Most dairy nutritionists have different philosophies when it comes to dairy cow ration formulation. But we can all agree on one thing: It’s not just crude protein and net energy of lactation anymore.
Dairy cow ration balancing has become much more sophisticated in the last 15 years with advances in nutrition research, ration formulation models and feed analysis methodologies.
CP versus AA balancing
One of the most impactful changes in dairy nutrition in the past 15 years was a move away from formulating dairy rations for crude protein (CP) content. Dairy cows do not have CP requirements; they have amino acid (AA) requirements. Amino acids are the building blocks of proteins.
Research and field experience have confirmed that when cows absorb essential AA in a profile that closely resembles the profile their bodies require, total essential AA requirements are reduced and efficiency of protein utilization is maximized.
Therefore, balancing rations for AA has allowed nutritionists to significantly reduce the amount of CP fed. Increased efficiency of protein utilization and lower CP in rations result in less waste nitrogen to the environment.
In the past, we overcame amino acid deficiencies in lactating cow rations by feeding high levels of CP, of which a high proportion came from expensive rumen-undegradable sources.
The strategy has shifted, and the first goal of balancing a ration for AA is to provide enough rumen-degradable protein to maximize the efficiency of microbial protein synthesis in the rumen. Energy supply – particularly fermentable carbohydrates like sugar, starch and digestible fiber – will drive microbial protein yield.
The second goal is to balance rumen-undegradable protein, including bypass methionine and lysine sources, to optimize metabolizable protein, which is the true protein absorbed in the intestine.
Microbial protein synthesized in the rumen, together with rumen-undegradable protein, comprise the majority of the metabolizable protein passage to the intestine. Concentrations of lysine and methionine in metabolizable protein have a great impact on protein content of milk and milk yield. New research data shows health and reproductive benefits when amino acid-balancing rations.
How about energy?
The energy system used for lactating and dry dairy cows is net energy of lactation (NEl). Determining feed and forage energy content beyond digestible energy at maintenance, although not completely accurate, is commonly done.
Dry matter intake has a huge impact on diet digestibility and, therefore, on the energy concentration of the diet. Most feed-testing laboratories calculate feed and forage energy concentration based on three times intake over maintenance.
This energy estimation is the most widely used by ration formulation programs, and it is often denoted as NEl-3X. Although a good approximation, nutritionists should not get hung up on balancing rations for NEl concentration but focus on the intake and characteristics of the individual energy sources, primarily ruminal neutral detergent fiber (NDF) and starch digestibility and type of fat (supplemental and native to the basal diet).
NDF and starch are the major contributors to dietary energy for lactating cows. Recent and ongoing research has shown that uNDF (the highly indigestible fraction of NDF) is a more robust predictor of digestibility of a feed than lignin. Also, uNDF has also become a very useful tool to benchmark forages in terms of potential rumen fill and dry matter intake constraints.
The difference between total NDF and the uNDF content of a ration (potentially digestible NDF) is the amount of fiber that has the potential to be fermented in the rumen and turned into available energy to the cow.
The extent of ruminal NDF degradation will be determined by the rates of digestion and passage. Nutritionists in the field rely on models that use laboratory-generated inputs of uNDF and rate of NDF digestion to predict the impact of rumen NDF degradation of feedstuffs on DMI and milk production of cows.
Another critical component of the NDF not reflected by just balancing for NDF content is the amount of physically effective fiber in a ration, which is important to promote cud chewing and rumen function and health.
Dairy cows are fed large quantities of starch because it is highly digestible and energy dense. Although cows do not have a requirement for starch, it is often a key nutrient to maximize production. When balancing dairy rations, nutritionists pay close attention to the types of proteins in corn silage and high-moisture corn as markers of ruminal starch digestibility.
Moreover, some nutrition models now rely on seven-hour in vitro starch degradability measured by commercial labs to benchmark the rate of starch digestion. Physical and chemical factors that affect starch digestibility include type of endosperm, particle size, kernel processing, storage method and moisture content.
Fat is another important energy source for dairy cows. As with CP, nutritionists should move away from relying exclusively on the total fat content of the ration. Instead, the focus should be feeding our dairy cows specific fatty acids (FA).
The composition of the fat, especially its unsaturated FA content, can have profound effects on rumen fermentation and production responses. Under certain dietary conditions, unsaturated FA can form unique trans-FA intermediates in the rumen that are potent inhibitors of milkfat synthesis.
Rumen unsaturated fatty acid load is one tool currently used in ration formulation to help determine how much fat can be fed to lactating cows. It is calculated as the sum of oleic (C18:1), linoleic (C18:2) and linolenic (C18:3) acids – the three main unsaturated FA consumed from all ingredients in a ration.
Forages and grains may account for the majority of the rumen unsaturated fatty acid load intake and, therefore, should not be overlooked. Ongoing research suggests that high-producing dairy cows can be marginally deficient in essential FA, primarily C18:2 and C18:3, which may impact reproductive function and performance.
Cows are always right
No doubt, feeding high-producing cows has become more complex and sophisticated. Nutritionists need to remember that cows let us know if the ration is optimal, not the computer.
An old adage says three different rations can be delivered to dairy cows: the ration the nutritionist balances, the ration the feeder mixes for the cows, and the ration the cow may sort through and decide to eat.
Nutritionists need to walk and read the cows, make sure the right feed is delivered to the right group of cows in a timely and consistent manner and strive to achieve a good-quality forage program as well as promote excellent cow comfort and management. Paying close attention to the fundamentals helps our dairy cows achieve the true potential of their nutrition program. 
-
Silvia Onetti
- Director of Nutrition
- Vita Plus
