The relationship between cation-anion differences in dairy cow diets and some blood indices levels and milk composition 1

In dairy cows, changes in dietary cation-anion differences (DCAD) induced significant differences in milk composition and in the blood Ca, Pinorg. and Mg content, as well as in some acid-base balance parameters. DCAD exceeding +350 meq/kg DM decreased the DM and protein content in milk. The same was found for lactose when DCAD were +330 meq/kg DM. The highest serum Ca level was observed when DCAD were about +350 meq/kg DM. Increases in DCAD decreased Mg and Pinorg. concentrations in serum and blood pCO2 and BB, however, the milk fat content and blood pH increased.


INTRODUCTION
and West et al. (1991) were the first to evaluate dietary cationanion differences (DCAD) for cows in lactation.Recently, the influence of a wide range of DCAD on dry matter intake, milk yield and composition (Delaquis and Block, 1995;Roche et al., 2003), acid-base status, blood and urinary mineral levels (Apper-Bossard and Peyrand, 2004) was evaluated.West et al. (1991) reported that milk yield increased when DCAD were +32 meq/100 g DM.Later, West et al. (1992) showed linear growth of dry matter intake when DCAD increased to +46 meq/100 g DM.In our study the influence of DCAD in dairy cow diets with different levels of hay and silages on milk composition and physiological status of animals was examined.

MATERIAL AND METHODS
Two experiments with Black-and-White cows in mid-lactation were conducted over a period of 3 months.The average milk yield per lactation was 5 000 l. Five protein-and energy-balanced rations with different ratio of hay and silages were tested (Table 1).There were 12 cows per group.The feed mineral composition was determined (AOAC, 1995).DCAD was calculated using the formula of Tucker et al. (1991) (meqK+meqNa)-(meqCl+meqS). Milk composition was determined on Milkoscan equipment, the levels of Ca, P inorganic and Mg in blood serum were determined photometricaly with the use of kits (POCH, Gliwice).Some indicators of blood acid-base balance (pH, pCO 2 , BB) were assayed by the Astrup method on Corning equipment.

RESULTS
DCADs in diets with wilted alfalfa-grass silage were significantly higher in comparison with diets containing hay or wilted Italian ryegrass silage (Table 1).When DCAD exceed +350 meq/kg DM (Figure 1), milk DM and protein contents declined.The lactose content decreased when DCAD equaled +330 meq/ kg DM.In contrast, the opposite relationship for milk fat was found.The fat level rose nearly linearly with increasing DCAD.The Ca concentration was highest when DCAD equaled +350 meq/kg DM, however, plasma Mg and especially P inorganic levels dramatically decreased when DCAD increased.The predominance of dietary cations over anions raised blood pH, while as DCAD increased, carbon dioxide partial pressure and buffer base decreased.

DISCUSSION
Many authors (Tucker et al., 1991;Delaquis and Block, 1995;Apper-Bossard and Peyeraud, 2004) demonstrated significant differences in the effects of anion supplementation on decreasing DM intake.This could be explained by the fact that in the study by Apper-Bossard and Peyrand (2004), the diet did not Figure 1.Influence of DCAD on milk composition and blood serum levels of some minerals, as well as on some physiological parameters in blood contain a buffer agent (NaHCO 3 ).Also in that study, cows fed diets with lower DCAD demonstrated lower milk yield, as well as a significant decrease in blood pH and blood buffer base.It could be stated that differences in DCAD levels between +100 and +400 meq/kg DM do not cause significant changes in feed intake and milk production when a dietary buffer is used.Elimination of NaHCO 3 from the diet induced an increase of feed intake and milk yield when the DCAD level rose to about +300 meq/kg DM.The increased milk fat level could be related to the difference in the C 2 /C 3 ratio in the rumen caused by a higher DCAD level (Apper-Bossard and Peyeraud, 2004) or with the S supplement (Tucker et al., 1991).
An increased blood pH level and simultaneous increase in the excess of cations over anions to over +200 meq/kg DM, similarly as in our study, was reported by Roche at al. (2003).They found that DCAD levels over +200 meq/kg DM induced a slight drop in milk yield and concentration of protein, fat and lactose in milk.This excess led to higher Na and lower Cl and S excretion in urine, as well as slight decrease in feed intake and body weight gain.

CONCLUSIONS
In lactating dairy cows, changes in dietary cation-anion differences caused significant differences in milk composition, the concentrations of Ca, P inorganic and Mg in blood serum, as well as in the level of some acid-base balance indicators.