Four supplementation treatments to investigate wet seasons responses were imposed on eight Brahman/Shorthom cross herds totalling 870 animals of all classes (cows and calves, bulls, heifers and steers). They grazed 12,300 ha of native pasture in the Darwin district of the Northern Territory. The treatments were: (i) control (no wet season supplement), (ii) Mineral (Ultraphos - supplementation with 13 minerals during the wet season), (iii) +Protein (Ultrapro-50 - the same 13 minerals plus non-protein nitrogen (WN) and true protein over the wet season), and (iv) Strategic (strategic use of salt, mineral and hTN supplements over the early wet, mid wet and late wet seasons respectively). All treatments received an hTN plus mineral supplement (Uramol) during the dry season. Supplement intakes by the herds were measured monthly for three years. Intake of Salt, Ultraphos and Ultrapro-50 supplements averaged 43, 124 and 149 g/animal/day respectively during the three wet seasons. Losses of the molasses-based block formulations due to rain were small (7%) during the wet season. However, up to one third of each pure salt block was eroded. Intake of Uramol during the three dry seasons averaged 182 g/animal/day across all herds. No urea toxicity problems were noticed over the three years of the supplement programme, despite high intakes during the dry season. All animals consumed some supplement. However, levels of supplement intake: (i) increased with the seasonal decrease in native pasture quality, (ii) decreased with an increase in proportion of paddock burnt, (iii) varied between animal classes, with lactating cows consuming 64% more supplement than non-lactating cows in two of the three years, and (iv) varied between individuals within classes. Individual intake varied widely (from 10 to 835 glanirnallday for lactating cows) with no significant correlation between individual intake and the pregnancy rate or average daily liveweight gain.

The effects of four wet season supplementation regimes on the pregnancy rates, mortality and liveweight of 870 Brahman x Shorthorn cross cattle (cows, bulls, heifers and steers) were investigated on Mt Bundey Station from 1980 to 1984. Treatments imposed were (i) Control (no wet season supplement), (ii) Mineral (supplementation with 13 minerals during the wet season), (iii) +Protein (the same minerals plus non-protein nitrogen and protein over the wet season), and (iv) Strategic (consecutive use of salt, mineral and non-protein nitrogen/protein supplements over the early wet, mid wet and late wet season periods respectively). Each treatment was replicated twice and all received a dry season supplement of non-protein nitrogen plus minerals while stocked at one breeder to 14 ha on native pasture. The pregnancy rate of lactating cows in the +Protein regime in 1982 was 77% compared with 60% for the other three regimes (P<0.05). The pregnancy rates in 1983 for both the +Protein (57%) and Strategic (54%) regimes were higher (P<0.05) than the Control (40%) and Mineral (31%) regimes. Mortality rates of breeders and heifers ranged from 2 to 5% in the Mineral, +Protein and Strategic regimes, compared with 8% (1982) and 12% (1983) (P<0.05) in the Control groups. Post pregnancy diagnosis losses were 8% in the +Protein regime compared with 16% in the other regimes (P<0.05). Breeders were 25 to 30 kg heavier in the +Protein than in Control regimes (P<0.05). Liveweight gain of steers in the +Protein regime was 11 to 31% higher than the other three regimes in two of the three steer drafts. Calf weight per breeder in the +Protein treatment was 55% higher in 1983 and 44% higher in 1984 than the other three regimes. It is concluded that wet season supplementation with non-protein nitrogen/protein and minerals has the potential to substantially increase herd productivity in the monsoonal tallgrass region. The response to the wet season +Protein regime, and the absence of a similar response to the Mineral regime, despite a phosphorus, sodium and sulphur deficiency in the pasture, was attributed to an overriding nitrogen requirement. Other factors affecting herd productivity were, paddock variability which was attributed to variation in the proportion of upland compared with floodplain/riparian land units in each paddock and seasonal differences related to the length of the preceding dry season.

Statistical models for conception rate were developed using data collected at a mid dry season (August-September) muster from a controlled-mated and supplemented herd of 900 Brahman cross cows at Mt Bundey from 1980-84. Average conception rates were 74% for maidens, 25% for first-lactation cows, 51 % for 4-10-year-old lactating cows, 54% for >10-year-old lactating cows and 96% for non-lactating cows. Although year effects dominated, they were partially explained by severity of the dry season and were consistent across levels of the other factors. Pregnant maiden heifers weighed 13 kg more than non-pregnant ones at the end of mating, after allowing for the weight of the fetus and gravid uterus. Conception rate of first-lactation cows was not affected by month of calving, adjusted mid dry season weight or condition score, possibly because of narrow ranges in these factors based on selection. For mature lactating cows the main effects on conception rate were month of calving (+11% for September to -16% for December or later), previous lactation (+6% for having reared a calf), adjusted mid dry season weight (-16% for <300 kg to +9% for >350 kg) and condition score (-17% for backward and store to +16% for good condition). Corresponding effects for aged lactating cows were +14% for September to -19% for December or later calving, +8% for previously rearing a calf, and -24% for backward store and store to +23% for good condition cows. The high conception rates for non-lactating cows corresponded with high liveweights and condition scores. A low cost, practical implementation of these models is based on previous lactation status and condition score in the mid dry season, with pregnancy diagnosis added at the next stage. Reproductive classes could be segregated on this basis for improved management and nutrition and to gain an economic return on capital invested in cattle control.

Extreme variability in cattle production between replicate paddocks in a large supplementation experiment on Mount Bundey Station in the Danvin pastoral district of the Northern Territory, led to a search for reasons for the differences. Soil type, land unit and pasture parameters were related to cattle production parameters.Pregnancy rates of lactating heifers were positively related to yields of Themeda triandra, Sorghum plumosum, Eriachne burkittii and Eragrostis spp., the f i s t three of which declined in yield following heavy grazing pressure. Quantity of pasture was more closely correlated with animal production than was quality due to the wide range in yields and narrow range in quality attributes. Pregnancy rate in lactating heifers was more sensitive to yield of pasture species than was lactating cow pregnancy rate, breeder mortality, calf weight per breeder mated or steer average daily gain. Heifers consumed less supplement relative to their requirements than mature cows and steers and their response was not confounded by previous history. These factors probably account for their greater sensitivity to pasture variations.Pregnancy rate of cows was positively related to the proportion of lowland soil types and negatively to proportion of lithosol (hills). An equation incorporating the percentages of these soils, explained 63%. 42% and 39% of the variation in lactating cow pregnancy rate in 1982, 1983 and 1984 respectively. Variation in pregnancy rate among paddocks was therefore largely attributed to variation in the proportions of "upland" and "lowland" soil types.Sensitivity to grazing pressure of species within the native pasture was indicated by localized damage to T. triandra and S. plumosum and a large reduction in yield of Sintrans after five years of continuous grazing with access to non-protein nitrogen supplements at one breeder to 14ha. Native pastures in the Adelaide River area would not sustain this stocking rate.

This survey examined the long term effects of regular helicopter use on cattle tractability and provided herd management information on mustering rate and efficiency in a tropical savanna woodland environment. Ten individual herds of high grade Bos indicus breeders were mustered from four to eleven times over a four year period as part of a supplementation experiment conducted on "Mount Bundey" station. Paddock size ranged from 744 to 2,224 ha. Mean annual mustering efficiency (proportion mustered) was 71% (64-85%), 92% (83-99%), 96% (92-100%) and 94% (78-98%) for bulls, calves, steers and breeders respectively. Paddocks were totally clean of all animal classes in 25% of musters. Helicopter mustering of breeders and steers in the last three years of the study was 12 percentage points more efficient than horse mustering (assisted by a spotter plane) in the first year. Mustering rate in April-May averaged 69 beasts/hour while the August/September round averaged 102 beastdhour. Mustering efficiency did not differ between April-May and August-September musters. Helicopter mustering did not adversely affect tractability of the cattle when carried out in conjuntion with horsemen. The data indicate that 100% clean musters could not be routinely achieved in this monsoonal woodland savanna. even under relativelv intensive conditions. This conflicts with current BTEC requirements and hinders animal husbandry practices.