Back to the Grind
By: Katie Raver, Animal Nutrition Technical Services Director
Fall’s arrival often marks the end of vacation season as school is back in session, harvest is in full swing, and we are getting back to the flow and the grind. In the spirit of harvest, particle size has been a frequent topic of discussion, not only as it relates to corn silage and kernel processing, but also corn grain, and the new kids on the block: sorghum silage and high oleic soybeans.
In the context of nutrient availability, we often think about the volume of each nutrient present in the feed, then multiply by how much is available for use by the animal. Seems pretty straightforward, right? Well, determining availability can be quite the daunting task. Although from the lab perspective we can give estimations on the disappearance of nutrients over time, it is more challenging to define the complex relationships between disappearance and in vivo digestibility: what a real cow does with this feed. Digestibility can be influenced by a number of factors, including eating behavior, rumen environment, and especially particle size of the feed.
Due to ruminants’ unique physiology, they employ a 2-step chewing process: the first upon ingestion and then, through rumination, regurgitate and rechew repeatedly. While this is a fairly simplistic concept, it actually creates further distinctions between how we prepare feed for ruminants, which is also unique. This, combined with the microbial fermentation of feed, makes particle size a critical assessment in understanding how a cow can utilize that feed.
Because ruminants ruminate, the initial chewing process is reduced as compared to monogastrics, which leads to less particle size reduction. We know that increased surface area improves microbial availability. However, the balance point is not to make grains too fine, where they ferment too quickly and cause a sudden drop in pH. For forages, much of the particle size reduction will happen over time through a process of cud chewing and microbial fermentation, until feed is broken down enough to pass through the rumen. For starches, sugars, fats, and proteins, enzymatic digestion continues in the post-ruminal digestive tract. This, too, is a balance, as we want to ensure we feed the microbes in the rumen to maximize microbial protein production and volatile fatty acid (VFA) production. But we know that there is a benefit to delivering precise nutrients directly to the cow via post-ruminal digestion. Although fiber may undergo some degree of hindgut fermentation, the amount is minimal compared to the rumen.
Thinking about these unique dynamics is a key part of determining what feed needs to look like - from a nutrient disappearance perspective and from its physical characteristics - to achieve the goal of delivering optimal nutrients to maximize the cow’s productivity. I’m often tempted to simply look at nutrient disappearance when assessing the quality of a particular feed ingredient, however, it’s critical not to fall into this trap. Assessing and considering particle size is also an essential component of knowing how a feed will perform.
Sorghum is an extreme example of this. For years, most of the industry has severely discounted the starch levels of sorghum silage. Without processing the berry, starch contained in the berry would be mostly unavailable to rumen microbes and enzymatic digestion in the hind gut. If we were to grind the sample in the lab and run it through in situ starch assays, we may see a 7-hour disappearance of around 75 percent. With starch content around 25 percent, we would calculate rumen degradable starch using the equation proposed by Fernandez et al. 2018 as 17.5 percent of dry matter (DM). Now, with new sorghum processing capabilities, this may be the new reality. However, without particle size assessment, we don’t have a trusted barometer of how cows will respond when rumen starch availability may range from less than 10 percent to over 75 percent.
Particle size doesn’t end at corn and sorghum. High oleic soybeans have entered the conversation. Decades ago, many producers fed these halved or cracked. However, new research from Michigan State University has suggested finer grinding to around 700-900 microns. This is an even more dynamic decision as we must assess the combination of roasting and particle size to determine how much protein will bypass the rumen. On top of that, we must consider the impact of grinding on the fat content. Both fat and protein are of primary interest. A finer grind will lead to higher availability, but could decrease the rumen undegradable protein (RUP) achieved by roasting. What’s the right answer? It depends on the other ration components and the farm’s goals. Regardless of these goals, particle size will always have an impact on the performance outcome.
In some cases, particle size is straightforward, and smaller is better. In other cases, more considerations need to be taken into account. One thing remains true: knowing the particle size of feed is essential to understanding how and where it will be utilized in the animal. It can be tempting to get hung up on a single value, but assessing digestibility requires multiple inputs. As we get back to the grind in our personal and professional lives, we can’t forget the importance of getting back to the nutrition basics and the importance of particle size.
