The transition from the dry period to lactation is the most stressful part of a dairy cow’s life. Physiological and hormonal changes accelerate during the eighth month of gestation as the milk secretion glands enlarge in the udder and the cow prepares to give birth.
Nutritional requirements increase significantly and play a pivotal role in the cow’s energy status and health both at the time of birth and in the early weeks of lactation when milk production is reaching its peak.
Managing calcium metabolism and reducing the prevalence of hypocalcemia (milk fever) in transition cows continues to challenge even the best dairy farmers and managers. Calcium is a macromineral necessary for bone formation and muscle function.
Cows that are hypocalcemic for extended periods of time have poorly functioning rumens that lead to other metabolic problems such as ketosis, displaced abomasums, laminitis, retained placentas, metritis and mastitis. Research has shown that cows with even subclinical levels of milk fever are likely to produce several hundred pounds less milk over the course of the lactation.
The beginning of lactation places a sudden and large demand on the calcium supplies and the mechanisms that keep it in balance in the dairy cow. A cow producing 22 lbs of colostrum will lose 23 grams of calcium in a single milking. This is about nine times as much calcium as is present in the entire plasma calcium pool of the cow.
Calcium lost from the plasma pool must be replaced through both increased calcium absorption in the intestine and calcium resorption from the bones. During the dry period, calcium requirements are minimal and these mechanisms for replenishing plasma calcium are relatively inactive and are slow to start up again at the time of calving.
Cows normally do a good job of keeping calcium balanced through a complex interaction of Vitamin D and parathyroid hormone (PTH) – except at the time of calving. Because the replenishing mechanisms needed to metabolize calcium are slow to respond at the time of calving, nearly all cows experience some degree of hypocalcemia during the first days after calving.
Intravenous calcium treatments have been the treatment of choice for decades to help the cow along while intestinal and bone mechanisms have time to adapt.
As far back as the 1980s, dairy researchers have recognized that macrominerals and their ions play a critical role in cellular metabolism. We know that at the time of freshening it is necessary for calcium to be pulled from the bones because dietary calcium and the mechanisms required to metabolize it from the diet are not operating at full capacity.
In order for this to occur, the cow’s system must be slightly acidified – which involves a negative ionic charge called anionic – to draw out the positively charged (cationic) calcium ions and get them into the bloodstream.
All ions have either a negative or positive charge. A simple way to understand the anionic-cationic dynamic is to think about the poles of a magnet. One pole is positive and the other is negative. Like poles repel each other and opposite poles attract each other. The same type of thing happens with ions in the bloodstream.
In the case of the positively charged calcium ions, there needs to be a negatively charged environment to pull them out of the bones and get them into the bloodstream. If the bloodstream is overloaded with other positive ions – remember, like charges repel each other – the calcium ions can’t get to the bloodstream because they are being held back.
This is often the case in a close-up cow diet and the troublemakers are most often sodium and potassium ions, which also have powerful cationic charges.
When balancing diets for dry cows that are close to calving, we need to get the system slightly acidified for approximately two to three weeks prior to calving. This acidification requires knowing the values of the two cations (potassium and sodium) and the two anions (sulfur and chloride) for the entire ration.
Therefore, an analysis of all the feedstuffs is required. The determination of whether a diet is anionic or cationic is calculated using a formula which measures the acid/base balance in milliequivalents per 100 grams of feed.
((Na/.023)+(K/.039))-((Cl/.0355)+(S/.016))=meq DCAD
The cation-anion balance is most often known as the dietary cation anion difference or DCAD. The DCAD formula will result in either a positive or negative value when the cations are added together and subtracted from the sum of the anions.
A positive value indicates that the diet is alkaline (more cations) or, if negative, acidic (more anions). A properly formulated anionic diet will result in a negative value typically around -1 to -6 meq per 100 g. (There are variations of this formula in use that incorporate other dietary minerals including phosphorus, calcium and magnesium, all of which have small additional impacts on the resulting calculations).
The importance of having accurate values for the four main minerals in the dry cow diet cannot be overstated. Particularly in forages, mineral values vary considerably.
Grass and legume forages are notoriously high and variable in potassium which is the primary cation affecting DCAD, resulting in a positive value. Chloride and sulfate salts added to a dry cow/close-up diet are the means by which a positive DCAD becomes a negative DCAD.
All milk cow diets are positive DCAD diets due to the high levels of potassium in forages. When formulating close-up diets, look for low-potassium forages.
The higher the potassium in the diet, the more chloride or sulfate that must be added, and there will come a point where the ration becomes both unpalatable to the cow and too expensive to be practical.
Table 1 , illustrates two simple dry cow rations that have been balanced for about 23 lbs of dry matter. One diet contains corn silage and a small amount of hay.
The other contains no corn silage and all forage comes from the same hay. A dry cow supplement of about eight lbs (as fed) is added to each diet. Dietary calcium in this example was kept close to 100 grams per day. PD
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John Hibma
- Nutritionist
- Central Connecticut Co-operative Farms Association
- Email John Hibma
