Most research evaluating diets with negative DCAD for prepartum cows has been done with multiparous cows. Given that multiparous cows have a greater incidence of hypocalcemia and other postpartum diseases that seems logical. However, nulliparous cows, also called first-calf heifers, represent approximately 30 to 40% of cows in the prepartum pen on most dairies. And on many farms nulliparous and multiparous cows share the same pen and diet prepartum. That means nulliparous cows are fed negative DCAD diets without fully understanding the benefits, or the potential negatives.
Nulliparous cows may indeed benefit from negative DCAD diets prepartum. But it could also be that feeding acidogenic diets, especially an excessively acidogenic diet, could result in reduced postpartum performance. Data with nulliparous cows is scarce, explains José Santos, professor of dairy cattle nutrition and reproduction at the University of Florida. So that answer remains unclear. The two recent meta-analyses (Lean et al., 2019 and Santos et al., 2019) only contained data on 151 nulliparous cows. That’s just not enough cows to definitively say yes or no if a change in diet caused a change in outcome.
New research from the University of Florida, and recently published in the Journal of Dairy Science, sheds new light on some of the questions surrounding acidogenic diets and nulliparous cows. The study was conducted at the University of Florida dairy unit in Gainesville. At about 250 days of gestation, 132 nulliparous cows were randomly assigned to 1 of 3 different DCAD diets: +200, -50, and -150 mEq/kg of DM until calving. All prepartum diets had the same forage-to-concentrate ratio and similar contents of NEL, crude protein and metabolizable protein. After calving all cows were fed the same lactation diet, milked 4 times daily during the first 100 days in milk and then 2 times daily through 305 days in milk to determine cumulative milk. All cows received timed A.I. for their first insemination.
The range of DCAD used in the study is similar to the range of DCAD reported in the two meta-analyses. In addition, current research recommends that multiparous cows be fed a prepartum diet with a DCAD between -50 to -150 mEq/kg to reduce the risk of hypocalcemia.
As expected, DMI declined linearly with the decrease in DCAD during the prepartum period. Because of the reduced DMI, the intake and balance of NEL also decreased linearly during the prepartum period. But during the first 100 days in milk there were no differences in DM or NEL intakes between dietary treatments.
Dietary treatment did not affect colostrum yield. Nor did it affect the concentrations or yields of fat, true protein, lactose, solids not fat in colostrum, Brix value or the total IgG concentration of the colostrum.
Milk yield also was not affected by dietary treatment. Nulliparous cows fed prepartum diets with +200, -50 and -150 mEq/kg produced an average of 80.7 lbs, 80.9 lbs and 79 lbs of milk, respectively, during the first 100 days in milk. Energy corrected milk and 3.5% fat corrected milk were also not affected by dietary treatment. Cows fed a moderate negative DCAD diet of -50 mEq/kg had the highest 305-day cumulative milk yield producing 22,057 lbs, but it was not statistically different from cows fed the other two dietary treatments.
“Our results clearly demonstrate that manipulating the DCAD of prepartum diets fed to nulliparous cows did not affect composition or yield of colostrum, productive performance in the first 14 weeks of lactation, or the cumulative milk yield at 305 days postpartum,” explains Santos. “We feel confident in telling producers that there is no advantage of feeding acidogenic diets to nulliparous cows if the objective is to improve productive performance.”
The study also looked at reproductive performance. Results showed there was no difference from dietary treatment in the pregnancy at the first AI. However, at the end of the 305-day observation period, there was a significant difference in the percent of cows pregnant by dietary treatment. Nulliparous cows fed the most acidogenic diet had the greatest percent of pregnant cows: 93.2% compared to 88.9%, and 76.7% for cows fed the -50 and +200 mEq/kg diets, respectively. While this experiment provides initial evidence that acidogenic diets may influence reproduction in nulliparous cows, more research is needed to replicate results.
The urine pH of nulliparous cows averaged 8.22, 6.67 and 5.41 for cows fed prepartum diets with a DCAD of +200, -50 and -150 mEq/kg. No cases of clinical milk fever were diagnosed. Results of both iCa and tCa showed that none of the cows’ calcium concentrations dropped below established thresholds for subclinical hypocalcemia.
Incidence of clinical diseases, which included retained placenta, metritis, displaced abomasum, mastitis, lameness and respiratory diseases, were tracked from calving through the first 100 days in milk. “We were able to detect quadratic differences in uterine disease, morbidity and the risk of multiple diseases,” says Santos. Cows fed the -50 mEq/kg diet had less risk of those issues than the mean of cows fed the +200 and -150 mEq/kg diets. Results for disease incidence for cows fed -50, + 200 and -150 mEq/kg diets were as follows: uterine disease, 25.6, 36.3 and 46%; for morbidity, 28.1, 41.4 and 55.6%.; and for multiple disease it was 8.9, 16.3 and 29.6%, respectively.
Although reductions in the risk of disease shown from feeding the -50 mEq/kg diet were large, the total number of cows in each treatment was limited to make inferences related to health and reproduction. As with any first of its kind experiment, the study will need to be replicated to improve the strength of the data and to be able to provide concrete recommendations for dairy producers.
At this point “we can say that feeding acidogenic diets to prepartum nulliparous cows up to -150 mEq/kg has no effect on milk production,” says Santos. “We can also say that overfeeding anions (causing excessive metabolic acidosis with a urine pH <5.8) is a bad idea for nulliparous cows.” Inducing excessive metabolic acidosis impairs energy metabolism in part because of the reduced intake but, more important, because of the negative effect uncompensated metabolic acidosis has on regulatory hormones that are important for energy metabolism.
So, if your multiparous cows and primiparous cows share the same prepartum pen and diet, you might want to consider feeding a DCAD diet closer to -50 than to -150 mEq/kg or provide separate pens and diets.