Dry-wet Transition Research Articles

Operational aspects of ley farming systems in the semi-arid tropics of northern Australia: a review

The ley farming systems proposed for the Australian semi-arid tropics involve rotating an annual pasture (usually legume) with crops sown using no-tillage. These systems were developed to overcome many of the climatic constraints that beset previous attempts at cropping in the region. However, difficulties in the timing of farm operations also contributed to past failures. No analysis of the operational aspects of ley farming had been made; this was the objective of this paper. During the transition between wet and dry seasons rainfall is extremely variable. These periods were shown to be the most critical time for operations in ley farming systems. During the dry-wet transition, management of ley pastures is very difficult because pastures must provide grazing for livestock as well as sufficient soil cover for timely sowing of a following crop. Legume pastures have reduced grazing value at this time due to spoilage by early rain. Similarly, during the wet-dry transition annual pastures must be allowed to set seed at a time when crops are yet to be harvested and stubbles are not available for grazing. It was suggested that including separate paddocks of perennial pasture could reduce the demand on annual pastures during the seasonal transitions. A limited number of days to sow a crop combined with severe yield penalties for late sowing restrict the area that can be sown in any season. Sowing opportunities were determined for 3 locations in the Northern Territory. No-tillage was shown to increase the potential number of sowing days. However, the time available to apply glyphosate before sowing using no-tillage was confined to the morning due to risk of afternoon rainshowers reducing efficacy. The number of seasons where a particular crop or cultivar could not be sown was independent of the method of tillage. The wet-dry transition was shown to be important for harvest operations. Our analysis of 3 sites in the Northern Territory suggests that for mungbean and sorghum, later maturity combined with prompt harvest will reduce the risk of poor seed quality due to weathering and could permit earlier sowing. However, for sorghum, an economic assessment of these practices required research that can quantify the interaction between sowing date, time-to-maturity and grain yield.

MORE GROWTH RATES OF BIRDS IN THE HUMID NEW WORLD TROPICS

Summary The growth curves of 40 species of lowland neotropical birds were fitted by logistic equation. The birds were mostly from Panama, Trinidad and Surinam. The growth constants of the fitted equations (asymptote A and growth rate K) were compared within and among species, and with previously published data on temperate species. Growth parameters of tropical passerines are about as variable within species as they are within temperate species. In both cases, variation in A and K between broods is greater than it is within broods. Panamanian birds breed during the dry-wet transition and conditions for growth apparently improve as rainfall increases. Asymptotes of growth curves are higher, and mortality within broods lower, as the breeding season progresses. Asynchronous hatching and the reduction of brood-size by selective starvation of young is a prominent phenomenon during the early part of the breeding season. Several instances are reported, however, of young persisting in nests with inadequate feeding and greatly subnormal weights. Slowed development under conditions of poor nutrition may be adaptive in the tropics if periods of low food availability are short and allow the possibility of recovery from undernourishment. As a group, neotropical lowland passerines (30 species) grow 23% more slowly than a sample of 51 temperate passerines. Variation of growth rates among these tropical species is similar to variation among temperate species, and it is related to adult body-size the length of the nestling period. Young of tropical and temperate species attain similar asymptotes, relative to adult body-weight, by the end of the nestling period. Hypotheses are advanced which might explain the slower growth rate of tropical species, and tested to the extent available data permits. (1) Because brood-size can be changed only by adding or removing whole young, changes in growth rate could provide finer adjustment of the energy requirements of the young to the feeding capacity of the parents. This model predicts different means and variances for growth rate within groups of species with different clutch-sizes, predictions not supported by available data. (2) Growth rate is shown to increase the maximum energy requirement of a nestling only if K exceeds some value determined by the energy requirement of the young, growth rate should vary in proportion to the level of basal maintenance metabolism. In a small sample of tropical species, rates of basal metabolism were 25% lower than in a comparable sample of temperate species. These data therefore support the hypothesis, although the cause of the lower metabolic rate of the tropical nestlings is not known. (3) Daily periods of hypothermia could reduce the energy requirement of the young and at the same time reduce their growth rate; but observations of body temperatures of tropical nestlings are contrary to this hypothesis. (4) The short day-length of tropical climates reduces the time during which young can assimilate energy relative to their energy expenditures. This model predicts that tropical nestlings would have less productive energy available, (consistent with their reduced growth rates), but it also predicts that arctic birds should grow faster than temperate species, which is not confirmed by available data. (5) The low nitrogen content of fruits may cause the slow growth of a few strictly frugivorous species (Oilbird and Bearded Bellbird), but among other tropical species growth rate is not correlated with the estimated proportion of fruit in the diet.