Effect of prepartum dietary cation-anion difference strategy and level of dietary calcium on postpartum blood calcium status and milk production of multiparous Holstein cows.

The transition from late pregnancy to early lactation in the cow is a critical period which often results in disruptions to homeostatic mechanisms, including a rapid increase in demand for Ca for synthesis of colostrum and milk. The rapid increase in Ca typically results in decreased circulating Ca concentrations, referred to as hypocalcaemia. Clinical hypocalcaemia, often referred to as milk fever, is detrimental to the health and productivity of dairy cows and over many years has resulted in extensive research. More recently, subclinical hypocalcaemia has been recognized as a much more prevalent than originally thought, as associations have been seen between low concentrations of blood Ca at four days in milk (DIM) and reduced milk production, poor reproductive outcomes, and increased health disorders such as retained foetal membranes, metritis, and displaced abomasum.

The concept of manipulating the dietary cation-anion difference (DCAD) of the pre-calf cow diet to mitigate clinical hypocalcaemia has existed for decades. Pre-calf manipulation of the cows’ acid-base balance to restore tissue parathyroid hormone (PTH) sensitivity needs to be low, some work has reported a pH less than 6.0. To achieve this, recommendations around anion supplementation for the close-up period have been mixed; some work has suggested that aggressive anion supplementation targeting low ranges of urine pH (UPH) should be discouraged to avoid losses in DMI and avoid potential negative health outcomes. In light of this, some researchers reported that optimal UPH should be between 6.2 and 6.8. Other researchers recommended a more aggressive approach suggesting UPH should be below 6.0 to increase Ca metabolism based on an increase in urinary Ca flux. In view of this uncertainty, the main objective of the study cited was to compare the effects of feeding between two levels of DCAD, between two levels of dietary Ca, and between their interactions on Ca metabolism, production of milk and milk components, and DMI of dairy cows during the transition period of early lactation. The authors hypothesized that cows fed a more negative DCAD pre-calf (as evaluated by UPH) and greater dietary Ca will improve post-calving Ca status as well as greater DMI and milk production.

Thus, the study examined two pre-calf DCAD levels, two pre-calf dietary Ca levels, and their interactions on post-calving Ca metabolism, DMI, and milk performance of Holstein cows. In a randomized block design, data from 98 Holstein cows enrolled in the study from 32 days before expected calving through 63 DIM were used for analysis. At 26 days before expected calving, cows were assigned to one of four treatments in a 2 × 2 factorial arrangement, with main effects DCAD (−2.61 mEq/100 g, partial acidification [PAS] vs. −10.26 mEq/100 g, full acidification [FAS]) and Ca (1.5% DM, high Ca [HCa] vs. 0.7% DM, low Ca [LCa]). Cows fed FAS and PAS diets targeted urine pH of 5.5 to 6.0 and 6.5 to 7.0, respectively. Cows fed FAS had lower urine pH and greater pre-calf urinary ammonium excretion than cows fed PAS (FAS 5.64 vs. PAS 6.71 ± 0.10 pH). Urine was collected for mineral analysis once per week upon enrollment until parturition and at one, two, and three DIM. Blood samples were collected once per week from enrollment until one week before calving, then at 0.5, 1, 1.5, 2, and 3 DIM for analysis of Ca, Mg, and P. Dry matter intake and milk production were recorded daily, and bodyweight data were collected weekly. Statistical analyses were conducted using SAS software (v. 9.4) using linear mixed models with covariates and repeated measures when appropriate.

Differences in pre- and post-calving DMI were not detected between FAS- and PAS-fed cows. Pre-calf DMI was not affected by dietary Ca level. Post-calving DMI and energy-corrected milk (ECM) were greater for cows fed HCa than LCa pre-calving. Circulating post-calving total Ca was not different from pre-calving Ca or DCAD. Cows fed FAS excreted more urinary Ca than cows fed PAS pre-calving and cows fed LCa excreted more urinary Ca than their HCa fed counterparts.

In summary: The study sought to evaluate the effects of dietary Ca, DCAD, and their interaction on Ca metabolism, milk production, and DMI. Cows fed HCa pre-calving had decreased post-calving ionic Ca concentrations, but greater postpartum DMI and ECM yield. Despite concerns about adding anions to target UPH below 6.0, cows fed FAS tended to have improved post-calving DMI as a percentage of BW and less BW loss during early lactation. Although interaction of Ca × DCAD were not generally significant, the improvement of performance outcomes based on HCa- and FAS-fed cows suggest that more work should aim to elucidate the effects of the interaction of high Ca diets and pre-calving DCAD strategies targeting UPH below or at 6.0. It was shown that feeding a more negative DCAD neither improved calcaemia nor milk yield, but it increased excretion of urine ammonia, supporting the concept that cows have multiple mechanisms to compensate for their systemic acid-base balance, particularly as UPH decreases below 6.0.