| Popis: |
Beef cattle in the Bowen Basin of Central Queensland may become exposed to drinking water that contains elevated concentrations of minerals, in particular sulphates, chlorides and metal ions Ca2+, Mg2+, and Na+ which are dissolved in rainwater after it accumulates in the pits of open cut coal mines. The current recommended mineral tolerances in drinking water for cattle (e.g. sulphate: 1000 mg/L) rely almost exclusively on the concentration of the mineral in the drinking water as the predictor of an animal's tolerance. These data have been arbitrarily defined from field observations or experiments where drinking water was prepared by spiking town (non-saline) water with minerals (e.g. sodium sulphate). The aim of this study was to generate empirical data to help explain the physiological changes which occur in cattle when they consume sulphates at high intakes, together with other minerals, in their drinking water. Three major studies were undertaken to investigate the effects of pit water (coal mine pit water diluted with town water) consumption on the physiology of cattle when offered as the sole source of drinking water. These studies focused on: (1) the total body clearance of sulphur and other minerals; (2) digestive function and water homeostasis; and (3) salivary secretions, rumen water balance and microbial nitrogen production. In each study, town water (low mineral) constituted the control drinking water treatment. The ingestion of the additional minerals in pit water (2000 mg SO4/L) by steers consuming a lucerne (Medicago sativa) hay diet (restricted intake; 14.9 g DM/kg LW/d) did not significantly alter drinking water intake (DWI), dry matter intake (DMI), organic matter digestibility (OMD) and the retention of N, S, Ca, Mg, Na, and Cl. Minerals contained in the pit water were either directly voided in the faeces or absorbed from the gut and excreted in the urine. Indicators of dehydration and movement of body solutes in steers were not influenced by drinking water treatment. When steers consumed drinking water with a higher concentration of sulphate (either 3780 or 2960 mg SO4/L) and other minerals, together with a roughage diet providing an approximate maintenance N intake (Rhodes grass (Chloris.gayana) hay, ad libitum, plus lucerne hay 0.45 kg DM/d), then the DMI and DWI were significantly reduced. However, drinking water containing 2960 mg S0.4/L did not compromise the in sacco rate of digestion and effective degradability of DM, rumen fluid kinetics, or the concentration of microbial-N and the OMD of the diet. A significant increase in both the concentration of sulphide-S in the rumen fluid and sulphate-S in blood plasma were indirectly implicated in causing the chronic effects on both DWI and DMI. In addition, a combination of a high total-S and an approximate maintenance N intake resulted in a reduced retention of absorbed N. The data describing water homeostasis were equivocal, with a reduction in water intake and an increase in urine volume indicating possible dehydration, but measurements of plasma and saliva osmolality, packed cell volume and rumen volume indicated normality. The ingestion by heifers of a higher osmotic load in the drinking water (pit water; 2690 mg SO4/L) while consuming Rhodes grass hay (ad libitum) did not influence fluid flow into or out of the rumen, thereby maintaining rumen water balance. Saliva secretions and liquid flow from the rumen to the omasum accounted for the greatest proportion of liquid entering and leaving the rumen, respectively. Drinking water which bypassed the rumen accounted for c. 70% of the water consumed in both treatments. Only a small proportion (< 10%) of the total fluid volume entering the rumen was derived from the drinking water. Consumption of pit water did not compromise the digestibility of N, OM, total carbohydrates or carbohydrate fractions. However, a significantly higher concentration of sulphide-S in rumen fluid was associated with an increase in the recycling of sulphate-S from the blood plasma, via the saliva to the rumen, together with a greater flow of microbial-N to the intestine. Despite a greater loss of body water via renal excretion, heifers consuming pit water did not appear to have a diminished hydration status. A companion study revealed that an intra-ruminal dose (16.5 mL/kg LW) of pit water (2690 mg SO4/L) significantly reduced methane production between 45 and 180 min post-dosing. A relationship has been proposed between methane output predicted using rumen fermentation stoichiometry and methane production measured in a closed circuit respiration chamber (r2 = 0.73). The studies undertaken showed that under the conditions imposed, growing cattle are capable of consuming sulphate, together with elevated levels of other minerals in the drinking water, at a concentration not exceeding 2.5-fold the present guidelines for sulphate, without chronic toxic effects. However, the use of concentrations of constituents in the drinking water as an indicator of an animal's tolerance can be misleading. It is proposed that an estimation of the total-S intake (feed + water), expressed on a liveweight basis, together with the proportion of sulphate in the drinking water which 'contributes to the total-S ingested, more accurately defines the physiological response by an animal when exposed to drinking waters containing high levels of minerals. Despite contradictory evidence, it appears that cattle consuming elevated quantities of S, mainly as sulphates in the drinking water, are capable of maintaining water homeostasis. Further investigations involving renal function studies would elicit the mechanisms that control this process. |
