Dairy efficiency is a commonly used term and is defined as either milk output per unit of dry matter intake or energy-corrected milk output per unit of dry matter intake. The problem with this measurement is that it is influenced by the energy density of the ration, among other factors.

As an example: Herd 1 is fed a ration with a metabolizable energy (ME) of 1.13 Mcal per pound, while for Herd 2 ME is 1.29 Mcal per pound. Production in either herd is identical at 75 pounds of energy-corrected milk. Dry matter intake is 57 pounds (Herd 1) and 50.1 pounds (Herd 2).

Cows in Herd 1 produced 1.32 pounds of milk per pound of dry matter intake, while cows in Herd 2 produced 1.5 pounds of milk per pound of dry matter intake. Which herd utilizes feed intake with greater efficiency?

Efficiency is defined in the Merriam-Webster dictionary as “the quality or degree of being efficient.” Sister Mary Elephant would not approve of this definition as it contains the root of the word being defined. Definition No. 2 is “the ratio of the useful energy delivered by a dynamic system to the energy supplied to it.”

Based on simple input/output relationships, one would conclude that Herd 2 utilized feed inputs more efficiently, producing approximately 14 percent more milk per unit of dry matter consumed. This is the ratio between ME for Herd 2 and Herd 1 (1.29 / 1.13 = 1.14).

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However, by using the second definition of efficiency, it may be determined that ME utilization is identical for either herd. Dairy efficiency is not really efficiency, by definition No. 2, but rather a rate.

Ratios have no dimension – that is, the numerator (top number) and denominator (bottom number) have the same dimension and therefore cancel out. Comparisons among herds are best made when milk production (numerator) is described as energy content and the denominator is ME.

How, then, can cows be fed to maximize milk energy output per unit of ME input? It has been known for some time and differences quantified that glucose, amino acids, volatile fatty acids and fat are used with differing efficiencies for synthesis of ATP (primarily for maintenance), for synthesizing major energy-containing milk components (lactose, butterfat and milk proteins), products of conception (not just the calf) and gain of body tissue.

Lactose comprises approximately 40 percent of milk solids, more than any other solid component, and represents approximately 25 percent of milk energy content. Lactose is a disaccharide, one molecule each of glucose and galactose (which is synthesized from glucose), so clearly glucose availability is important for lactose synthesis. Ruminal fermentation of cellulose and starch produce propionate, as well as other volatile fatty acids; propionate is converted in the liver to glucose.

It should be understood that, even if considerable quantities of starch are available for digestion in the small intestine, there is no net appearance of glucose from the gut of ruminants. Glucose must be synthesized from scratch (de novo) utilizing either propionate or those amino acids capable of being converted to glucose.

These processes do not come without a cost, as theoretical maximum efficiencies of synthesis of glucose from propionate have been reported as either 0.86 or 0.84; from 14 to 16 percent of the energy is lost.

Net synthesis of lactose from propionate was estimated to be either 76 percent or 78 percent efficient. It is difficult to calculate the efficiency with which amino acids are converted to glucose given the variable efficiency with which individual amino acids are utilized.

For alanine, more than 40 percent of energy content is lost as either heat or in disposal of nitrogen (urea synthesis) during conversion to glucose. Research in 2008 suggested that grains be well-processed (steam-flaked) in order to maximize ruminal disappearance of starch, thereby sparing amino acids, maximizing efficiency of glucose synthesis and therefore lactose synthesis.

Butterfat may either be synthesized de novo in the udder or taken up by the udder from the blood pool of circulating fat. Dairy cattle make butterfat in the udder from acetate, a volatile fatty acid from rumen fermentation.

Fat synthesis from acetate is a relatively inefficient process, capturing approximately 70 to 75 percent of the energy in precursors as energy in product. This compares to efficiency of fat synthesis in non-ruminants (from glucose) which is approximately 84 percent.

From 40 to 60 percent of butterfat is taken up directly from the bloodstream in the form of long-chain fatty acids. If these fatty acids are synthesized in adipose tissue from the same precursors as in the udder, long-term efficiency of synthesis is identical.

However, if dietary fatty acids are taken up and incorporated directly into butterfat, efficiency is increased to greater than 90 percent. As the content of long-chain fatty acids in feed increases, uptake by either adipose tissue or the udder may also increase; less heat is produced and more of the energy consumed is converted into product.

Obviously, if 100 percent of butterfat was synthesized from acetate, that process would be approximately 72 percent efficient. However, if 25 percent of butterfat was of dietary origin, then the efficiency would increase to 77 percent.

Milk protein is synthesized from amino acids; these may be directly absorbed from dietary sources or as microbial protein. Excessive dietary intake of protein reduces efficiency, and insufficient dietary protein reduces digestibility. Assuming that all amino acids are available in the amounts and proportions required, protein synthesis is from 77 to 82 percent efficient.

Providing amino acids thought to be limiting milk protein synthesis (lysine and methionine) has shown limited success. Researchers in 2010 reported that feeding supplemental lysine (ruminally protected) alone had no effect on milk production nor milk protein content, while a combination of ruminally protected amino acids did produce positive results.

An assumption basic to the use of ruminally protected amino acids is that degradability of an individual amino acid in a feed is the same as overall crude protein degradability. A study undertaken by the California chapter of ARPAS showed that for those amino acids evaluated (in alfalfa hay), ruminal degradability of an individual amino acid in a sample was more likely to be different from the average degradability for that sample than it was to be similar.

This observation, if shown for other feeds, makes it difficult at best to predict which protected amino acid should be fed to optimize supply.

As forage intakes increase to greater than 70 percent of dry matter, the efficiency of energy utilization is reduced. There are several explanations for this: The primary volatile fatty acid produced from fermentation of forages is acetate (about 50 percent); lesser amounts of propionate are produced (about 20 percent).

Acetate is used for maintenance purposes or for fat synthesis; some glucose is also required. When insufficient propionate is available for glucose synthesis, amino acids are converted to glucose for this purpose, reducing efficiency.

Similar to the hypothetical situation posed at the beginning of this article, researchers in 1991 fed energy-equalized rations which were either 75 percent alfalfa hay or 75 percent concentrate. Feeding a mostly forage ration resulted in greater energy expenditure when compared to the 75 percent concentrate ration. Increased energy costs of gut metabolism accounted for two-thirds of the differences between groups when fed near maintenance and 84 percent at twice maintenance.

When forced to deal with increased feed volume (either greater forage intake or simply more feed), gut tissue increased in mass; accounting for less than 10 percent of body mass, the gut accounts for more than 20 percent of whole-body energy expenditure.

Researchers in 1980 reported similar findings: Going from a non-lactating to lactating state, the digestive tract, liver and heart all increased in relative size by 30 percent. While not done in lactating dairy cattle, we have analyzed energy input/output data for growing beef cattle and found that, as energy intakes increase, costs of maintenance also increase.

Cattle fed at twice maintenance had 32 percent increased maintenance costs compared to cattle fed at maintenance. Feeding so that gut energy expenditures are minimized while maintaining productive function increases efficiency.

Estimates of efficiency for individual milk components may be used to estimate overall efficiency. Based on estimates from 1968, the maximum efficiency of milk synthesis is approximately 0.76. Researchers in 2014, using more modern data, estimated maximum synthetic efficiency to be 0.74.

Direct incorporation of dietary fat into butterfat (25 percent of total) increases overall efficiency of milk synthesis by 4 percent, reducing dry matter intake by approximately 1 pound at theoretical maxima. How well do real cows compare to theoretical as regards to efficiency of milk synthesis? Observed efficiencies are in the range of from 0.57 to 0.65; cows utilize metabolizable energy for milk synthesis from 74 to 88 percent of maximum.

Identifying cows with inherently lesser efficiencies and culling them is important, as is determining if your herd is operating near maximum efficiency. While greater production, in and of itself, increases energy expenditures, providing cows with feeds that are more efficiently turned into milk will keep more bucks in your pocket. PD

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PHOTO: Photo byPDstaff.