Starch Digestibility
Mary Beth de Ondarza, Ph.D., Technical Services Nutritionist
F.A.R.M.E. Institute, Inc., Homer, NY
When the feed man drops off a ration nutrient analysis on the farm, typically it will include a fraction called Non-Fiber Carbohydrate (NFC) which usually makes up 35-40% of the dry matter in a ration designed for high production. NFC is calculated by difference [100-(%NDF + %CP + %Fat + Ash)]. NFC is generally more rapidly digested than fiber. Unfortunately, NFC is a non-uniform fraction. Portions of the NFC ferment faster than other portions. Because of this non-uniformity, NFC can ferment very differently depending its ingredient source.
Non-Fiber Carbohydrate is made up of different amounts of simple sugars, beta-glucans, galactans, pectins, and starch. Simple sugars (like in maple syrup and molasses) are rapidly fermented in the rumen to the stronger acids, propionic acid and lactic acid. Simple sugars are not present in large amounts (4-6% of the ration DM) in traditional dairy cattle diets. Beta-glucans and galactans (like in kidney beans – the musical fruit!) are present in small amounts in some grains and beans fed to cows. Pectins (like in jelly) are present in feeds like beet pulp, citrus pulp, and alfalfa. Beta-glucans, galactans, and pectins, also referred to as soluble fiber, must be fermented in the rumen since starch enzymes in the intestine cannot break them down. Soluble fiber is fermented rapidly but yields a weaker acid called acetic acid, as a by-product. Thus, soluble fiber generally does not lower rumen pH and cause acidosis.
Most of the NFC is made up of starch (28-32% of the total ration DM). For this reason, starch digestibility, both in the rumen and intestine, has a crucial impact on milk production. Cereal grains, such as corn and barley, provide the greatest proportion of the starch in a cow’s diet. Starch is made up of units of a sugar, called glucose, which are bonded together. Depending on the starch source and processing, the glucose units may be very tightly bonded and compacted together or they may be weakly linked together. For this reason, different starches may be either rapidly or slowly fermented in the rumen.
Factors Affecting Starch Digestibility:
A. Starch Type:
Rate of starch digestion is known to depend in part on its source.
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Fast >---------------------Intermediate ---------------------------> Slow
Wheat ------Barley ---------Oats-----------Corn ---------------Sorghum
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B. Starch Processing:
1. High Moisture Ensiling-
Ensiling high-moisture grains (28-32% moisture) increases starch digestion by breaking down the protein structure of the grain and disrupting the crystalline structure of the starch. This allows the microbes to more easily burrow into and digest the starch.
2. Grinding -
Grinding increases the amount of surface area that the rumen microbes can attach to. Thus, grinding increases starch digestibility in the rumen and in the intestine. Because of the non-crystalline nature of the starch in high-moisture corn (HMC) (28-32% moisture), it is usually recommended that it be rolled rather than ground. Grinding would usually make HMC degrade too rapidly and cause acidosis. However, sometimes we must deal with HMC that isn't 28-32% moisture, its drier. In that case, it must be ground finer. Miner Institute in Chazy, New York reported an increase of 5 lb/cow/day when corn at 23% moisture was ground with a hammer mill through a 1/2" screen.
Cornmeal may either be coarsely or finely ground. Finely ground cornmeal will be digested to a greater extent. However, grinding too fine may cause more acidosis.
Recommended Particle Size of Corn Products (Sniffen et al., 1995)
% Retained on a 1/8" screen
Cornmeal ----------------------------------------- 25
HMC, 21%--------------------------------------- 30
HMC, 25%--------------------------------------- 40
HMC, 30%--------------------------------------- 50
If the amount of cornmeal retained on the 1/8" screen is only 5-10%, NEl can be adjusted upward by 5%. But, watch out for acidosis with this corn! If 30-35% of the cornmeal is retained on the 1/8" screen, NEl can be discounted by 5 to 10%. Remember, however, that the amount of effective fiber in the ration can impact the amount of energy in the corn. Coarsely ground cornmeal will stay in the rumen longer and be more completely digested if there is a good fiber mat in the rumen.
3. Gelatinization or "Cooking" –
Gelatinization is defined as the irreversible destruction of the crystalline order in a starch granule, so that the surface of every molecule is made accessible to solvents or reactants, including the rumen microbes. Gelatinization in feed is brought about by a combination of moisture, heat, mechanical energy, and pressure. It increases the speed at which enzymes and microbes can break down the linkages of starch to yield energy and microbial protein.
Steam-flaking, extrusion, and pelleting all cause starch to gelatinize. However, the degree of "cook" is highly dependent on the amount of moisture, pressure, and heat actually obtained during each of these processes.
4. Chemical Treatment –
Sodium hydroxide (NaOH), a caustic chemical, is being used by some farmers to disrupt the protein matrix of grain and to gelatinize starch. It is used on whole grains as an alternative to grinding. However, rumen digestibility of this grain is generally lower than that of ground grains and this may lower microbial protein synthesis.
Who Cares About Starch Digestibility?
When feeding today’s high-producing dairy cows, we need to provide the best blend of starch sources to ensure optimum rumen efficiency and maximum energy utilization. The starch sources that you purchase should complement your particular protein sources and your home-grown starch sources.
The Rumen's Needs.....
1. Energy:
Starch and sugars are used as an energy source for the rumen microbes. The microbes convert the starch and sugars into volatile fatty acids, primarily propionic acid, which are absorbed across the rumen wall. The volatile fatty acids are used for energy or for glucose production by the cow.
2. Microbial Protein:
Carbohydrates, such as starches and sugars, and degradable protein are used by the rumen microbes to make microbial protein. As more microbial protein is made in the rumen, there is less need for dietary bypass protein (UIP). Rumen microbial proteins are high quality. They contain a blend of amino acids similar to fish meal and blood meal, which are easily converted to milk protein. Thus, increasing rumen microbial protein increases milk production.
For efficient growth of the rumen microbes to occur, the availability of carbohydrate and protein to the microbes must be synchronized throughout the day on a minute by minute basis. If too much protein is supplied without an available source of carbohydrate, the microbes will use the protein as a source of energy and waste the nitrogen found in protein. For example, alfalfa silage and raw soybeans contain a large amount of readily available protein. When those feeds are included in the diet, it is wise to feed a portion of the carbohydrate in a readily available form. Barley, flour and high-moisture corn are all considered sources of rapidly available carbohydrate. If only slowly degradable carbohydrate, such as cornmeal, was fed with the alfalfa silage and raw soybeans, milk production could suffer.
The use of the soluble (SIP) and degradable (DIP) protein fractions has helped nutritionists to provide a more appropriate balance of proteins to the rumen microbes. The soluble protein is rapidly available (within 1-2 hours), while the remaining degradable protein is available over a longer period of time (2-16 hours). Now, we are starting to think more about our NFC’s, primarily starch. The goal is to provide a portion of both rapidly digestible starch and more slowly digestible starch and not to provide too much of either one.
3. Rumen Acidosis:
Starches and sugars are primary culprits of rumen acidosis. Excessive starch and sugar fermentation can result in excessive amounts of lactic acid which dramatically reduces rumen pH, hindering the growth of the rumen microbes (especially the fiber-digesting microbes), reducing feed intake, reducing milkfat production, reducing milk production, and causing laminitis. Rumen pH can fluctuate throughout the day depending on how fast the starches and sugars in the diet ferment.
Generally, it is recommended that NFC levels not exceed 40% of the total ration DM. This is due to the development of acidotic rumen conditions commonly at higher levels of NFC. However, this recommendation does not consider how fast the starch ferments in the rumen. For example, if the starch is primarily from flour, barley, or high-moisture corn, acidotic conditions could be more prevalent than if the starch is from cornmeal, which is more slowly fermented in the rumen. With rapidly available starches, the maximum NFC level may actually be less than 40% of the ration DM.
The acceptable amount of total NFC and rapidly degradable starch will be related to other aspects of the ration and how it is fed. If adequate fibrous particles are provided and they are long and stimulate good cud-chewing, more NFC and rapidly degradable starch may be allowed in the diet. If bunk space is limiting and cows are slug feeding, high levels of NFC and rapidly degradable starch will not be tolerated. If grain is only fed twice per day rather than many times per day or in a TMR, one should limit the total amount of NFC’s and rapidly digestible starch. Finally, if stall comfort is poor and cows are standing on concrete too many hours of the day, NFC and rapidly degradable starch levels should be limited to avoid laminitis.
Starch Digestion in the Intestine:
Optimization of rumen efficiency is the first criteria for enhancement of milk production. We must meet the remainder of the cow's energy needs with starch and fat directly absorbed from the intestine. We want to make sure that all of the starch that is not used by the rumen microbes is taken up by the cow at her intestine and is not passed out in her manure.
Summary:
With these objectives in mind, our goal when purchasing starchy ingredients and processing starches should be to provide maximum overall digestibility while, at the same time, providing an ideal balance of rapidly and slowly degradable starch to optimize rumen efficiency and maximize the production of microbial protein. Providing all starch in a highly digestible form that totally degrades within a few hours in the rumen would create too much acid and hinder the rumen microbes. On the other hand, a slowly degradable starch may prevent an acid rumen but it may also limit microbial protein synthesis and end up on the barn floor at the other end of the cow.
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