described by first-order mass-action models. These models consider that crude protein (CP) consists of multiple fractions that differ widely in their rates of degradation, and that ruminal disappearance of protein results from the combined effects of degradation (Kd) and passage rates (Kp; NRC, 2001). Degradation represents the proportion of nutrient that disappears at a given moment in time, whereas Kp represents the proportion of nutrient that flows from the rumen to the intestine. Therefore, both processes can be expressed as %/h, and both Kd and Kp can be used to calculate rumen degradable (RDP) and rumen undegradable protein (RUP) fractions in feed ingredients (NRC, 2001).

To determine Kd, researchers typically incubate feedstuffs in the rumen of cannulated cows and model the degradation and disappearance of nutrients over time. Based on these degradation kinetics, protein can be classified into three main fractions: A, B, and C (expressed as % of CP). Fraction A is considered rapidly degradable, whereas fraction C is considered undegradable in the rumen. Fraction B degrades more slowly than fraction A but can still be degraded, provided it remains in the rumen long enough and does not pass to the intestine. As the reader can infer, the determination of protein fractions in feeds is labor- and resource-intensive, limiting the routine estimation of ruminal degradation kinetics of nutrients in feed ingredients by researchers and nutritionists. Therefore, diet formulation typically relies on modeling approaches and library values for protein fractions.

According to NASEM (2021), fractions A, B, and C in raw soybeans are 26%, 74%, and 0%, respectively, whereas in roasted soybeans these fractions are 18%, 77%, and 5% (Table 1). Although these numerical differences may appear small, they can translate into substantial differences in ruminal degradation kinetics. Using the NASEM (2021) library values and assuming these soybeans have the same particle size, it is possible to model degradation and disappearance (i.e., 100 – degradation) for both soybean types (Figure 1A and 1B, respectively). Assuming a Kp of 7%/h for both soybeans, the calculated Kd values are 8%/h for raw soybeans and 5%/h for roasted soybeans. Correspondingly, RDP is 65.5% and 50.1%, and RUP is 34.5% and 49.9% for raw and roasted soybeans, respectively.

¹SD = standard deviation.
²N = number of samples used for the analysis.
³Relative change, % calculated as Whole Roasted ÷ Whole Raw × 100.
⁴ADIP = acid detergent insoluble protein; NDIP = neutral detergent insoluble protein; TFA = total fatty acids.

It should be noted that roasting soybeans has minimal impact on CP and total fatty acid concentrations (expressed as % of DM). The major changes occur in the distribution of protein fractions, particularly soluble protein and the acid detergent– and neutral detergent–insoluble protein variables (ADIP and NDIP, respectively). Thermal processing decreases soluble CP (i.e., protein that is rapidly solubilized in the rumen) and increases ADIP and NDIP, which are commonly used as laboratory indicators of protein solubility in feedstuffs. In practice, conventional laboratory analyses cannot directly quantify changes in RDP and RUP in feeds. Instead, they provide indirect indicators (e.g., soluble CP, ADIP, and NDIP) that are difficult to translate into expected animal responses during ration formulation. Therefore, more specific laboratory measurements are needed to accurately evaluate the effects of thermal processing on soybeans. These methods will be discussed in the third article of this series.

At this point, the reader may ask: What are the implications of changing the RUP concentration in soybeans on milk production? To explore this question, we considered two diets formulated for a multiparous Holstein cow weighing 694 kg, at 157 days in milk, producing 50.2 kg/d of milk with 3.8% fat and 3.2% true protein, and consuming 28.2 kg/d of dry matter (NASEM, 2021). The only difference between the diets was the inclusion of 1.4 kg (3.1 lb; DM basis) of either raw or roasted soybeans. As shown in Table 2, a 0.3-percentage unit difference in RUP content between diets resulted in a 5.5% increase in the contribution of RUP to metabolizable protein (MP; the sum of amino acids digested and absorbed in the intestine), but only a 1.5% increase in total MP supply and a 2 g/d increase in metabolizable lysine supply, according to NASEM (2021) predictions.

¹Calculated or estimated using NASEM (2021).
²Diets included 1.4 kg (3.1 lbs; DM basis) of raw or roasted soybeans.
³ME = metabolizable energy; NEL = net energy for lactation; MP = metabolizable protein.

When considering the predicted (NASEM, 2021) milk yield and milk component yields for these diets, there were no differences in NEL-allowable milk, nutrient-predicted milk, or nutrient-predicted milk true protein (Table 3). Only MP-allowable milk was 0.9 kg/d greater for roasted compared with raw soybeans. It should be noted, however, that NASEM (2021) advises against the use of ME- or NEL-allowable milk and MP-allowable milk to predict the performance response of dairy cows to dietary changes. According to NASEM (2021), both equations exhibit severe bias, and the most appropriate metric for predicting changes in performance is nutrient-predicted milk true protein. Based on this metric, no production differences would be expected when feeding roasted versus raw soybeans; however, this may not reflect the true biological response. This highlights an important point: nutritional models are useful tools, but they do not always precisely or accurately capture the full biological response of dairy cows. Therefore, nutritionists are encouraged to rely on high-quality research in which similar dietary treatments have been directly compared.

¹Variables are expressed as kg/d.
²Diets included 1.4 kg (3.1 lbs; DM basis) of raw or roasted soybeans.
³NEL = Net energy for lactation; MP = metabolizable protein.

Reports in the literature indicate that, when diets are isoenergetic and isonitrogenous, replacing raw with roasted soybeans is expected to increase milk production in dairy cows. For instance, Faldet and Satter (1991) reported an 8 lb/d increase in milk yield for dairy cows supplemented with roasted soybeans compared with cows fed raw soybeans or soybean meal. These same authors also summarized the literature and reported overall increases of up to 1.5 kg/d (3.3 lb/d) and 2.9 kg/d (6.4 lb/d) in milk yield or fat-corrected milk when heat-treated soybeans were compared with raw soybeans or soybean meal, respectively. More recently, Bales and Lock (2024) demonstrated that feeding roasted versus raw high-oleic soybeans increased energy-corrected milk in dairy cows (101.4 vs. 108.0 lb/d).

Overall, thermal processing of soybeans increases the amount of protein that escapes rumen degradation, resulting in greater RUP and more amino acids available for absorption in the intestine. However, these benefits depend on having enough RDP in the diet to support proper rumen fermentation and microbial protein production. In the next article of this series, I will discuss how to properly heat-treat soybeans, and which laboratory tests can be used to evaluate roasting quality. Excessive heating may decrease intestinal digestibility of RUP (i.e., amino acids), which could compromise the benefits of roasting soybeans.

References

Bales, A. M., and A. L. Lock. 2024. Effects of raw and roasted high oleic soybeans on milk production of high-producing dairy cows. J. Dairy Sci. 107:10869–10881. doi.org/10.3168/jds.2024-25092.

Faldet, M. A., and L. D. Satter. 1991. Feeding Heat-Treated Full Fat Soybeans to Cows in Early Lactation. J. Dairy Sci. 74:3047-3054. doi.org/10.3168/jds.S0022-0302(91)78490-7.

NASEM (National Academies of Sciences, Engineering, and Medicine). 2021. Nutrient Requirements of Dairy Cattle. 8th rev. ed. The National Academies Press. doi.org/10.17226/25806.

NRC. 2001. Nutrient Requirements of Dairy Cattle. 7th rev. ed. Natl. Acad. Press.