The objective was to compare Ca serum concentration and frequency of severe [Ca <2.0 mmol/L] and moderate [Ca <2.125 mmol/L] subclinical hypocalcemia (SCH) in cows supplemented according to the dietary cation-anion differences (DCAD) principles or with synthetic zeolite. The trials were performed in 2 large commercial farms in US and blood samples were taken within the first 24 h after calving. The test consisted in comparing Ca serum concentration when the farms were using DCAD management for preventing hypocalcemia –in Farm 1 at -14.7 mEq/100 g DM (full DCAD) and in Farm 2 at 0.62 mEq/100 g DM (partial DCAD) - with the Ca serum concentration levels when synthetic aluminum silicate was supplemented by using the commercial product X-Zelit (Protekta Inc./Vitfoss, Graasten, DK). The differences in the plasma calcium concentrations between treatments were tested within farms with ANOVA using the lm procedure in the R package. Fisher’s exact test was used to assess differences in incidences of SCH. The plasma calcium concentration was significantly (P < 0.001) higher in the X-Zelit group compared with the DCAD group on both farms (Table 1). The frequency of animals having severe SCH was 57 and 65 percent in farm 1 and 2, using full and partial DCAD approaches, respectively. During this period, the frequencies of moderate SCH were 83 and 88 percent in farms 1 and 2. During the treatment with X-Zelit the frequency of severe SCH were reduced by 5 and 0 percent in farm 1 and 2, respectively, while the reduction of incidence in moderate SCH were 20 and 13 percent respectively in farm 1 and 2, in both cases significant (P < 0.001). Results suggest a lower SCH frequency using X-Zelit compared to low or neutral DCAD feeding.

Table 1. Least squares means ± error of mean of blood parameters in multiparous cows

Farm	DCAD			X-Zelit
n	Plasma Ca, mmol/L	n	Plasma Ca, mmol/L
1	54	1.98 ± 0.02		40	2.26 ± 0.02
2	76	1.92 ± 0.02		84	2.26 ± 0.01