OMAFRA UPDATE

Grinding it Out:

Acidosis Risk Factors, Why Does Particle Size Matter to Feedlots?

By Megan Van Schaik, Beef Cattle Specialist, OMAFRA • www.omafra.on.ca; Dr. Katie Wood, Associate Professor, University of Guelph; and Emily Conlin, University of Guelph

With small cereal harvest behind us for the year and corn harvest in the midst, it’s a good time to be thinking about how grain processing impacts cattle health and performance. The suite of management tools to address and prevent digestive upsets and correct performance issues needs to include an evaluation of grain processing. There is a fine balance between over and under processing. Grinding grain too finely can lead to digestive upsets and related conditions such as bloat, ruminal acidosis, and laminitis, and a lack of processing can reduce feed utilization efficiency.

Both grain type and degree of processing influences the rate and extent at which grain is fermented in the rumen. As a rule, wheat ferments the fastest of the grains in the rumen and corn ferments the slowest, influenced by the type and extent of processing (Figure 1).

Figure 1: Figure 1. Rate of digestion as influenced by grain type and processing method. Adapted however this hierarchy is from Stock and Erickson (2006)

Grain is processed to achieve increased starch availability and digestibility in feedlot rations. Generally, greater processing is related to improved dry matter and starch digestibility, which can translate to improved feed conversion efficiencies. Processing grains through steam-flaking, rolling, or grinding with a hammermill enhances total tract starch digestibility. However, in any processing scenario, particle size distribution (the range of particle sizes in a sample) and specifically the extent of fines produced must be monitored to manage risk of reduced rumen pH and digestive upsets.

It is not possible to identify an overarching “sweet spot” for grind size and extent of processing because ideal particle size is a function of a number of variables in a feeding program, including overall composition of the ration, type of grain processing, and moisture levels. The level of roughage inclusion in the ration influences the impact of fine grinding, where increased levels of effective fibre can offset some of the negative impacts of extensive processing. The effect of roughage level in the diet was demonstrated in a study conducted by North Dakota State Researchers where improved feed efficiencies were observed when fine-grinding corn was fed in a ration with adequate forage levels (greater than 15.5% DM). Some studies have also shown that the impact of processing method for corn grain may be influenced by inclusion of wet distillers grains, suggesting that there is a positive effect of wet distillers in binding fines and improving uniformity in the diet. However, more research is needed to fully understand the interaction between various feed ingredients and grain grind size.

The processing method in itself can impact starch digestibility and risk of digestive upsets. For example, steam flaking and ensiling can increase the overall total tract digestibility of starch. Steam flaking and ensiling grains tend to shift digestion to the small intestine, which not only contributes to increasing digestive efficiencies, but also reduces risk of ruminal acidosis. Further to this, University of Nebraska research suggests that feeding a combination of different grains or like grains subjected to different processing methods, such as a combination of high-moisture grain and dry grain, can have positive benefits to performance and can reduce risk of acidosis.

Monitoring Particle Size

In a study to investigate acidosis risk factors in Ontario feedlots in 2019, particle size of processed corn used in feeding programs was assessed (Figure 2). Flaked corn and high moisture corn tended to have a higher average particle size with less variability while corn screenings tended to have a lower average particle size with higher variability. While these results are not surprising, what is important to note is that there was variability in particle size even within each processing category, which emphasizes the need to monitor grain particle size of grains for each individual feeding program. For example, fines can still be produced during the steam-flaking process. Flakes that are thinner and are subjected to a longer steaming time before flaking results in a flake that is less brittle and less likely to produce fines.

Figure 2. Average particle size of processed corn samples across participating Ontario feedlots, 2018-2019

In the absence of hard and fast recommendations for particle size distribution across all feedlot cattle ration scenarios, it is important to have a sense of grain particle size distribution for ration formulation purposes and when troubleshooting animal performance and health issues. The underlying principals are that an abundance of fines from processing in the ration puts cattle at higher risk of digestive upsets and grains that are under-processed can negatively impact performance by reducing feed utilization efficiency. Keep in mind that intended particle size distribution may differ from actual particle size distribution, and settings/maintenance on processing equipment must be monitored.

Particle size can be monitored using a number of different methods. RoTap Sieving is the standard laboratory method for determining particle size distribution. This method involves a sieve stack and an automated sieving motion. This method generates a geometric mean particle size but the equipment tends to be cumbersome and impractical for on-farm use. Alternatively, the relative corn index can be generated using a sieve stack that can be brought on farm. The sieve stack (Figure 2) consists of sieve mesh sizes of #4 (4.75 mm), #8 (2.4 mm), #16 (1.01 mm), #30 (0.54 mm) and a bottom pan. An index is used based on the weights of grain particles on each sieve (Figure 3). The Relative Corn Index was developed by Dr. Mike Hutjens (University of Illinois) and can be used as a relative measure to monitor particle size distribution in grains.

Figure 3. Sieve stack and index used to determine the Relative Corn Index, developed by Dr. Mike Hutjens (Selger, Bill)

This is a quick assessment that can be done on farm and is particularly helpful in monitoring changes in particle size distribution. Data collected from our feedlot studies in Ontario in 2018 and 2019 show a close relationship between the mean particle size generated from the Ro-Tap and the Relative Corn Index generated by the specified sieve stack, further emphasizing that the Relative Corn Index is a useful and practical tool to monitor changes to grain particle size over time (Figure 4).

Figure 4. Relationship between the geometric mean particle size (determined by Ro-tap) and Relative Corn Index of processed corn samples across participating Ontario feedlots, 2018-2019

Feedback Mechanisms

Particle size of grain should be monitored proactively, but reassessment is warranted when performance issues or digestive upsets occur. Monitoring fecal starch is an effective tool for monitoring starch digestion, as it provides a measure of the concentration of undigested starch. This unutilized starch represents a direct cost to the feedlot operator. Where over-processed grains may lead to digestive upsets, a high fecal starch value may indicate insufficient processing.

In summary, it is important to monitor grain particle size to ensure grains in your feedlot rations are not being over or under processed. Processed grains can be proactively monitored for particle size, and a fecal starch tests can also provide insight into starch digestion. Work with you feed representative to set targets for and monitor particle size.

For a complete listing of references, please contact one of the authors of this article. OB