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Energy/Carbohydrates

First, we will cover energy and carbohydrate requirements. An often used, but outdated, energy term is total digestible nutrients; these are usually expressed as either a percent or a pound. A more commonly used term, in expressing energy requirements for dairy cows, is net energy for lactation or NEL. NEL is expressed as megacals per pound on a concentration basis or megacals per day on an absolute basis.

This is a diagram of the energy partitioning of a ration consisting of 60% alfalfa and 40% corn grain and it shows the losses at different points of digestibility or different points of the digestion in the GI tract.

The gross feed or the gross energy value in alfalfa and corn diet is equivalent to that of most other feedstuffs or even non-feedstuffs such as saw dust. So this diet, the cow would digest 70% of the energy and 30% would be lost in the feces. The first loss is that is indigestible. The second loss is energy in urine and in gas and this would account for 10% of the total energy, leaving the cow with 60% as metabolizable energy. TDN would come in somewhere between digestible energy and metabolizable energy. TDN accounts for the loss of urine but not the loss in gas. TDN will often over-estimate the energy value of forages, because a considerable amount could be lost as gas. TDN usually predicts energy value of grains fairly closely, if you compare it to NE or net energy. The next loss along the energy diagram is losses as heat, which accounts for about 20% of the total energy. And this is heat lost during the fermentation in the rumen and also heat lost or heat used during nutrient metabolism, i.e. absorption. Of this diet, a cow can actually utilize about 40% of the total energy for maintenance, milk production and other physiological functions.

Cows partition energy differently during different physiological states. During early lactation and during late lactation, the cow's primary or first goal is to meet maintenance requirements. In early lactation, the second goal is to maintain milk yield. Third, or three items tied for third, would be growth, if the animal is still immature, weight gain, if the animal lost any during early lactation, and then reproduction. In late lactation, maintenance is still number one, but second on the list would be nutrients for reproduction, tied for third would be milk yield and growth and if any nutrients are left over, ranking fourth would be weight gain.

The maintenance requirement for energy or NEL for a lactating dairy cow, is about 10 megacals of NEL per day. Or if we look at common rations fed to dairy cows, the first 13 pounds consumed per day would go toward meeting maintenance energy requirements. If cows are on pasture, the energy requirements increase by about 10 to 15 percent per day and also animals under cold stress. Those animals in first or second lactation also have a requirement for growth and this would be an additional 1.3 pounds of ration dry matter going towards growth if these animals were immature. For milk, the hosting cow needs about .31 or about a third of a megacal per pound of milk produced. She would need to consume about .4 of a pound of ration to produce this pound of milk.

Energy requirements for reproduction do not become a concern until generally the last two months of pregnancy. At this point, about 4 pounds of the ration dry matter would go towards pregnancy and weight gain. The cost to put a pound of weight back on would be about 2.33 megacals per pound of gain or the cow would need about three pounds of ration dry matter to gain a pound of weight. If we look at the total dry matter intake and where this is going about 45% would go to nonproductive states or physiological functions other than milk production. Reproduction, maintenance, growth and weight gain would account for about 45%. The other 55% of intake can go towards milk production.

What provides energy to cows? It is mainly carbohydrates. In ruminant nutrition and also in dairy nutrition, we usually separate fiber carbohydrates out into two fractions or we separate carbohydrate fractions into two. One would be fiber carbohydrates, which we can further separate into three fractions cellulose, hemicellulose, and lignin. And then, we have the non fiber carbs, which generally come from grains, this would be starch, sugars, pectin and organic acids. If we look at the plant cell wall, inside the cell are soluble contents such as protein, sugar, fat, starch and pectins. Where we find fiber is actually within the cell wall. And we said that fiber is made up of hemicellulose and cellulose and lignin. And although we call it (lignin) a carbohydrate, it is actually not a carbohydrate. And it works somewhat similar to cement, it holds the cellulose and hemicellulose together.

Commonly, what we evaluate in rations are two different, with respect to fiber, are two different fiber factions. One is ADF, which is composed of lignin and cellulose, of course these are in the plant wall. Digestibility can be quite low of these components. We can actually use ADF to predict the energy level of the feed and ADF increases as the plant matures. This is depicted in a graph of the relationship of NEL and ADF in corn silage.

You can see that as ADF increases in corn silage from 20 to 40 percent the energy content, in megacals per pound, decreases from about .7 to .6. We actually lose about a third of a pound of milk per pound of dry matter as corn matures from 20% ADF to 40% ADF. As forages mature, in general, ADF content increases and the energy value decreases. If we put all the fiber carbs together, which would be cellulose, lignin and hemicellulose, so the total cell wall this is called NDF. This fraction has a modest digestibility, so it is greater than the digestibility of ADF, and we use it to predict intake. Also, as the plant matures NDF will increase. This graph depicts optimal levels of NDF in the ration of dairy cows.

There is some optimal level, somewhere between 25 and 33 percent of dry matter should be NDF, it depends a bit on milk yield. As you get above 33%, intake will decline and this is depicted as high or low on the Y-axis. Intake declines because it is limited by rumen capacity, remember that NDF is moderately digested, so these feed particles stay in the rumen longer and so intake is simply limited by capacity. However, if NDF gets too low, that means that we do not have enough fiber in the diet. Generally, we have too much grain and intake is actually limited by excess fermentation of starch and sugars which can lead to acidosis. NDF should not be too low, less than 25%, nor should it be too great, greater than 33%. In either event, you can decrease intake. Acidosis is a condition which results in lower rumen pH, a pH something less than six. The optimal rumen pH for a dairy cow should be somewhere between 6.5 and 7. The two causes are one is the type of the diet one low in fiber or high in concentrate and the other is the physical form of the feeds. Actually, we could grind forages to a very small particle size and not have any grain in the diet, but actually induce acidosis, simply because of fermentation of the feeds as the particle size decreases, increases dramatically. The effect of acidosis is that we shift the rumen microbial population and actually the VFA pattern. The rate of passage slows to try to prevent excess fermentation and the feeds, due to the condition of the rumen, have a lower digestibility.

Next, we will look at the composition of some feeds, the fiber composition and the energy value.

First, our two corn silages differing by five points in ADF, 25 versus 30, and eight points in NDF, 41 versus 49. We can see that the NEL value, in megacals per pound, decreases from .68 to .65. The same can be shown in two different alfalfas, one with 37% ADF and one with 28. So as ADF increases, from 28 to 37, energy value decreases from .68 to .56. If we look at our high energy feedstuffs, or grains such as barley and corn, you will notice that they are relatively low in NDF and ADF compared to forages, but have a relatively high energy concentration.

Not all NDF or fiber is equal. Another thing that we commonly look at in dairy diets is effective NDF or NDF that is effective in stimulating rumination. We know that long particles stimulate rumination or simply rumen motility. Rumination, by regurgitation boluses from the rumen through the esophagus, back to the mouth and re-chewing leads to saliva production, which in turn buffers the rumen. Small particle sized forages actually act very similar to grain in the rumen, where we get rapid fermentation. Our goal is to have the majority of the NDF in the ration come from forage. We would like about three quarters of NDF from forage and also we would like to have the forage of some long particle length, not finely ground. So the next table depicts NDF and effective NDF of different forages and by-products.

The first one in the table is corn silage that has been cut at a theoretical length of 3/8 of an inch and the second is corn silage that was finely chopped. Now, notice that both of these have very similar NDF values, yet, the corn silage with the 3/8 of an inch chop is more effective at stimulating rumination than the finer cut corn silage. You will notice that alfalfa hay which is 38% NDF has a very high effective NDF value or 92% of the NDF is effective in stimulating rumination.

One of the reasons that we look at NDF, is the use of high fiber by-products in dairy diets. Two good examples are brewer’s grain and distiller’s. You will notice that both of these have higher NDF values versus alfalfa or corn silage. Yet, look at how effective they are stimulating rumination. 18% of the NDF in brewer’s is effective and 4% in distillers is effective. A unique feed is actually whole cottonseed, which is very high in NDF, yet 100% of the NDF is effective in stimulating rumination. Another common high fiber by-product added to dairy diets, is soy hulls which has the highest NDF in the table, yet it is relatively ineffective in stimulating rumination because of the small particle size. One carb fraction is fiber, the other is non-fiber carbs and these are mainly composed of sugars, starch, pectins. Pectins are actually unique in that they are non-fiber, but they are not fermented to propionic acid and potentially lactic acid and these are relatively high in citrus and beet pulp.

The last component of non-fiber carbs is organic acids and these are generally found in silages or fermented feeds.

Non-fiber carbs is determined by a difference method. Simply, 100 minus the NDF in the feed minus the crude protein minus the fat minus the ash. The nice thing about non-fiber carbs and NDF is that they vary inversely. If we keep everything in the ration constant, crude protein, fat and ash remain constant, what happens when you increase NDF? Well, NFC must decrease. What happens if you decrease NDF? Non-fiber carbs must increase. Generally, if NDF is balanced correctly, non-fiber carbs will be balanced correctly. The optimal range that they found in research, is somewhere between 35 and 45 percent of the ration dry matter should be NFC.

If you look at the non-fiber carbs of feeds, versus the NDF content, we see that the forages, which are relatively high NDF, have relatively low non-fiber carb values.

Compare that to two common high energy supplements, corn and barley, which are low in NDF and very high in non-fiber carbs. We also find that our high fiber by-products or forage replacements, such as soy hulls, which is very high in NDF, is very low in non-fiber carbohydrates.

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