Dan McFarland - Penn State Extension
All cows go through a period of negative energy balance after calving, when the cow cannot consume enough dry matter to meet their body maintenance requirements and the increased nutrient demands of the udder. As a result, the cow must mobilize body fat and protein to meet these requirements in order to maintain a high level of milk pro-duction. This period of negative energy balance in fresh cows lasts for approximately five weeks after calving.
Higher-producing cows have been shown to consume more dry matter per day than lesser-producing animals, and they also partition a higher percentage of their nutrient intake to the udder to support milk production. These high-producing cows also mobilize larger amounts of body fat and protein. However, this still does not provide enough nutrients to support the higher level of milk production in early lactation cows, and these animals experience a greater negative energy balance than average-producing animals.
Iowa State University conducted a 20-year genetic selection project that used either breed average or breed high sires chosen only for predicted differences in milk production. This study provided useful information that directly applies to the issues dairy producers face every day with ketosis and metabolic disease in today’s high-producing dairy cows.
LACK OF ENERGY
The average feed intake as a percentage of bodyweight was the same between both groups for the first two weeks postcalving. This would obviously result in a greater negative energy balance for the high-producing group. The non-esterified fatty acids (NEFA) were higher for the high-producing group and peaked during the first week of lactation. Likewise, the beta hydroxybutyric acid (BHBA) levels were higher for the high-producing group of cows and peaked during the second week of lactation.
The common cutoff point for subclinical ketosis is 1.2 mmol/liter of BHBA in the blood. Animals above this number are considered subclinically ketotic and are usually treated. There have been numerous testing protocols established and most have proven to be quite effective in identifying those animals with subclinical ketosis, as well as determining the overall incidence of ketosis in fresh cows within a specific herd.
AN ENERGY SOURCE
BHBA is produced in the liver as a result of incomplete fat oxidation. Body stores of fat and protein are mobilized at the same time when the cow is experiencing a period of negative energy balance. Even though BHBA is produced above normal levels when higher amounts of fat is mobilized, it can also be an energy source to the dairy cow. One molecule of glucose will result in the production of 248 Kcal of energy,while one molecule of BHBA will result in the production of 279 Kcal of energy.
It is impossible for a high-producing dairy cow to eat enough dry matter postcalving to meet the requirements for the udder to produce high volumes of milk. The cow has no choice but to mobilize fat and protein to try and meet this requirement. The higher amounts of BHBA that are produced during the conversion of NEFA to energy are the primary underlying cause of ketosis.
SHOULD YOU TREAT?
In general, the highest producing cows in any given herd will have higher levels of both NEFA and BHBA postcalving. The dilemma is what to do with animals that are above 1.2 mmol/liter yet appear to be clinically normal, are eating well and do not appear to be suffering from any of the negative effects associated with elevated BHBA levels in fresh cows.
When a ketosis monitoring program is in place, it is important to clinically evaluate these animals that are above the 1.2 mmol/liter cutoff point. Is their head buried in the feed bunk and eating everything they can? Is their udder full of milk? Do they appear well hydrated, healthy and not suffering from low energy levels? Are they producing a high level of milk for being early in lactation? I would propose having a specific cutoff point for all dairy cows is not feasible, and that physically evaluating these animals should also be part of the ketosis monitoring program to determine if they should actually be treated or not.
It’s important we do everything we can to provide the maximum amount of energy to these fresh cows to try and minimize the amount of fat and protein that has to be mobilized. There are multiple energy supplements, bypass fat and glucose precursors that can be added to the fresh-cow ration to help meet these elevated requirements for high-producing cows. Bypass choline can also be used to help move fat out of the liver and allow it to be used in the mammary gland as an energy source and/or increase the fat content of milk.
Obviously, good feed bunk management, not overpopulating the fresh cow pen, high-quality forages, well-balanced diets and excellent palatability of the ration are all important in maximizing nutrient intake and reducing the amount of fat that has to be mobilized by the fresh cow. However, we also have to realize high-producing cows will have to mobilize more fat and protein and as a result their BHBA levels will also be elevated. This does not mean all these animals must be treated just because their BHBA level is above 1.2 mmol/liter. Careful observation of these animals should be part of the ketosis monitoring program to avoid the unnecessary expense and treatment of high-producing animals that are perfectly healthy.