Soybeans for Dairy: Heat Treatment and Antinutritional Factors

There are two major aspects to consider when processing soybeans for dairy cows: thermal processing and physical processing. This series of articles will explore both, focusing on how heat treatment (thermal processing) and particle size (physical processing) affect dairy cattle performance and nutrient use efficiency. In this first article, I highlight the presence of antinutritional factors in raw soybeans and discuss their potential negative effects on nutrient utilization in dairy cattle.

The nutritional value of soybeans is largely determined by the amino acid composition of their protein and their rich fatty acid content. However, the full nutritional potential of soybean protein can only be realized after an appropriate degree of heat treatment has been applied. Raw soybeans contain antinutritional factors that may interfere with efficient protein utilization. These naturally occurring compounds can inhibit digestive enzymes and slow nutrient digestion in the gastrointestinal tract. Although their effects are often overlooked in ruminant nutrition—since the rumen is generally capable of neutralizing these antinutritional factors—they become particularly relevant when the goal is to optimize nutrient supply for young calves (especially during the preweaning phase) and potentially for high-producing dairy cows.

Antinutritional factors in soybeans can be divided into two main categories (Table 1; Liener, 1981). The first group includes heat-labile compounds such as protease inhibitors (e.g., trypsin inhibitors), ureases, hemagglutinins, goitrogens, antivitamins, and phytates. These factors are effectively reduced, or even fully inactivated, through heating. The second group consists of heat-stable compounds, such as saponins, estrogens, allergens, and certain flatulence factors, which are largely unaffected by roasting. A brief note is warranted regarding flatulence factors. One might be tempted to assume that feeding soybeans would cause cows to produce more gas from their "lower end” and thus contribute to global warming. In reality, this is not the case. Cows do not contribute to greenhouse gas emissions from flatulence, but primarily from enteric methane (CH₄) released through eructation (belching); and feeding soybeans may, in fact, help mitigate CH₄ intensity, expressed as grams of CH₄/kilogram of energy-corrected milk (ECM). Flatulence factors may be instead a concern for humans, particularly those shifting from animal protein to plant-based protein sources. Humor aside, the most important early effect of heating soybeans is the rapid decline in protease inhibitor activity and ureases as heating progresses. In practical terms, this improves intestinal availability of dietary protein, a point that will be further demonstrated later in this article.

¹Adapted from Liener et al., (1981).

 When considering preweaning calves, Ansia and Drackley (2020) reviewed the literature on the use of soy products in milk replacers and concluded that processing methods play a critical role in the effective removal of antinutritional factors, thereby influencing calf performance. Conventional processing techniques, such as heat treatment, are limited in their ability to fully eliminate or inactivate these compounds for dairy calves. Instead, they primarily serve to increase the proportion of bypass protein (rumen undegradable protein, RUP). Alternative extraction methods (e.g., soy protein isolates or soy protein concentrate) have shown more promising results when incorporated into milk replacers (Ansia and Drackley, 2020). These forms of soy protein contain minimal or negligible amounts of antinutritional factors and may perform comparably to milk proteins.

The most recent nutrient recommendations for dairy cattle published by the National Academies of Sciences, Engineering, and Medicine (NASEM, 2021) account for the potential negative effects of feeding plant-based proteins in milk replacers. For example, in a 4-week-old calf weighing 132 lbs and consuming 1.43 lbs (dry matter basis) of milk replacer containing 20% crude protein and 20% fat, along with 1.48 lbs/day of starter, the models predict a 0.15-lb lower average daily gain when plant proteins are included in the replacer compared with milk proteins (i.e., metabolizable protein allowable growth prediction). While these models are only predictive tools—and calves may still grow adequately when consuming plant-based milk replacers—they illustrate the potential performance penalty associated with feeding young calves with such protein sources.

¹CP = crude protein; ME = metabolizable energy; MP = metabolizable protein

In lactating cows, antinutritional factors are generally not a major concern, as the rumen is typically able to neutralize them effectively. Petzel et al. (2024) investigated the effects of soybean-derived trypsin inhibitors on nutrient utilization and lactational performance, specifically testing whether inhibitors from raw soybeans impair protein use. Roasted and unroasted soybeans were included at 10% of dry matter intake (DMI), and different forms of casein were infused directly into the abomasum to assess intestinal protein digestibility. The unroasted diet contained 8,273 units/g DM of trypsin inhibitor compared with 373 units/g DM in the roasted diet.

Overall, the researchers found no strong evidence of an interaction between soybean type (roasted vs. unroasted) and casein infusion. Importantly, the presence of trypsin inhibitors did not negatively affect nutrient digestibility or plasma amino acid concentrations, suggesting minimal impact of these compounds under the study conditions (Petzel et al., 2024). However, it is worth noting that the cows in this trial were in a positive protein balance, leaving open the question of whether trypsin inhibitors might play a greater role when cows are fed lower-protein diets that rely more heavily on nutrient digestibility. Additionally, cows fed roasted soybeans exhibited significantly greater DMI, which may have contributed to the lack of treatment differences in nutrient digestibility.

The study by Bales and Lock (2024) raised new questions about the potential negative effects of antinutritional factors in high-producing cows. Using an elegant experimental design, they evaluated the effects of feeding raw and roasted high-oleic soybeans. Their study tested: (1) replacing soybean meal in the control diet with 16% high-oleic soybeans, (2) the effects of roasting compared with feeding raw soybeans, and (3) the effects of feeding raw soybeans supplemented with a soybean meal–based bypass protein. Results showed that cows fed roasted (high-oleic) soybeans produced more ECM than those fed diets containing soybean meal only (108.0 vs. 97.0 lbs/day). Although the study did not directly compare raw soybeans plus bypass protein with roasted soybeans, mean estimates were 102.3 and 108.0 lbs/day of ECM, respectively, with only a tendency for bypass protein supplementation to support greater production than raw soybeans alone (100.5 lbs ECM/d). These findings led me to speculate that antinutritional factors may have impaired, to some extent, protein utilization when feeding raw soybeans, even with bypass protein supplementation.

Overall, antinutritional factors in soybeans have long been recognized for their negative impact in young calves but are often considered inconsequential in lactating cows. However, as nutritionists continue striving for greater precision feeding and improved nitrogen efficiency, the potential influence of these compounds in high-producing cows warrants renewed attention. Feeding roasted soybeans remains an effective strategy to minimize or eliminate antinutritional factors while also increasing bypass protein supply—a subject that will be further explored in the next article of this series.

References

Ansia, I., and J. K. Drackley. 2020. Graduate Student Literature Review: The past and future of soy protein in calf nutrition. J. Dairy Sci. 103:7625–7638. Doi.org/10.3168/jds.2020-18280.

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.

Liener, I. E., 1981. Factors affecting the nutritional quality of soya products. J Am Oil Chem Soc 58, 406–415. Doi.org/10.1007/BF02582390.

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.

Petzel, E. A., S. Acharya, E. C. Titgemeyer, E. A. Bailey, and D. W. Brake. 2024. Effects of heating soybeans on postruminal amino acid bioavailability, performance, and ruminal fermentation in lactating cows. J. Anim. Sci., 2024, 102, skae084. Doi.org/10.1093/jas/skae084.