Expert Answers - Jan. 16, 2009

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Answer provided by Mike Hutjens, extension dairy nutritionist at the University of Illinois

Q: If manure is tested for starch content, what guidelines can be used to estimate optimal values and potential losses? 

A: Testing manure for starch content remains controversial, but may be a tool to evaluate grain and corn silage processing, forage particle size, rumen fermentation, and site of starch digestion. University of Pennsylvania researchers developed an equation to estimate total tract starch digestibility based on 72 samples from various groups in eight herds. The equation listed below has an R-squared value of 0.73 (which explains 73 percent of the observed variation in starch digestion).

Pennsylvania formula: Apparent digestibility of starch = 0.9872 - (0.0176 times percent fecal starch).

All values are expressed on a 100 percent dry-matter basis. For example, if a composite group or herd fecal starch analysis contains 10 percent starch (on a dry-matter basis), the apparent digestibility of starch is 81 percent [0.9872 – (0.0176 x 10 percent starch or 0.1760)].

University of Illinois researchers collected fecal samples from 19 herds in southwestern Illinois. The average fecal starch content was 6 percent, with a range of 3.9 to 9.9 percent. This resulted in an average starch digestibility of 84.6 using the following equation:

Illinois formula: Apparent digestibility of starch = 93.73 - (2.61 times fecal starch percent) + (0.91 times fecal lignin percent).

All values are expressed on a 100 percent dry-matter basis. The Illinois equation had an R-squared value of 0.73 when including percent fecal starch and percent fecal lignin.

Other variables evaluated, but not declared significant, were: percent feed starch level, percent feed NDF, percent feed lignin, milk yield, level of corn silage, dry matter intake, percent fecal NDF, percent fecal dry matter, inclusion of straw (four herds), and inclusion of molasses (seven herds).

The Illinois test costs $13 per sample for fecal starch. However, this does not include fecal lignin, which can be difficult to test in commercial labs and adds $16 to the starch analysis.

We recommend testing for fecal starch using the Pennsylvania equation.

Pennsylvania researchers suggest that fecal starch be under 5 percent, and that each additional one-percentage-point decline in fecal starch can support 0.67 pound of additional milk at the same dry-matter intake.

When taking a composite sample, be sure to sample five to 10 cows and send in one to two cups of manure for testing (check with your lab first).

Fecal-starch testing may be another tool to use when evaluating your clients’ nutritional management.

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