The first priority of dairy rations should be to maintain rumen health. Traditionally, forages of sufficient particle size are used to stimulate cows to chew feed during eating and cud-chewing (the latter is called rumination). Dairy cows spend about 10 hours/day chewing. Besides being the major source of buffer, chewing the cud is the universal sign of cow contentment. However, with todays hard-working bovines, fine-tuned rations are needed to balance rumen health with high productivity.
Because of the variation among various forages, most people balance rations for fiber concentration in the diet. Many use acid detergent fiber (ADF) because it is an easier lab procedure. The NRC recommends a minimum of 19-21% ADF, 75% of which should be from forage. However, neutral detergent fiber (NDF) is more accurate. The dairy NRC recommends 25 to 28% NDF, with 75% of this from forage. Therefore, 28% NDF x .75 = 21% minimum forage NDF in the diet. Forage NDF is a better measurement of fiber requirements unless you use only alfalfa as the major forage source because requirements by the NRC for 21 and 28% ADF and NDF more closely approximate the ADF to NDF ratio of alfalfa (about 30:40) than corn silage (28% ADF and 51% NDF) or other grasses. Of course, this requires periodic testing of forages for fiber concentration for best results. Estimation of fiber concentrations of pasture actually consumed is more tricky but can be done.
Forage particle size also has a big role in maintaining rumen health by stimulating rumination and therefore salivation. Because cows tend to chew only well enough during eating to swallow feed, larger particles of forage in the diet cause particles in the rumen to be larger, which increases time spent ruminating. As forage particle size decreases, fibrous byproducts, especially cottonseed, probably become more effective (stimulate more chewing during rumination) compared with byproducts in diets with large forage particles. Also, dietary buffers are more beneficial and are good insurance in low forage diets, especially with corn silage as the major forage.
Corn silage needs to be chopped more finely than alfalfa to break up the cob to reduce its wastage and also to at least partially open the kernel to increase starch digestibility. Finer chopping also allows better packing. Kernel processing should allow the corn to be chopped more coarsely while still disrupting the kernel and cob for good digestion.
Alfalfa haylage in bunkers can be chopped more coarsely than alfalfa in upright silos (coarser chop may plug unloaders) or corn silage. However, as particle size of forage increases in length, an increasingly larger particle size causes a progressively lower increase in additional chewing (i.e., a limiting returns). Long hay has only a minor advantage compared with coarsely chopped haylage. Conversely, very finely chopped haylage has considerably reduced effectiveness compared with moderately chopped haylage. Forages with moderate particle sizes can be used effectively so long as there is enough forage NDF, the particle size is not too fine, and starch is reduced compared with the same diets containing forages of larger size.
Even if forage particle size is appropriate, farmers need to be careful to follow the manufacturers mixing times for mixer wagons to prevent a drastic reduction in particle size during mixing. Conversely, total mixed rations help reduce slug feeding of highly digestible grains over the day. Component feeders should strive to feed grain about four times per day if there is a lot of rapidly degradable starch (e.g., finely ground or steam-flaked corn).
Rumen degradability of carbohydrates is another major factor influencing rumen pH. While saliva from chewed forage helps buffer pH, fermentation of carbohydrates produces the acids that decrease pH in the first place. In typical diets with only forage and grain (no byproducts), research at Ohio State and other universities has shown that you should have not much more than 30% starch. Because starch is difficult to measure, most ration balancing programs calculate nonfiber carbohydrates (NFC) as 100% - ash - fat - protein - NDF. Alfalfa has about 10% pectin, which is part of the fibrous cell wall but which gets dissolved in NDF solution. Because NDF underestimates fiber in alfalfa, the pectin is actually included in the NFC portion. An alfalfa-based ration containing 40% NFC may have only 30% starch. However, with all corn silage in the diet, 40% NFC may be over 35% starch, because corn silage has essentially no pectin. Although highly digestible, pectin apparently does not ferment to lactic acid. Therefore, a 40% NFC diet with corn silage would have more fermentable starch than a 40% NFC diet with alfalfa and probably would be more likely to cause acidosis.
Corn grain has a lot of variation in starch digestibility in the rumen Coarsely rolled corn has a similar digestibility in the rumen as does NDF from good forages.However, when corn is ground or rolled very finely, steam-flaked very aggressively (<25 lb/bu density), or fed as high-moisture corn (especially if rolled), then ruminal digestibility may be increased by 20 or 30 percentage units in high producing cows One study that progressively replaced starch from barley (which is very digestible) with starch from corn (all had 33% starch and >33% NDF) linearly decreased feed intake from 50.2 to 42.9 lb/day of dry matter and reduced milk production by over 11 lb/day. The high rumen degradability of barley starch increased production of acids, which are important regulators of feed intake. With corn silage diets, besides having more starch relative to NFC than with alfalfa diets, you automatically provide high moisture corn in the silage. With corn silage as the sole forage, you should consider using fibrous byproducts to reduce ruminally digested carbohydrates to lower levels (say 35 to 38% NFC compared with 38 to 40% NFC for alfalfa diets) to help maintain rumen health.
Cows on pasture can select the forage they want to eat. This forage is typically lower in fiber and higher in rapidly degradable sugars than harvested forage. Reducing NFC concentration by feeding less corn, more fibrous byproducts, or fat within guidelines should help prevent a lower rumen pH and butterfat depression. Butterfat test is a relatively accurate indicator of rumen health within individual farms (compared with previous history) but is less accurate across farms because of the variety of factors besides rumen pH that affect it.
Good managers could reduce forage NDF down to about 18% (or possibly lower). However, this does increase the risk of laminitis. We have noted very low rumen pH values in some of our trials with corn silage as the sole forage. Therefore, reducing forage below NRC recommendations should be done with extreme caution with diets high in corn silage. NFC should be reduced 1 to 1.5 percentage unit for every 1 percentage unit decrease in forage NDF percentage below NRC recommendations or values found to be previously acceptable on that farm. Fresh cows (first 3 to 4 weeks) should avoid low forage diets altogether. They are most prone to acidosis-related problems because their rumens and rumen microbes are not yet adapted to low forage, high grain diets.