Nobody likes to walk through a freestall pen and see manure laden with flecks of yellow corn pericarp. To any dairy manager, herd manager or nutritionist, “pretty poo” is all the rage – and pretty means consistently fleckless. This pretty manure is not only important to the diet but also a key part of every dairy’s bottom line. As a major quantifier of the TMR’s starch digestibility, fecal starch percentage can weigh heavily on the make-or-break of a dairy’s feeding program.
Although fecal starch content is a major indicator of the diet’s success, it doesn’t provide information regarding specific feeds or forages. The key is assessing the individual starch sources in the diet to determine what is influencing a suboptimal fecal starch percentage.
Protocols
Whenever you seek to understand a complex situation with multiple factors at play, a thoughtful protocol is required. Evaluating fecal starch is no exception. Fecal starch should be analyzed at least once per year and at least three months after putting fermented starch feeds (such as corn silage, high-moisture corn, snaplage, etc.) into storage. Significant changes to corn grain particle size, feed moisture, feed maturity, etc., should also motivate fecal starch analysis.
Fecal starch samples should be taken in the high-cow or early-lactation pens. We recommend taking 10 to 12 samples from fresh, undisturbed manure piles of healthy, high-producing cows. Samples are best collected in a jar supplied by your laboratory. Maintain room for air between the samples and the cover to keep the jar from exploding in transit (lest your friendly package carrier deny future shipments).
This protocol may seem intense. However, when you consider the dollars you could potentially save by improving fecal starch content, it becomes a very worthy investment of time.
Results
Now the fun part: results receipt and tracking. How does your herd compare?
If the fecal starch content is less than 3% and total tract starch digestibility is greater than 96%, you should continue to track starch sources to maintain this optimal starch digestion (i.e., pretty manure).
Conversely, if fecal starch is greater than 3%, opportunities for improvement exist. Evaluate the starch feeds in your diet to identify the causes of suboptimal fecal starch content.
I typically recommend checking corn silage kernel processing scores (KPS) next by sending a sample to the laboratory. Samples with a laboratory KPS score of at least 70% (that is 70% of the starch passes through a 4.75-millimeter screen) are considered optimally processed. If your corn silage returns a KPS score of less than 70%, no immediate actions can be taken. However, long-term, you can use this information to discuss targets with your harvesting team during a pre-corn silage harvest meeting. Adjust the processor rolls accordingly and continually monitor kernel processing as harvest progresses.
Fecal starch content can also be elevated due to coarse particles in dry corn or high-moisture corn. In this case, you can take immediate action to regrind and process more aggressively, depending on the storage structure currently in use.
The economics
How does fecal starch content impact the bottom line? As fecal starch increases above the 3% target, many variables are suddenly at play. Research has shown that, with each percentage point increase above 3%, milk yield can be reduced by 0.7 pound per cow per day.
A team at the University of Wisconsin – Madison has calculated the value of lost starch in manure based on an equivalent amount of digestible starch in ground dry corn. The MILK2024 calculator takes into account fecal starch, dollars per bushel of corn grain, dry matter intake (DMI), dietary starch concentration (on a dry matter [DM] percentage basis), corn grain starch concentration (DM percentage basis) and in vitro or in situ ruminal starch digestibility of grain corn (as a percentage of starch).
To appropriately utilize the calculator, first determine how much starch is not being digested by the cow and is instead excreted into the manure. Next, estimate the amount of digestible starch for each unit of dry corn. This step requires predicting starch rate of disappearance (kd, percentage per hour), total tract starch digestibility of corn grain (as a percentage of starch) and concentration of digestible starch (percentage of DM). Starch kd is predicted using in vitro or in situ starch digestibility and a lag time of one hour. Based on rates of disappearance and passage (kp; 12% per hour), ruminal starch digestibility is predicted and total tract starch digestibility is estimated. The model calculates the amount of extra corn grain (pounds per cow per day) needed to replace the starch lost in manure using digestible starch (pounds per pound of corn). Based on corn prices, the cost per cow per day is estimated and provided.
For example, consider a herd with a dry matter intake (DMI) of 56 pounds eating a 26% DM starch diet. The diet includes high-moisture corn with 72% starch and 78% starch digestibility. Dry corn is valued at $4.45 per bushel. In this scenario, the loss of corn grain is 1.07 pounds per cow and the cost is 8 cents per cow per day.
Reality check
From January 2023 through mid-April 2024, we conducted a fecal starch survey of 35 herds. The fecal starch content across all herds ranged from 0.26% to 12.68% and averaged 3.78%. Total tract starch digestibility averaged 95.28%. This indicates some room for improvement. The average corn silage KPS was 72.93%, the average dry corn micron size was 372, and the average high-moisture corn micron size was 841.
Four of the farms achieved a fecal starch content of less than 1%, and 13 farms achieved between 1% and 2% fecal starch. This is outstanding, and we will continue to monitor harvesting and management practices to maintain these scores.
Variation in fecal starch across herds exists due to differences in grain and corn silage management. One of the herds surveyed is a great example of this. The farm started with 12.68% fecal starch. The high-moisture corn was harvested at 75.41% DM and stored in a bottom-unloading upright silo. This corn was processed at feedout with an older roller mill that produced a 1,310-micron size. Milk production was at 90 pounds per cow per day, but farm managers observed digestive upsets from hindgut fermentation challenges as a significant amount of starch was passing through the rumen to the hindgut.
This farm purchased a newer hammermill and processed the corn to 322 microns. Fecal starch retested at 0.31%. This contributed to a milk production increase of 5 pounds per cow per day. The farm also reported significantly fewer digestive upsets.
Manure may not be a pretty thing to look at or speak about, but its significance in determining starch efficiency and, in turn, the ability to dial in diet economics is priceless. Work with your nutritionist to regularly analyze your herd’s starch efficiency to bolster your bottom line. In today’s dairy industry, these efficiencies aren’t a want, they are a need.
