---

Dry matter (DM) intake is the single most important factor that determines milk production of dairy cows and the cost of producing milk. For nutritionists, the first step in formulation of any dairy ration is to know the DM intake of the average cow in the herd or pen. All of the nutrients, except water, needed for milk production are in the DM. In general, 1 pound of DM from a good TMR will support between 2.0 to 2.5 pounds of milk. More nutrient dense or highly digestible rations will support milk closer to the 1 pound of DM to 2.5 pounds milk ratio than low nutrient dense rations. Cost of rations generally follow nutrient density with high nutrient dense rations costing more than low nutrient dense rations that contain more forages and fibrous byproducts. Therefore, maximizing DM intake of a ration matched to the nutrient needs of the cow will result in the best cost ration and most efficient ration. Limiting DM intake leads to reduced feed efficiency, higher costs/unit of milk produced and less milk and milk income.
A few weeks ago, I had the opportunity to visit with Dr. Mike Allen from Michigan State University about feed intake of dairy cows and what regulates it. Over the last few years, he has extensively studied feed intake in dairy cows and how nutrient metabolism regulates it. I emphasize regulation or metabolic control of feed intake and not prediction of feed intake. Predicted feed or DM intake is used extensively in formulation of rations and often as a measure in evaluation of cow performance. However, cows will eat to their milk production nutrient needs and low milk production equates to low DM intake and high milk production to a high DM intake. Because of the multiplicity of factors (cow, diet, environment, feed management and facility) that affect DM intake, the best anyone can do in predicting DM intake of cows with any accuracy is within 1.5 to 2 pounds of DM per day. I, as many other nutritionists have done over the years, have used the predicted DM intake as a benchmark to evaluate the efficiency or performance of cows. In either situation, a predicted DM intake has not told us a lot and telling the cows to eat more or less DM has not worked. The way to changing DM intake is through understanding the metabolic factors than regulate DM intake and then changing the ration to provide more or less of the nutrient factors that regulate DM intake.
Dr. Allen has proposed metabolism of propionate, amino acids and fat in the liver is what signals either hunger or satiety in the cow. He calls it the HOT (hepatic oxidation theory) regulation of DM intake. The control of feed intake is very complex and involves the integration of multiple chemical signals to the brain as to whether the cow eats or not.
In over simplifying HOT, the basic premise is the liver signals for hunger or satiety based on the production of glucose from propionic acid produced in the rumen and the metabolism of long chain fatty acids coming from NEFAs and the ration. On a practical basis, here is how Dr. Bill Weiss from The Ohio State University explains the glucose production portion of HOT and how it influences DM intake.
• When rations contain moderate to high starch (25 to 30 percent of the DM) from highly fermentable starch sources (high moisture corn, wheat, barley, steam flaked corn), DM intake will be lower than expected. Milk production from these rations can be very good, but the liver is converting more propionate into glucose than the mammary gland can remove for milk production. This excess glucose has a negative effect on DM intake.
• Replacing half or more of the high moisture corn fermentable starch with dry corn will increase DM intake. The conversion of dry corn starch to glucose is slower than with highly fermentable starch and the utilization of glucose for milk production probably matches glucose availability closer. This does not imply milk production will be any different between high moisture corn and dry corn, but feed efficiency may be slightly lower with dry corn as cows eat more to produce the same milk.
• Feeding low starch, highly digestible fiber byproduct feeds would likely increase DM intake. The digestible fiber byproduct feeds are not going to ferment to propionate in the rumen and therefore, as with dry corn, excess glucose is not likely. However, because fibrous feeds are lower in energy than starchy feeds, milk production could be lower and DM intake higher on a low starch, high fiber ration than a moderate to high starch ration.
How feeding fat fits into HOT is more complicated and confusing than the conversion of propionate to glucose. The liver processes NEFA, the non-esterified fatty acids, and dietary long chain fatty acids (fat supplements) into a form than can be incorporated into milk fat. Portions of the fats passing through the liver also are utilized for energy. If the amount of fatty acids coming from NEFA and/or the ration is less than the amount that can be secreted in milk or needed for energy, fat metabolism in the liver functions normally. However, when the negative energy balance causes NEFAs to increase, the metabolism of NEFA in the liver induces a satiety effect lowering feed intake (typical ketosis situation). Therefore, the more NEFA the liver processes, the greater the satiety (feed intake depression) leading to increased release of NEFA from adipose tissue further reducing DM intake. This is the ketosis where cows need energy, but do not eat or eat only low energy forages. The goal is to supply enough fatty acids from NEFA and ration supplemented fat along with propionate to meet milk fat yield and energy requirements. The key points related to HOT and fatty acid metabolism in the transition/early lactation cow are:
• Fat can be used by the liver and some other tissues as an energy source leaving more glucose for milk production in the early lactation cow where DM intake is lagging.
• Cows can utilize some fat supplementation in rations in early lactation to increase milk production and minimize body weight loss, but they must be in compliment to NEFA and not in excess.
• Unsaturated fatty acid supplements tend to depress DM intake whereas saturated fatty acid supplements do not.
• Dr. Allen believes HOT as it relates to fat is applicable to fresh cows the first three weeks of lactation that are fed a relatively high forage-NDF (~26 precent DM basis) ration with small amounts of fat supplementation. The high forage-NDF ration provides rumen fill to help prevent displaced abomasums. However, the ration also needs to contain enough grain to supply the glucose cows need for milk production.
For mid to late lactation cows, Dr. Allen suggests feeding a ration that is low in starch (< 24 percent of the ration DM) and has more forage and byproduct digestible fiber. This will increase feed intake and provide a more consistent supply of fuels from the liver (glucose and fatty acids) for milk production rather than for body weight gain. Dry ground corn and nonforage fiber sources (beet pulp, corn gluten feed, soyhulls) are good feeds to feed for lower fermentability in mid to late lactation rations. Highly fermentable starch sources (aged corn silage, high moisture corn, bakery waste, ground barley, wheat) should be low or eliminated from mid to late lactation rations as they direct more fuel to body condition leading to over conditioned or fat cows.
The liver is an important pathway for energy metabolism directing both the amount of energy consumed (DM intake) and how the energy is utilized in the cow. Glucose, from the digestion of starchy feeds, and long chain fatty acids are key nutrient metabolites the liver uses to stimulate or inhibit eating by the cow. Measuring DM intake of cows on farms is essential to good nutrition and must continue as a best management practice. However, knowing how the metabolism of feeds affects DM intake can help in feeding cows to meet production and health requirements. There is a lot to learn about DM intake of ruminants, but understanding the chemistry of feed intake and metabolism will help nutritionists formulate better rations in the future.
[[In-content Ad]]

---