By: Sandra R. Stokes
Dairy farmers can improve long-term milk production by having a well-managed program for dry cows during the last 3 weeks before calving. In herds with high incidences of metabolic disorders, anionic salts may be a key to reducing milk fever, retained placentas and abomasal displacements.
Even in well-managed herds, with cows in proper body condition and with low incidences of clinical milk fever and ketosis, an additional 500 to 1,000 pounds of milk may be gained in subsequent lactation by avoiding subclinical hypocalcemia.
The most critical time in a dairy cow’s production cycle is the first few days postpartum. How she performs during this time sets the stage for her complete lactation performance. A major factor is how she adapts from the dry cow group into the fresh pen. Cows that experience metabolic disorders at this time produce less and are more susceptible to secondary health disorders such as ketosis, mastitis, retained placenta, displaced abomasum and uterine prolapse. A smooth transition from the dry period into lactation is critical in achieving high peak milk production.
A primary management goal for this period is to control subclinical hypocalcemia and “droopy cow syndrome.” Cows with these disorders expel fetal membranes late, have weak appetites and milk poorly. Consider the changes cows experience around the time of calving: hormonal profiles shift; the fetus relocates before birth, decreasing the cow’s gut capacity; and the mammary gland prepares for lactation. And then she is expected to increase milk production from none to more than 70 pounds a day within a few weeks.
At calving, a cow producing 20 to 25 pounds of colostrum loses 23 grams of calcium in a single milking, or about nine times the amount in her entire plasma calcium pool. Two-thirds of all mature cows are estimated to be hypocalcemic at calving. Drops in plasma calcium pools cause hypocalcemia.
Calcium is vital to smooth muscle contraction. Therefore, postpartum subclinical hypocalcemia affects the reproductive tract’s involution process, and gut motility. Normal blood calcium is 8 to 12 milligrams per decaliter (mg/dl). Blood calcium reduced to 7.5 or 5 mg/dl can lower abomasal motility by 30 and 70 percent, respectively. Reduced abomasal motility from low blood calcium lowers feed intake and displaces the abomasum.
Traditionally, hypocalcemia was thought to be caused by improper dietary calcium levels in the dry period. High calcium intakes in the dry period (more than 70 grams of calcium per day) were thought to suppress the cow’s ability to use bone stores at calving, thereby inducing milk fever.
However, recent research suggests that potassium may play a significant, if not primary, role in subclinical and clinical hypocalcemia. If prepartum diets are high in calcium or potassium, consider an anionic salt program in the close-up dry-cow ration.
Anionic salts are minerals with a high proportion of anions. Anions are negatively charged; cations are positively charged. Living tissue maintains a balance of anions and cations to achieve neutrality. Thus, the balance of anions and cations in a feed should be near neutral.
However, certain cations and anions greatly affect the body’s metabolic processes. In particular, the cations sodium and potassium and the anions chloride and sulfur are considered to greatly influence the acid-base status in the body. The dietary cation-anion difference (DCAD) concept quantifies the major cations and anions in diets. A negative DCAD diet contains more equivalents of anions than cations; a zero DCAD diet indicates balanced equivalents; and a positive DCAD diet contains more cation equivalents.
Research recommends a DCAD of 10 to 15 milliequivalents per 100 grams of dry matter for close- up dry cows. To calculate DCAD in milliequivalents per 100-gram ration of dry matter, use the following formula:
[(% sodium/0.023) + (% potassium/0.039)] — [(% chloride/0.0355) + (% sulfur/0.016)]
A ration using typical forages and concentrates generally has a positive DCAD. Adding anionic salts (magnesium sulfate, calcium sulfate, ammonium sulfate, calcium chloride, ammonium chloride and magnesium chloride) is the only way to achieve a negative DCAD. Use these salts only for the close- up dry-cow group. Do not feed heifers anionic salts. The potential lowering of dry matter intake that can occur when anionic salts are fed outweighs the possible benefits from feeding anionic salts to heifers. Springing heifers normally have fewer problems with milk fever and hypocalcemia, making anionic salts less beneficial to them.
Determining DCAD helps producers estimate a diet’s influence on an animal’s acid-base status. An- ionic diets induce a subacute metabolic acidosis. How anionic salts improve post-calving calcium status is unclear. However, several body systems respond to this dietary manipulation and work together to stabilize plasma calcium pools, including the kidney (controls urinary calcium excretion), intestine (absorbs dietary calcium) and bone (stores calcium).
Feeding negative DCAD diets the last 3 weeks before calving can increase blood calcium. Research has shown that cows fed negative DCAD diets eat more feed in early lactation (Figure 1). University of Idaho research found that cows fed a negative DCAD diet took in less feed prepartum, but more postpartum than cows fed a positive DCAD diets. Furthermore, the negative DCAD diet increased blood calcium, apparently offsetting any harm from reduced intake prepartum.
When selecting anionic salts, consider the source. Some may have higher bioavailability, which increases the likelihood that enough minerals will be absorbed. Do not use sodium chloride or potassium chloride because they are neutral salts that contribute both anions and cations to the balance. They do not affect DCAD.
Although dietary DCAD is relatively easy to calculate, monitoring urine pH of close-up dry cows is a more accurate way to determine the diet’s impact on an animal’s acid-base status. The goal in feeding anionic salts is to lower blood pH, which usually raises blood calcium. Because urine pH drops with blood pH, a good indicator of blood pH is the more easily determined urine pH.
Producers can quickly and easily monitor urine pH on the farm using pH paper or a pH meter. The urine pH of close-up cows eating negative DCAD rations should be between 5.5 and 6.5. If urine pH is higher than 6.5, lower the DCAD level, either by decreasing dietary potassium levels or increasing sulfur or chlorine in the diet. A urine pH lower than than 5.5 means that anionic salt intake is too high; lower salt levels to avoid reduced feed intake, displaced abomasum and kidney overload. Table 1 matches urine pH range with metabolic status.
Because problems can occur when anionic salts are fed, producers absolutely must monitor the close- up pen and control feeding of anionic salts precisely. Use a total mixed ration to ensure that intake of an- ionic salts is adequate. Feed the diet free choice, giving animals access to feed through*out the day. In general, do not pasture dry cows being fed anionic rations because you cannot control or monitor the total feed intake.
Field reports indicate that some herds have had serious health problems when fed anionic salts. In severe cases, animals have died when anionic salts were misfed, probably because they ate too little dry matter because of excessive anionic salt feeding. These cases stress the importance of accurately monitoring feed intake of prepartum cows.
Unless you monitor dry matter intake and anionic salts consumption, do not feed anionic salts. Dry matter intake normally declines as parturition nears, so anionic salts can further depress dry matter intake to the point that metabolic disorders arise. If animals take in significantly less dry matter near parturition, they can be predisposed to such metabolic disorders as displaced abomasum, milk fever, and ketosis. Feed anionic salts only to close-up dry cows. This necessitates having a minimum of two dry cow groups. Also, as mentioned before, separate springing heifers from cows and do not feed them anionic salts.
Feeding negative DCAD diets the last 3 weeks of the dry period can help reduce droopy cow syndrome. But remember to follow the guidelines, particularly regarding farm-specific ration formulation, intake monitoring and urinary pH evaluation, or you may do more harm than good. Proceed only under the guidance of a nutritionist or veterinarian.
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