With the increased cost of grain and forages in the USA, dairy producers have turned their attention to the feed efficiency of milk production. Low on-farm starch digestibility contributes to reduced feed efficiency. Several researchers have demonstrated that measuring fecal starch (FS) is highly correlated to total tract starch digestibility in dairy cows. One factor affecting the digestibility of starch in lactating dairy cows is the extent of the processing of the corn kernel during the harvesting of corn silage. Kernel processors are employed on the forage harvesters to break the corn kernel into smaller fractions. The extent of kernel processing varies based upon the management of the kernel processing equipment, kernel maturity, hardness of the corn, and extent of corn silage fermentation. A kernel processing score (KPS) was developed by Mertens (2005) which involves submitting a corn silage sample to a laboratory where it is dried and sifted through variable sized mesh sieves. A starch analysis is performed and the portion of the starch that passes through a 4.75 millimeter sieve is determined more digestible by the lactating dairy cow. Guidelines for KPS are >70%, excellent; 50 to 70%, adequate; and <50% poor.
Materials and methods
From December, 2012 to July, 2013, 35 dairy operations ranging in size from approximately 1,200 to 6,800 cows participated in a study to evaluate the effect of KPS of ensiled corn silage on the content of starch in the feces of cows fed the corresponding corn silage. These dairy operations which represented a total of approximately 58,000 cows were located in the states of Wisconsin, Iowa, Minnesota, and Illinois. A minimum of at least 6 core samples from the face of corn silage bunkers or drive-over piles was consolidated and a sub-sample sent to a commercial lab (Rock River Laboratories, Watertown, Wisconsin) for KPS determination using the method of Mertens (2005). On the same farms and same day, fresh floor fecal samples were collected from at least 20 cows per pen from pens with cows less than 120 days-in-milk with a composite sample analysed for FS by Rock River Laboratories (Watertown, WI). Some dairies were sampled more than once as they changed sources of corn silage.
Results and discussion
Figure 1 plots the KPS score compared to the respective % FS (DM basis) of each dairy arranged from high to low KPS. Linear lines were plotted for both KPS and % fecal starch. A negative relationship between % FS and KPS % (R2 = 0.58, P = 0.001, Figure 2) was observed. Ferguson (2003) established a positive relationship (R2 = 0.782) between FS and total tract starch digestibility (TTSD) in lactating dairy cows. Also, Ferraretto and Shaver (2012) demonstrated a negative relationship between FS and total tract starch digestibility (R2 = 0.94). Ferguson (2003) estimated that for each one percentage unit increase in FS above 5% (DM basis), a corresponding decrease of 0.35 kg of milk yield per cow per day can be expected.
This excess of starch in manure decreases feed efficiency of milk production, adds to the manure load on a dairy farm, and represents wasted money on feed that is not digested.
Figure 1. Percent fecal starch plotted against respective corn silage kernel processing score (%) for each dairy sample (n=47).
Figure 2. Relationship between kernel processing score and % fecal starch (DM basis). % fecal starch prediction equation y = 12.90 (±1.04) – 0.15x; R2 = 0.58; P=0.001.
There was a wide variation in kernel processing scores (34-76%) on the Midwest USA farms that participated in this research. A high negative correlation between corn kernel processing score and fecal starch in high producing lactating dairy cows suggests that improving corn kernel processing during corn forage harvest is a management tool that can increase total tract starch digestibility and increase milk production efficiency.
Presented at the 2015 International Silage Conference.
Mertens, D.R. 2005. Particle size, fragmentation index, and effective fiber: Tools for evaluating the physical attributes of corn silage. Proc. 2005 Four-State Dairy Nutr. and Management Conf. pg 211-220.