Estimating and Managing Protein and Fat Digestibilities

Dr. Kirby Krogstad, Assistant Professor, Department of Animal Sciences, The Ohio State University

Introduction

The most costly macronutrients included in rations fed to dairy cattle are fat and protein. Because of their expense and their necessity to the cow, we must maximize their degradability and absorption. Much like carbohydrates, maximizing the digestibilities of protein and fat require accounting for multiple factors. We will explore these factors and look at how they can be managed to benefit both the cow and the bulk tank.

Protein - Heating and Processing

When managing protein feeds, like soybean meal, distillers’ grains (DDGS), bloodmeal, or canola meal, the processing is critical. The manufacturing process for each of these protein sources includes some level of heating. The level, duration, and process of heating directly affect the degradability, site of degradation, and quality of the protein in the feed. Under-heating these feeds can leave anti-nutritional factors intact to reduce feed degradation and animal performance, while overheating can reduce degradability and degrade specific heat-susceptible amino acids. When procuring protein feeds, a thorough understanding of how they’re processed may help you maintain your animal’s performance by avoiding problematic, poorly processed feeds.

High oleic soybeans have been a hot topic in dairy nutrition for a couple of years and partially because it reminded us of the importance of heat-treatment, roasting in this case, in preparing feeds for dairy cattle. Roasted is clearly superior to non-roasted soybeans (Bales and Lock, 2024). In this experiment, the soybeans were roasted at 157°C (315°F) for 2 hr, but of course, there are many combinations of roasting times and temperatures used. The University of Wisconsin-Madison lists best practices for roasting as heating the beans to 295-300°F and steeping them for 30 minutes. Both under and overheating present risks. I suggest reading their Extension article on the topic for more detail (McCarville and Goldsmith, 2025).

Overheating is also a concern for DDGS. A paper in Journal of Dairy Science demonstrated that heating DDGS samples reduces the digestibility of intact and rumen incubated feeds. For example, Lys was only 10.8% digestible after overheating compared with a range of 56.5 to 78.2% for other typical sources of DDGS (Figure 1; Boucher et al., 2009).

Figure 1. Overheated DDGS have much lower digestible Lys than low-heat counterparts.

An additional protein feed where heat and processing can have profound impacts on feeding value is bloodmeal. In one study, the processing of the bloodmeal changed the total tract disappearance of protein from 44 up to 85%, with the lowest digestibility having been exposed to the greatest temperature and longest cook time (van Ryssen and Schroeder, 2003).

When procuring protein ingredients, or processing them yourself, take great care to source the highest quality ingredients and to process them correctly. An important metric to assess is the rumen undegradable protein which can be estimated through in vitro or in situ analyses at commercial laboratories. Additional analyses can be done to estimate total-tract disappearance.

Fatty Acids

Fatty acids are expensive; if you’re purchasing and feeding them, ensure you’re getting all the value that you can. To maximize the value, you must maximize the digestibility. The digestibility of fatty acids is influenced by the specific fatty acid fed, the blend of the supplement, and even the form of the supplement. The simplest way to demonstrate the variability in fatty acids is to refer you to NASEM (2021) Table 4-1; they list estimated fatty acid digestibility coefficients that range from 31 to 76%.

First, the specific fatty acid matters. Increasing the flow of stearic acid (C18:0) to the small intestine reduces fatty acid digestibility while increasing palmitic acid (C16:0) reduces digestibility less than additional stearic acid (Boerman et al., 2015). Recently, additional research has also demonstrated that incorporating oleic acid at 17 to 30% of the total FA supplement increases fatty acid digestibility (de Souza et al., 2019). This echoes another factor which is that highly enriched supplements, that is supplements composed of more than 90% palmitic or stearic acid, tend to have lower digestibility and should be avoided.

Conclusions

Protein and fat are best in the bulk tank. To maximize them, we need to get the details right on digestibility. For protein feeds, its all about processing and heat. Too much processing and heat, and we are at risk of losing digestibility. Not enough and we may not provide the desired undegradable protein and we also risk leaving anti-nutritional factors in the feeds in tact, which is especially important for soy-based feeds. For fats, think fatty acids. Stearic acid is less digestible than palmitic acid, but including oleic acid can improve digestibility. Also, avoid purified highly enriched fatty acids as they’re less digestible than blends.