Effect of phosphate solubility on phosphorus kinetics and ruminal fermentation activity in dairy goats

This study was carried out to determine the impact of phosphorus (P) solubility on its kinetics in liquid and bacterial ruminal phases and also on rumen fermentation activity. For the first time, the fractional disappearance rates of ruminal P were estimated and postprandial variations of bacterial P content were studied. Two groups were formed, each with four ruminal-canulated goats weighing 60±1.4kg, and were housed indoors within individual stalls. Their days in milk (DIM) ranged between 42 and 52. During adaptation and experimental periods, goats received twice a day (08:00 and 15:00h) a ration consisting of 400, 300 and 300g/kg of dry matter (DM) of sugar beet pulp silage, grass hay and experimental semi-purified concentrates, respectively. The concentrates supplied 0.71g of P/g of dietary P either as soluble source (monocalcium phosphate, HSP group) or insoluble one (dicalcium phosphate, LSP group). Liquid and solid ruminal contents were obtained at time 0, 90, 180 and 360min after morning feeding. Dry matter intake (DMI) and P intake (Pint) were recorded at each sampling time. Phosphorus in ruminal liquid, ruminal bacteria and volatile fatty acid (VFA) contents were quantified. DMI was 1.22 times higher for the HSP group. Available P concentration was 1.3 times higher for the same group at time 360. Fractional disappearance rates of P from rumen liquid content were not different between groups (-0.007h⁻¹). Postprandial changes (times 0-90) of P concentrations in liquid-associated bacteria (LAB) from LSP group suggested a relative P deficiency status (HSP=1.6, LSP=1.4, ±0.04g/100g of organic matter (OM)). Phosphorus content of solid-associated bacteria (P-SAB=0.59±0.02g/100g of SAB) presented less variation suggesting a capacity to use P from feedstuffs. The total and individual VFA kinetics were affected by P solubility. Under extreme conditions, where P supply derives mainly from inorganic sources, P solubility affects the kinetics of DMI and P concentrations in ruminal liquid, as well as fermentation activity. Phosphorus solubility significantly affects P content of LAB, whereas SAB appeared to adapt to variations of P concentrations in rumen liquid. However, further research is needed in these areas.