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SUMMARYThe results of six experiments on growing cattle weighing 140–480 kg, and with liveweight gains of —0·46 to 1·11 kg/day, were reanalysed to provide estimates of their phosphorus (P) requirements. The 158 data sets were from individually penned cattle offered barley straw-based diets ad libitum with dry matter digestibilities of 0·53–O·65, or from cattle grazing tropical pastures with in vitro dry matter digestibilities ranging from 0·50 to 0·62. Various concentrations of dietary energy, nitrogen (N), calcium (Ca) and P were imposed during the course of the experiments with the penned cattle and various rates of application of P fertilizer changed the botanic and nutrient composition of the forages available to the grazing cattle. The P balances and P kinetics of the cattle were studied using 32P as a tracer.Over the range of P intakes normally observed in cattle consuming forage diets (10–60 mg/kg LW), the coefficient of P absorption was high and not affected by age or liveweight. The regression coefficient relating P intake to P absorption was 0·77 for unsupplemented grazing cattle and 0·82 for penned supplemented cattle. When the plasma inorganic P concentrations were < 50 mg/1, urinary P excretion of the penned cattle was low, as were the endogenous faecal P losses of both the penned and grazing cattle. These losses were concluded to represent obligatory losses and were related to dry matter intake (r = 0·73) in the range 9–17 mg P/kg LW.The total P requirements of growing cattle were estimated as g/day and g/kg DM intake from this data. The requirements of cattle consuming forage diets were 40–50% lower than those published by the Agricultural and Food Research Council (AFRC 1991), even though the same equation for the net requirements for growth was utilized.

SUMMARYStudies were conducted to elucidate the nature and cause of the drastic losses of live weight of cattle grazing buffel grass pastures after first rains at the end of the dry season in northern Australia. This paper examines trends in weight and body composition during the dry and early wet seasons; it shows that although most of the loss in fasted live weight occurred just after first rain, most of the loss of body solids, mainly fat, had already occurred by this time. Losses of body solids in the dry season were not fully reflected in live-weight losses because of increases in total body water and in gut ‘fill’. The greatly accelerated weight loss in the period following first rain appears to have been due mainly to a large reduction in gut contents. Empty-body weight actually increased during this period owing to increases in tissue water. Continuation of this trend in tissue water into the early wet season resulted in the rate of live-weight gain greatly exceeding that of body solids.

SUMMARYFollowing the measurement of tritiated water (TOH) spaces, 31 cattle were slaughtered and chemically analysed in this study. They included several breeds, both females and castrate males, and were of varied nutritional history. Their body-fat content ranged from 4 to 21% of fasted live weight.Total body water (including the water in the gut contents) was reliably estimated from TOH space, measured after allowing an overnight 16 h waterless fast for TOH equilibration. Following this regime, residual D.M. in the gut contents amounted to 1·75% of fasted live weight. The relationships of body fat to body weight, and body fat to body water when both were expressed as percentages of body weight, were too variable to be used in any predictive fashion. Equations were derived, using fasted live weight, allowing the accurate estimation in vivo of the quantities of the chemical components in the whole body (i.e. total body minus D.M. in gut contents).It was demonstrated that the sum of total body water and total body fat constituted virtually 80% of total body tissues, and that total body protein closely approximated 80% of the fat-free dry matter, in cattle varying widely in body condition. These relationships constitute the physiological basis of the equations presented.Comparable principles appear to apply to sheep, and a range of other mammalian species.

SUMMARYThe determination of feed phosphorus content using oesophageally fistulated cattle is reported in this paper, from an experiment in which salivary phosphorus was labelled with 32P.An intravenous infusion of Na232PO4 to cattle produced an immediate increase in the specific activity of salivary phosphorus, which then fell rapidly to an essentially linear asymptote by 3 h after the infusion. The phosphorus content of consumed feed was calculated from the degree of reduction in salivary specific activity by the feed phosphorus, expressed as the ratio of the specific activities of bolus and saliva phosphorus.A dose of 100 /μCi 32P allowed the accurate prediction of phosphorus content ranging from 0·07 to 0·25% in various feeds, at intervals from 3 to 24 h after the infusion; the predicted and actual phosphorus concentrations were highly correlated (r = 0·95). Similar observations for feeds ranging from 0·14 to 0·25% phosphorus suggested that accurate prediction was also possible 144 h after infusion. Comparison of estimated feed phosphorus content of grazed material with that measured in hand-plucked herbage indicated that this approach is applicable to grazing studies.

SUMMARYCattle grazing buffel grass pastures during the dry season at Katherine lose large amounts of live weight during a short period following first rain. In Part 1 we showed that these losses were due mainly to reduction in gut contents. Part 2 examines the changes in pasture, diet and grazing behaviour to assess the probable change in nutritional status of cattle following rain.During the dry season the most valuable pasture component, green leaf, was very scarce and had a nitrogen concentration of about 1%. Diets of oesophageally-fistulated cattle contained less than 0·5% N. Shortly after first rain diet N doubled owing both to rapidly increasing amounts of green leaf and to the increase in N concentration in young leaf to over 3%. Although intake was not measured, literature is cited to show that this increase in dietary N would be expected to increase rumen digestion rates with a resultant marked increase in D.M. intake and a reduction in gut contents.Although availability of high quality herbage increased very rapidly following first rain, a period of increased nutritional stress immediately following rain could not be ruled out. However, any such decline in nutrition appears to be short-lived.

In an earlier paper (Little, McLean & Winter, 1977) we reported a technique for the measurement of the phosphorus content of the diet consumed bygrazing cattle, which recently has been used successfully to measure the dietary intake of phosphorus by sheep grazing Mitchell grass (Astrebla spp.) pastures in western Queensland (Beale & McMeniman, 1978). In this technique, phosphorus of salivary origin is distinguished from that of plant origin in samples extruded by oesophageally nstulated animals by prior labelling of salivary phosphorus with the isotope 32P following the intravenous infusion of Na2H32PO4. The difference in specific activity between saliva and extruded bolus is used as the basis of the calculation of the phosphorus content of consumed herbage. Oesophageally nstulated animals may graze experimental pastures for an hour or more while samples are collected from them. It is important to have an accurate measure of saliva speoific activity, representative of that secreted during the grazing period; therefore, application of the technique is facilitated if the rate of change in saliva specific activity during collection periods is slow and linear.

A coring technique (6), that may be used for estimating the mean amount of herbage on closely grazed pastures, has been adapted to include visual scoring. Two methods are described, both of which use sets of pasture cores from the sward as reference standards. The observers score the herbage on view at random sites against the standards. In the first method the scores are converted to herbage yields directly by reference to the yields of the standards. In the second method a double sampling regression technique is used and the set of standards is used as a visual guide only.

The addition of superphosphate increases the legume content of grass-legume pastures and the growth rate of beef cattle (Shaw, 1978). It has generally been assumed that the increase in beef production following the application of superphosphate is related directly to greater amounts of legume in the pasture consumed (Norman, 1970; Shaw, 1978) but there is no direct evidence on this subject. This paper reports a study of the influence of small quantities of superphosphate on preference for legume by cattle grazing Stylosanthes-native grass pastures.