اثر منبع کاتیونی و DCAD جیره گاو شیری بر فراسنجه‌های تخمیر شکمبه در شرایط برون تنی

Introduction Minerals are an essential component of all biochemical reactions of the animal's body and microorganisms. The difference between specific anions and cations has a greater contribution to the physiological performance of the livestock than their individual effects. In the last decade, several studies have conducted on the use of diet minerals and their interaction on increasing dry matter intake. Recent studies have shown that high-yielding cows in early lactation respond well to raising the level of dietary anion-cation difference in order to increase milk production. Published results are much fluctuated regarding to mineral source and the changes scouring in rumen ecosystem. The aim of this study was to evaluate the effect of DCAD level and cation source on rumen digestion kinetics.
Materials and methods In order to investigate the effects of cation source and dairy cow diet’s DCAD level on microbial fermentation properties, the gas production and batch culture experiments were used. The experimental diets were prepared for use in the gas production method based on the metabolic needs of dairy cows using NRC system software. Then, the samples were milled and screened by one-millimeter mesh, and dry matter, crude protein, crude fat, ash, NDF and ADF were determined according to AOAC methods. In addition, the concentration of calcium, sodium, potassium and magnesium elements was determined by atomic absorption spectroscopy method, phosphorus, and sulfur were determined by colorimetric method and chlorine by gravimetry. Finally, the dietary cation-anion difference (DCAD) was calculated based on the Goff equation. The gas production parameters, the time corresponding to half of the maximum produced gas (t0.5), rumen acidity and dry matter degradability were measured in a 2 × 2 × 3 factorial experiment. Main factors were included of DCAD (+150, +250 and +350 meq/kg DM), potassium sources (Potassium carbonate (KC)) and Potassium carbonate sesquihydrate (KCS)) and magnesium sources (magnesium oxide (MO) and magnesium carbonate (MC)). 
Results and discussion Based on the results, there was a significant difference between treatments in gas production parameters, t0.5, ruminal pH and dry matter degradability. Treatment with DCAD of  +150 with two sources of KC and MC showed the highest amount of gas produced during 120 hours of incubation, with the highest potential for gas production and the highest gas production rate. In this treatment, since the fermentation rate was higher, t0.5 was reduced. The main effects of two sources of potassium have a statistically significant difference, so that potassium carbonate has higher gas production and fermentation rates than potassium carbonate sesquihydrate. The interactions of DCAD, magnesium and potassium sources on the pH of the culture medium and the surface under the pH curve were not significant. Among the main effects, the effect of magnesium source on pH of culture medium was not significant, but the potassium source had a significant effect on pH and the surface under the pH curve, so that the potassium carbonate source had higher ruminal pH than the Potassium carbonate sesquihydrate. There was no significant difference in ruminal acidity with increasing DCAD level during 12 hours of incubation. However, at time 24 and 72, the acidity index increased with increasing DCAD. Dry matter degradation was influenced by different experimental treatments. In general, in different treatments, the apparent digestibility of dry matter has increased linearly with increasing levels of DCAD. The increase in DCAD results in an increase in rumen pH and rumen buffering capacity, which leads to an increase in the concentration of rumen-derived volatile fatty acids and improved rumen function, and increases the degradability. The difference in gas production from dry matter degradation between different levels of DCAD was also statistically significant, So that DCAD +250 and +350 may increase the degradability of dry matter and consequently, increased gas production by improving the culture medium status. The use of the KCS source reduced the fermentation rate and increased the fermentation time, which increased the digestibility of dry matter.
Conclusion Generally, it can be concluded that DCAD increases the rumen pH and buffering capacity, and therefore the use of KC and MC sources with DCAD +250 levels improves rumen fermentation, But the use of the KCS source specifically increases dry matter intake by supplying potassium for the microbial population and increasing digestibility. Based on this, potassium carbonate sesquihydrate and magnesium oxide with DCAD +250 seems to be an appropriate supplement to dairy cow.