ARS | Publication request: CHEMICAL AND PHYSICAL CHARACTERISTICS OF CORN SILAGES AND THEIR EFFECTS ON IN VITRO

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 20, 2005
Publication Date: August 10, 2005
Citation: Ferreira, G., Mertens, D.R. 2005. Chemical and physical characteristics of corn silages and their effects on in vitro. Journal of Dairy Science. 88:4414-4425.

Interpretive Summary: It is important for farmers feeding corn silage to know the availability of starch so they can harvest silage effectively and formulate cow diets optimally. Starch is a major contributor to the digestible energy of corn silage for dairy cows. However, if starch is in intact kernels or large kernel fragments, it is poorly digested when animals do not chew corn silage thoroughly. Corn silage is sometimes processed, by passing it through rollers when it is chopped at harvest, to fracture kernels and make the starch more available for digestion. We developed a method to measure minimally fragmented starch in intact kernels and large fragments by sieving dried corn silage and measuring the starch retained on sieves with openings that were greater than 4.75 mm. To identify factors affecting digestion, we related minimally fragmented starch and chemical composition to the digestion of corn silages using an artificial rumen in vitro system. When silages were finely ground before in vitro digestion, only the effects of fiber composition were important in determining digestion. The more fiber and lignin in the fiber, the lower the digestibility of ground silages. However, particle size and proportion of minimally fragmented starch were important in determining the in vitro digestibility of unground silages because these factors measure the effects of poorly digested starch in kernels and large fragments. Our proposed method for measuring minimally fragmented starch provides a quantitative meaure of the effectiveness of kernel processing in corn silages that will be useful for improving the nutritional evaluation of corn silages. Our research demonstrated that starch in intact corn kernels is poorly digested, and our method will help farmers adjust harvesting procedures and improve ration formulations to insure the maximum utilization of starch in corn silage.

Technical Abstract: Estimating the available energy in corn silage provides a unique challenge because it contains variable proportions of grain and stover, each of which can differ in availability due to chemical composition and physical form. The objectives of this study were to investigate relationships among chemical components and their relationships with in vitro disappearance of ground and unground dried silages, and to quantify minimally fragmented starch in corn silage and investigate its impact and that of mean particle size (MPS) on in vitro disappearance of unground silages. Thirty-two corn silages were selected to provide diversity in DM, protein, fiber and MPS. Detergent fibers were highly correlated with each other and with non-fiber carbohydrate, and were used to develop prediction equations between these constituents. Sieves with apertures of 4.75 mm and bigger were used to isolate intact kernels and large kernel fragments, which were collected and analyzed to measure minimally fragmented starch (Starch>4.75). Dividing Starch>4.75 by total starch defined the proportion of minimally fragmented starch (Starch>4.75/Total), which ranged from 9 to 100% with a mean of 52%. Starch>4.75/Total was positively correlated with MPS (r = 0.46). The inverse of Starch>4.75/Total is an index of kernel fragmentation, and a method for measuring it was proposed. Silages were prepared as whole material or ground to pass through a 4- or 1-mm screen of a cutter mill. In vitro DM disappearance (IVDMD) was greater for ground than for whole samples (71.7 and 61.2%, respectively). Increased IVDMD for ground samples was attributed to greater in vitro neutral detergent fiber (NDF) and neutral detergent solubles (NDS) disappearance. The IVDMD of ground samples was related to NDF and acid detergent lignin (ADL) (R2 = .80). The IVDMD of whole corn silage was related to ADL, Starch>4.75, MPS, and DM, but not to NDF. When IVDMD was partitioned in digestible NDS (dNDS) and digestible NDF, the dNDS of whole was not uniform or completely fermented. The difference in dNDS between ground and whole was related to Starch>4.75/Total. In conclusion, the proportion of minimally fragmented starch provides a corn silage fragmentation index that is related to the in vitro digestion of whole silages and, when validated by in vivo trials, may be a useful quantitative substitute for the qualitative processing adjustment factor that is used currently in summative equations for estimating the total digestible nutrients (TDN) of corn silages.