2019 Survey of Fecal Starch in Michigan Feedlots

Corn is the predominant source of energy in the beef cattle diets of Michigan feedlots. Starch typically represents 68-72% of corn grain dry matter and is the primary energy contributor. Reducing undigested starch excreted in manure may represent a significant opportunity to improve feed efficiency and farm profitability.

Several management factors influence the extent of starch digestion. Harvest % dry matter (%DM), kernel particle size reduction (processing), and ensiling are the three most impactful variables.

Measuring the amount of starch secreted in manure is an indicator of lost opportunity in feed energy. While target fecal starch levels are relatively consistent in the study of dairy nutrition, beef feedlots tend to have more variability in measured fecal starch. A portion of observed feedlot fecal starch variability can be attributed to varying levels of starch in the diet, while variability in corn grain %DM and processing represent the other primary driver.

During the spring of 2019, a survey was conducted of Michigan beef feedlots to assess the status of starch digestion and identify opportunities to enhance feed efficiency. Fecal and TMR samples were collected from 10 feedlots, representing 10 finisher and 9 grower diets. Nine of 10 farms were feeding Holstein steers. Fecal starch was measured through Dairyland Laboratory. Manure samples were also washed over a strainer for visual assessment. TMR samples were processed through a Penn State particle separator. Starch sources were identified. Extent of corn grain processing was labeled as either extensive (+) or modest (-) based on the kernel processing cup for corn silage and visual appraisal for other corn grain sources.

Fecal starch content was highly variable, ranging from 1.5% to 39.5% of fecal dry matter. Ranking finisher samples by fecal starch content (Figure 1) showed a relationship to moisture content and processing of corn sources in the diet. Grower diets tended to not follow the same pattern.

It was noticed that the Penn State particle separator typically captured whole and minimally processed corn kernels in the middle pan (<19mm,>8mm) (Figure 2). Very few particles resided in the top pan (>19mm). All but one farm’s diets showed modest to low forage fiber content. The lone high forage diet was excluded as an outlier in this particle size analysis.

Finisher fecal starch tended to correlate most closely with % as-fed in middle pan of particle separator (Figure 3). This suggests kernel particle size, i.e. processing, is strongly correlated to fecal starch content.

Extent of starch digestion is highly variable between feedlots. TMR starch levels at estimated 40-60% of dry matter contribute to much of the differences, while management of corn sources explain another large portion of the variability. As expected, increased dependency on high moisture, ensiled corn products and more extensive processing related to lower fecal starch levels. Washed manure samples (Figure 4) have some, but limited value in evaluating fecal starch.

Opportunity may exist to increase feed efficiency through the more extensive processing of corn. Greater dependence on high moisture, ensiled sources of corn grain also enhances starch digestion. Maintaining rumen health and optimizing rate of gain should also be considered when enhancing rumen availability and digestion of starch.