Effects of dietary cation-anion difference in lactational performances of dairy ewes

Dietary cation-anion difference (DCAD), defined as the mathematical difference between cations and anions present in a ration, is calculated as mEq per 100 g DM equal to (Naplus plus Kplus) – (Cl− plus S2−). There are no available data on the effects of DCAD of dairy ewes. Thus, the objective of this study was to determine the effects of diets differing in DCAD values on the lactational and acid-base performance of dairy ewes. A total of 40 ewes from 2 dairy breeds (Manchega, n equal to 20, Lacaune, n equal to 20) at mid lactation were used. Ewes were allocated to 8 groups of 5 animals and blocked by breed, BW and milk yield. Dietary treatments were applied for 10 wk and consisted of total mixed rations in which the DCAD value was modified from anionic to cationic values being: 6.6 (AN-5), 26.3 (AN-25), 45.2 (CA-45) or 64.1 (CA-65) mEq per 100 g DM. Individual dry matter intake (DMI) was measured at wk 5 and 10 using PEG as indigestible external marker. Milk yield and milk composition were recorded weekly and biweekly, respectively. Jugular blood samples for acid-base balance were taken and analyzed at week 4, 8 and 10. The DMI, DMI per kg BW0.75, milk yield and the energy corrected milk (ECM) showed a quadratic response by effect of DCAD. Maximum intake was calculated between 40 and 45 mEq per 100 g DM, while maximum milk yield was calculated between 50 and 65 mEq per 100 g DM. Moreover, we observed differences in acid-base blood indicators according to breed. Manchega ewes showed a neutral compensated stage, while Lacaune ewes passed from metabolic acidosis to a neutral stage, depending on DCAD value. Urine pH showed a quadratic response by effect of DCAD and was a representative indicator of the DCAD values in the ration. Blood urea and Cl- increased in anionic treatments as a consequence of the greater contents of NPN and Cl- in the anionic salts used. Blood Na+ increased in order to maintain the cell electroneutrality. Finally, hematocrit and hemoglobin in the cationic rations decreased by effect of DCAD, which was attributed to an increase in water intake. According to the obtained results, diets containing DCAD values in the range of 35 to 45 mEq per 100 g DM are recommended for lactating dairy ewes. High yielding dairy ewes were more sensitive to DCAD than low yielding ewes