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Roasting Soybeans for Dairy Cattle Diets
Shared from Inside the ZONE® newsletter, Pioneer Nutritional Sciences
Why Roast Soybeans?
There has been an uptick in the interest in roasting soybeans since research (Bales and Lock, 2023) findings showed that Pioneer® brand Plenish® high oleic soybeans can be incorporated into dairy diets at relatively high levels (upwards of 10 lbs/cow/day) compared to commodity soybeans. Feeding roasted Plenish soybeans provides dairy cows rumen undegraded protein (RUP) along with energy from the high oleic (low linoleic) oil, without adversely affecting butterfat levels in milk (Weld and Armentano, 2018).
Protein in raw soybeans is highly soluble and readily degraded by rumen microbes, leading to loss of valuable amino acids, high ruminal ammonia, and limited access to a potential supply of RUP. The primary reasons for roasting soybeans are to improve utilization of the protein and remove anti-nutritional factors such urease or trypsin inhibitor.
Feeding unheated (raw) soybeans may lower its feeding value to animals in general due to the presence of compounds like trypsin inhibitors, urease, lipases (Schingoethe, 1982), and hemagglutinin which can cause gastrointestinal disturbances initiated by immunological mechanisms (Sissons et al., 1982).
Soybean RUP can be increased by roasting soybeans. Since the effect of heat treatment is a function of both temperature and time, roasting conditions must be closely monitored (Hsu and Satter, 1995); as excess heat beyond the optimal has the potential to decrease RUP and intestinal digestibility of RUP-contained amino acids, particularly lysine (Faldet et al., 1992; Stern et al., 1985).
The amount of protein escaping rumen degradation is a function of retention time in the rumen (rate of passage) which is affected by feed and ration attributes like dry matter intake (DMI), particle size, particle gravity (sink/float) concentrate to forage ratio, and rate of digestion.
Research: Roasted vs. Raw
Study #1: Tice et al. (1993) compared feeding raw and roasted soybeans of different particle sizes to lactating dairy cows. Overall, cows fed raw soybeans tended to have higher ruminal pH and ammonia which can be explained by higher protein solubility. Roasting increased duodenal N flow and its digestibility, which authors related to trypsin inhibitor destruction by heat. In addition to increased total tract digestibility no other effects of roasting or particle size were reported for fat.
Smaller particle size led to increased ruminal ammonia and increased intestinal N digestion because of increased surface area exposed to proteases. In addition, smaller particle size was linearly associated to declining RUP (whole 48.3%, cracked 44.2%, and ground beans 28.8%), possibly due to microbial protease action facilitated by reduced particle size. Authors concluded that roasting soybeans is justified at high intakes (e.g., >10% of DMI) or when flow of essential amino acids might be limiting (e.g., early lactation), and production could benefit from RUP. Feeding cracked or quartered beans appears to be more advisable, as whole beans may present a challenge because tend to separate in concentrate mixtures, on the other hand grinding reduces value as a RUP source.
Cows fed roasted beans produced 6.8 lbs/d more milk than those fed raw soybeans (42.7 vs. 35.8 lb/d), while protein content tended to be higher (3.37 vs. 3.36 %). Smaller particle size was associated to DMI but did not affect milk yield significantly.
Study #2: Faldet and Satter (1991) compared feeding raw or roasted full fat soybeans to early lactation cows from 15 to 119 DIM. Authors reported heating to increase over 2x RUP (28 vs. 61% of CP), increased RUP resulted in more available lysine (2.02 vs. 2.31 % of DM), both factors could help explain increased milk protein yield. The authors observed no differences in intake, but increased milk yield (85.6 vs. 75.2 lb/d), and protein (2.42 vs. 2.22 lb/d) and a tendency for higher milk fat yield (2.88 vs. 2.68 lb/d).
Study #3: Hsu and Satter (1995) worked with early lactation cows that were fed one of 7 alternative heating by time combinations. After roasting, beans were processed in a hammer mill to produce quarter and half pieces. Maximum RUP values were observed for samples heated to above 295°F and held without cooling for more than 15 min. Also, heifers fed soybeans heated at 295°F for 30 min and 307°F for 30 min had higher plasma EAA concentrations than heifers fed raw soybeans. No statistical differences in DMI, BW or fat corrected milk were observed. However, cows fed soybeans roasted at 295ºF for 30 minutes produced numerically more FCM than cows fed raw soybeans (84.3 vs. 80.1 lb/d, p=0.08).
Particle Size of Roasted Soybeans
Research indicates that particle size affects the protein degradability characteristics of roasted soybeans. The particle size can affect how a high-producing dairy cow utilizes protein. The primary concern is that small particle protein is more likely to degrade rapidly in the rumen than large-particle protein. The results of several reputable studies indicate that, in order to retain the RUP value of the feed, the optimal particle size of roasted soybeans are whole/half and half/quarter (Weld and Armentano, 2016). In TMRs, the whole/half particle size should result in little to no separation. However, in grain mixtures or supplements, a half/quarter particle size may work better. If the goal of feeding roasted soybeans is to supply RUP, then fine grinding and pelleting are not recommended (Ishler and Varga, 2008).
Quality Parameters
Protein Dispersibility Index (PDI): PDI has been used in the feed industry for over 25 years but has only recently gained traction to distinguish the quality of roasted soybeans. PDI relates to the solubility of a feed ingredient. The protein in soybeans is highly soluble, and this solubility will decrease with heat exposure (time and temperature increases).
A PDI of 9-12% corresponds with high RUP without over-roasting, while PDI above 14% is underheated and RUP can be greatly reduced. PDI analysis involves grinding the sample to 1 mm and is an indicator of adequacy of the heat treatment. Therefore, PDI is a rough proxy for RUP because the actual dietary RUP of roasted soybeans is also affected by level of intake, particle size, particle specific gravity (sink/float) concentrate to forage ratio, and rate of digestion. Research reported a positive correlation (r2= .82) for PDI and RUP when estimated in situ (Hsu and Satter, 1995) versus a lower correlation of 0.28 between PDI and RUP estimated in vitro (Tremblay et al., 1996).
| Score | Interpretation | Comments |
|---|---|---|
| PDI <9 | Potentially over-roasted | Decrease heat; analyze lysine for heat damage |
| PDI 9-11 | High quality roast | Continue to monitor PDI, high RUP |
| PDI 11-14 | Marginally under-roasted | Increase heat or steep time; monitor PDI |
| PDI >14 | Significantly under-roasted | Evaluate UA, remove urea from diet if needed, will have significantly lower RUP |
Data from samples processed in a commercial drum roaster at 12 temperatures by time combinations (Hsu and Satter, 1995) was used to compile the chart below allowing for easier interpretation of the relationship between PDI and RUP. Note that acid detergent insoluble nitrogen (ADIN) does not effectively measure over-roasting but post ruminally available lysine (PRAL) can be used as lysine is the most susceptible to heat damage per Stern et. al., (1985) who reported that the lysine side chain is highly reactive and susceptible to non-enzymatic browning.
Urease Activity (UA): Soybeans contain urease, the enzyme that converts urea to ammonia. Destruction of the urease enzyme in soybeans by heating is correlated with destruction of trypsin inhibitors (Batal et al., 2000). UA measures presence of residual urease in soybean after heat treatment.
UA values of 0.05–0.30 are reasonable evidence that the beans have been properly cooked. If the beans are used in a TMR or a high-moisture grain mix containing urea, a range of 0.05–0.10 is preferred (Ishler and Varga, 2008). As comparison, UA of SBM has been reported to range from 0.19 pH units for under processed SBM to 0.01 and 0.02 pH units for overprocessed and ruminal escape SBM (Grieshop et al., 2003). It has been suggested that the combination of PDI and urease test would help better monitor soybean meal quality in particular (Batal et al., 2000). Urease is deactivated very quickly at 300°F, therefore properly roasted soybeans with PDI <11 should have easily deactivated urease. If improper or marginal roasting is done and PDI>11, the UA assay can be used to determine if urea needs to be removed from the diet.
Post Ruminal Available Lysine (PRAL): PRAL is a simply a calculation of RUP x available lysine. Measurement of available lysine is warranted when “over-roasting” may have occurred. Samples with a PDI>9 should not have issues with heat damaged protein, but samples with a PDI<9 may need to be evaluated.
Temperature & Steeping Time
The temperature of soybeans coming out of the roaster is very important as is the steeping time. Some processors are not getting the soybeans hot enough out of the roaster, and others are not steeping the beans. Steeping allows for temperature to uniformly reach the core of the soybean. Roasters that only heat soybeans to 250oF, with no steep, are not effectively processing soybeans resulting in lower RUP. Hsu and Satter (1995) recommended heating soybeans to 295oF and doing a 30-minute steep; thus, processors should monitor soybean temperatures out of the roaster, targeting ~300oF and provide an adequate steeping time. Another variable in a quality roasting is the temperature of the soybeans going into a roaster. Colder soybeans take longer to effectively heat compared to warmer soybeans. On-farm roasting protocols likely need to be adjusted throughout the year in environments with large temperature changes in summer vs. winter.
For more information, and a full list of references cited contact: Dr. Nelson Lobos or Dr. Adam Krull.
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