No clinical or subclinical hypocalcemia was observed prepartum or postpartum as plasma iCa and tCa concentrations never declined below 1.76 mmol/L or 7 mg/dL, respectively. Feeding a -180 mEq/ kg DM DCAD diet prepartum reduced urine pH which is consistent with previous reports (Santos et al., 2019). The decline observed in our trial is slightly greater than reported in most trials at this DCAD concentration. Anionic salts have been reported to prevent hypocalcemia in multiparous cows at or near calving (Horst et, 1997; Moore et., 2000; Block, 1984). In the current study, cows consuming an acidogenic diet increased serum iCa and tCa concentrations. As a result, limits were set for iCa and tCa at 48 and 48 hour before and after calving for iCa, respectively. The current study is contrary with the results of Moore et al. (2000), who found no change in postpartum Ca concentrations in cows fed an anionic diet. In our current trial, serum Ca reached a nadir 2 d postpartum comparable with previous reports by Joyce et al. (1997) and Romo et al. (1991). Acidogenic diets raise serum Ca concentrations by increasing calcium mobilization from bone, as evidenced by enhanced serum hydroxyproline, a marker of bone resorption, which agrees with Goff et al. (1991), and is mediated by increased serum PTH concentrations (Horst et al., 1997). It was hypothesized that as blood acidity increases, tissue response to PTH increased (Romo et al., 1991; Horst et al., 1997). Because hydroxyproline is a marker of bone resorption (Abu Damir et al., 1994), greater serum hydroxyproline concentrations suggest that cows were better equipped to respond to Ca homeostasis challenges around the time of calving because of their increased ability to mobilize calcium. Our results are similar with previous studies (Block 1984; LeClerc et al., 1989) in which hydroxyproline concentrations increased as the dietary DCAD decreased. Cows fed the − 180 prepartum DCAD had lower serum PTH concentrations 2 d after calving (P < 0.001) than those fed either 0 or -120 DCAD.
The negative DCAD (-200 mEq/ kg DM) diet resulted in a lower urine pH, which was consistent with (Van Soest et al., 1991; Santos et al., 2019). Anionic salts have been proven to prevent hypocalcemia in multiparous cows at or near calving (Horst et,1997; Moore et., 2000; Block, 1984).
Cows fed DCAD diets (-200 and − 120 mEq/kg DM) had lower serum PTH concentrations than cows fed the 0 DCAD diet. The blood PTH levels are usually inversely proportional to iCa concentrations (Jonsson et al., 1980). The rationale for the higher concentrations of blood PTH in cows fed 0 and − 120 DCAD compared to those fed − 200 DCAD is that the parathyroid gland is particularly sensitive to any decrease in blood Ca levels (it responds to changes in ionized Ca levels, but we always measure total Ca which is going to be about 2 times the ionized Ca). Each cow had a normal serum Ca concentration in her blood. A young calf's total Ca level might be 10.5 mg/dl, while an older cow's level might be 9.0 mg/dl. When blood Ca falls below the normal level for that animal, the gland secretes more parathyroid hormone. The PTH concentrations in the blood will increase 10 to 20-fold. If a cow is fed anions correctly, the increase is only transitory and returns to baseline levels by day 2 or 3 (Goff et al., 1989). The target tissues react to PTH and work on bone and kidney cells to bring blood Ca levels back to normal. Although the PTH concentrations increased when blood Ca falls in cows not fed anions, the cow’s blood and urine are more alkaline, and tissues are resistant to the effects of PTH. This is due to the PTH receptor failing to recognize the hormone effectively. As a result, blood calcium levels do not rise rapidly or at all, and the parathyroid gland secretes significant amounts of hormone for a prolonged length of time. The PTH concentrations in the blood of cows with milk fever are extraordinarily high; however, this does not help them maintain adequate calcium levels (Goff et al., 2004).
The current study agrees with Apper–Bossard et al. (2010) who reported no differences of ruminal fluid pH and NH3. The rumen has a strong buffer system to maintain a stable rumen environment by keeping any sudden rise or fall in ruminal fluid pH within a normal range (Tucker et al., 1992). The negative DCAD diets had no negative effect on ruminal content fermentation since the VFAs rose and the ruminal fluid pH did not change considerably. Given the difficulty of interpreting ruminal fluid VFA concentrations, which vary substantially depending on sampling site and rumen dilution rate due to rate of passage and total rumen contents, we would avoid attempting to assign a direct influence of DCAD on ruminal fluid VFA. Because rumen dilution rate and rate of passage can be altered by osmotic changes generated by additional anions with negative DCAD, the apparent increases in VFA concentration with negative DCAD in cows fed − 120 and − 200 mEq/kg DM were difficult to understand.
Feeding a negative DCAD diet to dairy cows in late gestation had no effect on colostrum yield and IgG concentration. This is in line with previous findings (Weich et al., 2013; Diehl et al., 2018; Lopera et al., 2018). This is consistent with the findings of Weich et al. (2013) who reported no difference in birth weight between calves born to cows fed a diet containing − 160 mEq/kg DM (41.1 kg) and calves born to cows fed + 120 mEq/kg DM (41.1 kg) (44.6 kg). Similarly, Diehl et al. (2018) found that feeding a diet containing − 200 mEq/kg DM for 28 days before to delivery had no influence on calves' birth weight when compared to a control diet containing − 30 mEq/kg DM.
Colostrum quantity and composition were unaffected by lowering DCAD in prepartum diets from about + 130 to -130 mEq/kg DM. Brix levels are closely connected with colostrum IgG concentrations (Martinez et al., 2012), and Brix readings of 21% are a good indicator for colostrum high enough IgG concentration to transfer passive immunity, as observed in the current study (50 g IgG/L; Quigley et al., 1998).
Results for production were similar to previous studies (Weich et al., 2013; Balbir et al., 2017; Silva 2015) that reported feeding negative DCAD prepartum did not affect postpartum milk yield. Moore et al. (2000) fed anionic salts and observed no depression on milk yield from 7 to 70 days in milk. In contrast to our findings, Beede et al. (1991) observed that addition of anionic salts to the prepartum diet increased milk yield in the following lactation by 3.6%. Our results are not consistent with those of others (Roche et al., 2005; Ju et al., 2007a,b) who have reported a positive association between DCAD and milk fat test. Several studies have shown that addition of dietary rumen buffers such as NaHCO3 and K2CO3 increase milk fat percent, especially when depressed milk fat occurs (Hu et al., 2007a,b). West et al. (1992) reported an increase in the milk fat percent by in-creasing DCAD, without any adverse impact on milk yield. Conversely, some studies have reported an increase in milk production without changes in milk fat percent (Tucker et al., 1991; West et al., 1991). No difference in percentage of lactose associated with altered DCAD agrees with other studies (West et al., 1992; Tucker et al 1988). Martinez et al. (2018) evaluated daily milk production and weekly milk fat and protein content of 79 cows and did not find an effect of DCAD treatment effect on milk yield, ECM, or true protein content, but they reported approximately 0.23 percentage points higher fat in the first 49 postpartum when cows were fed negative DCAD prepartum. Our results differ from those of Leno et al. (2017) who compared three DCAD levels (+ 183, + 59 and − 74 mEq/kg of DM) and noted an increase in milk yield and ECM in the first 3 weeks postpartum by decreasing prepartum DCAD. Our results agree with those of Santos et al. (2019), who found in multiparous cows that fat content was not different between DCAD groups, but milk protein content increased by feeding an acidogenic diet prepartum.