Net Energy Lactation as a measure of feed energy in dairy cow nutrition (Part 1)
Dairy farmers have become used to Total Digestible Nutrients (TDN), Digestible Energy (DE) and Metabolisable Energy (ME) as systems for expressing the energy content of the feeds eaten by dairy cows. Net Energy Lactation (NEL) is yet another system that can be used for this purpose. In the first article of two on this topic, we will describe (a) the basic concepts underpinning each of these systems, and (b) the relative advantages and disadvantages of each. In the second article (in the next issue of To the Point) we will look at the technological innovations that Meadow Feeds are phasing in to make NEL a practical, dynamic way of describing the energy value of feeds for lactating dairy cows. Ultimately, Meadow Feeds are aiming to meet the energy requirements of dairy cows in an accurate, predictable and consistent manner, to make dairy farming more efficient, and more profitable.
All energy in feed is derived from three chemical fractions, namely carbohydrates, lipids and proteins. The total energy content of feed (gross energy, or GE) is measured in the laboratory with a bomb calorimeter, as the heat produced when a sample of the feed is combusted in the presence of oxygen. Measured in this way, feedstuffs on opposite ends of the quality spectrum in energy terms, like wheat straw and maize, have more or less the same GE. Yet we know that maize is far superior to wheat straw as a source of dietary energy. Clearly then, GE is practically useless as a measure of feed energy in animals. This disparity between the “useful energy” and GE of different feeds (and for that matter, the same feed at different times, from different sources, etc.) relates to major differences in their physical and chemical make-up, which leads to a wide range of energy losses during the processes of digestion and absorption from the digestive tract, and subsequent metabolism at cellular level.
Most of the energy losses that occur after ingestion of the feed can be explained by energy losses via the dung. Structural carbohydrates (cellulose, hemicellulose) are difficult to digest, and thus more prone to faecal losses of undigested carbohydrate energy. Non-structural carbohydrates (starches, sugars) are easily digested, and thus less prone to losses of undigested carbohydrate in the dung. Generally, lipids (fats and oils) and proteins are digested relatively easily, compared to structural carbohydrates (although many factors can have a negative influence on lipid and protein digestion, e.g. low levels of endogenous lipase enzyme or the presence of trypsin inhibitor respectively). Animal species also have a major effect on the digestibility of feed, particularly in fibrous feeds (ruminants have a clear advantage over monogastric species here). The TDN and DE systems basically differentiate the energy value of feeds on the basis of measuring differences in the digestibility of nutrients (i.e. the difference between the quantity of nutrients or energy present in the ingested feed and in the dung excreted).
Further energy losses occur during digestion and metabolism, namely via the production of urine and combustible gases (mainly methane, which is significant in ruminants, but negligibly small in chickens and pigs). Methane losses will be higher with high roughage diets than with high concentrate diets, and will vary with plant associated factors. An average of 8% of the GE in roughages (but as much as 15% of the GE in poor quality tropical forages) can be lost as a result of belching up methane. Urinary energy losses equate on average to about 5% of the GE, but may be higher when animals are fed too much protein or diets with imbalanced essential amino acid ratios. The ME system differentiates the energy value of feeds by quantifying the energy lost in dung, urine and gases.
Finally, a portion of the feed energy is lost as (unnecessary) heat, called the heat increment (HI). The NE system represents the final step in a logical progression of quantifying energy losses – it differentiates the energy value of feeds by quantifying energy lost in the dung, urine, gases and as heat. Hence, the NE system is essentially the ME value minus the heat increment of feeding. NE represents that portion of the dietary energy that is actually available for maintenance and production (theoretically, the most accurate value).
Several factors influence the HI of a feed, including the chemical composition and animal species (see Table 1), the functional purpose that energy is used for (maintenance, lactation, growth, pregnancy – see Table 2) and the feeding level. It is important to note that within the carbohydrates, structural carbohydrates have much higher HI values than non-structural carbohydrates. Since ruminants are fed diets varying between low and high fibre content, these animals derive very different quantities of NE from the feeds with similar ME content. Hence, description of the energy value of feeds in NE terms is of most importance to ruminant animals.
The main advantages of the TDN and DE systems lie in the relative ease and low cost of obtaining information on digestibility of feeds, and consequently, the accumulation over many years of large databases with TDN and DE values for the majority of commonly used feeds. On the down side, these systems do not account for all the losses of energy that occur during digestion and metabolism. These systems are very well known and still widely.
Whilst the ME system accounts more comprehensively for energy losses during digestion and metabolism than the TDN and DE systems, it does not account for energy lost as heat. It is more cumbersome, time-consuming and thus expensive to measure ME than DE, especially in ruminants, since urine and gas production must be measured and analysed. [In birds, it is easy to measure the output of faeces and urine, since it is voided as a common excrement. Gas production can be ignored for practical purposes. Hence, actual ME values are generally available for poultry feedstuffs.]
Due to the cost and difficulty of measuring ME in ruminants, far less information has accumulated, for far fewer feeds, than is the case with TDN and DE. In fact, a lot (if not the majority) of the ME values of feeds tabulated for ruminants are based on simplistic calculations, assuming a set relationship between ME and TDN or DE (e.g. ME = 0.82 X DE), which does not hold in practice. Depending on the feed in question, ME values can range from less than 50% of the DE value (e.g. poor quality roughages), to over 90% of the DE (e.g. dietary fats). Extreme variation in ME as proportion of DE is possible even for the same feedstuff. Dealing with ME for ruminants in this manner negates much of the potential advantage associated with the more comprehensive accounting for energy losses theoretically vested in this system. Perhaps the most damaging criticism leveled against ME for ruminants is that the system overestimates the energy value of roughages relative to concentrates. Furthermore, ME is not used with the same efficiency for maintenance and production purposes.
The NE system gives the most comprehensive account of energy losses during digestion and metabolism, and is theoretically the most accurate way to describe the useful energy in the feed, particularly for ruminants. Since the energy requirements of animals are fundamentally described in NE terms, the NE system makes it theoretically possible to deal with feed energy values that are directly compatible with the energy needs of the animal. However, the facilities and procedures to measure and analyse all the variables involved in an actual NE measurement, turn it into a very time-consuming and expensive exercise. Hence, even less data have accumulated on actual NE value of feeds than is the case with ME. The theoretical exercise of (simplistically) calculating NE from ME is clearly as open to legitimate criticism as calculating ME from DE!
In the next issue of To the Point you can read how Meadow Feeds harnass technology to derive NEL values for dairy feedstuffs, in order to describe the energy value of feeds in a practically meaningful way.
Date published: 2005-05-16