Literatura científica selecionada sobre o tema "Ewes South Australia Fertility"

Reproductive wastage is a major economic burden in sheep production globally, especially within Australia as livestock production systems face increased pressure from climatic variability (e.g. prolonged droughts or flooding). Sheep are sensitive to acute changes in their environment such as heat stress, which if not adequately monitored will result in significant production losses such as reproductive failure, increased parasite and worm burden, morbidity and mortality risks. Through basic and applied research in the areas of stress and reproductive physiology our team has made significant advancements in the understanding of sheep behaviour and physiological responses to acute and chronic stressors. Using minimally invasive hormone monitoring technology in combination with field based assessment of sheep health and productivity traits, our team has delivered new knowledge on how sheep react to acute environmental stress and how it impacts on sheep reproduction. In this chapter, we evaluated the fertility rates and embryo quality of Merino ewes under AI breeding program. We discuss factors such as heat stress that can impact on ewe and offspring quality.

Demographic transition refers to a shift in reproductive behavior from a state of high birth and death rates to a state of low birth and death rates. This transition takes place because of advances in agricultural technology and medical science or improvement in hygiene environment, all of which result in corresponding declines in the mortality rate. In this first phase of the demographic transition, population growth rises because the decline in mortality rate has not been coupled with any significant change in parents’ fertility decisions. Then, in the second phase of the transition, parents begin to reduce their fertility as they realize that their ideal number of children can be more easily achieved with fewer births. The widespread use of contraceptive techniques facilitates parents’ attempts to reduce fertility, which in turn causes a decline in the population growth rate. Eventually, the population growth rate converges to a new level, which may be higher or lower than in the pretransitional stage. To facilitate comparison, we can use figure 11.1 to characterize the time and process of the transition. In figure 11.1, Tα marks the apparent starting point of a continuous decline in mortality. Tβ, which normally occurs later than Tα, refers to the time at which the fertility rate begins to decline. Tγ, is the point of lasting return, with an average rate of natural increase equal to or less than that of the period preceding the date of Tα. The convention is to define D = Tγ - Tα as the duration of the transition period. Chesnais (1992) separated the observations of world demographic transition into several types. The first type includes developed countries in Europe and Japan; the second type consists of countries with immigrant European populations, such as the United States, Australia, and Argentina; late-developing countries, such as India. South Korea, and Jamaica, belong to the third type. For countries of the first type, the mortality decline process is closely related to the development of medical technology, which was gradual and spread out over time; hence, the demographic transition is also long. Late-developing countries and those with large immigrant populations were able to adopt the already-developed medical technology from the advanced countries at one time.

The cerrados of central Brazil have long been designated savannas without sufficient understanding of the structure and functioning of the different vegetation forms in the region. Excessive emphasis on identifying similarities with other savannas in Africa and Australia, and even within South America outside Brazil, prevented researchers from recognizing the cerrados’ special features and interdependence among themselves in the landscape where they occur. The more extensive cerrado sensu stricto on dystrophic soils, and to a lesser extent the gallery forests known locally as matas de galeria or matas ciliares, dominated the attention of most botanists and other researchers (Ratter and Dargie 1992, Furley 1992, Furley and Ratter 1988, Furley et al. 1992). Even with increasing interest in biodiversity and ecosystem functioning, very little ongoing research is reported on nutrient cycling from the cerrado region (Solbrig et al. 1996). Information available in the literature is restricted to isolated attempts to describe and quantify specific processes like litterfall and decomposition (Peres et al. 1983), rainwater composition (Schiavini 1983, Delitti 1984), soil fertility gradients (Lopes and Cox 1977), leaf nutrient concentrations (Haridasan 1987, 1992, Araújo and Haridasan 1988), primary productivity of the ground layer (Batmanian and Haridasan 1985, Meirelles and Henrique 1992), effects of burning (Coutinho 1990, Kauffman et al. 1994, Miranda et al. 1996c) and activities of soil fauna (Constantino 1988, Egler and Haridasan 1987, Oliveira Jr. 1985) at specific sites within a particular vegetation. Results of long duration experiments from permanent plots or watersheds are not yet reported in the literature. Very little information is available on the food webs or the role of fauna in nutrient cycling. Research on specific processes like CO2 emission on an ecosystem basis is quite recent (Miranda et al. 1996a, b, Mier et al. 1996). The following discussion is therefore restricted to the occurrence of different vegetation forms in the cerrado region and environmental factors affecting their distribution and functioning in relation to nutrient availability and nutrient cycling processes. One of the difficulties in getting information on research already carried out in Brazil is that the dissertations of graduate students in the universities are seldom published in indexed journals.

Low quantities of soil nitrogen limit plant growth in the Chihuahuan Desert (Ettershank et al. 1978; Fisher et al. 1988; Lajtha and Whitford 1989; Mun and Whitford 1989) and in other deserts of the world (Wallace et al. 1980; Breman and de Wit 1983; Sharifi et al. 1988; Link et al. 1995). Indeed, although deserts are often regarded as water-limited systems, colimitation by water and N may be the more general rule (Hooper and Johnson 1999; Austin and Sala 2002). In a broad survey of desert ecosystems, Hooper and Johnson (1999) found evidence for colimitation by water and N even at the lowest levels of rainfall. In arid ecosystems, water is delivered in discrete events separated by drier periods, which restrict biological activity and uncouple plant uptake of nutrients from decomposition. Local variations in net primary production in arid and semiarid ecosystems are largely determined by processes that control the redistribution of water and soil nutrients across the landscape (Noy-Meir 1985; Schlesinger and Jones 1984; Wainwright et al. 2002; see also chapter 11). In this chapter we focus on the N cycle in different plant communities of the Jornada Basin with the recognition that after water, N is the most likely resource to determine the plant productivity of this ecosystem. Where arid environments are dominated by shrubby vegetation, the distribution of soil properties is markedly patchy with strong accumulations of plant nutrients under shrubs and relatively infertile soils in the intershrub spaces (Noy-Meir 1985). These islands of fertility are particularly well described in the Chihuahuan Desert and other areas of the American Southwest. Local accumulations of nutrients under vegetation are also documented for desert habitats on other continents, including Europe (Gallardo et al. 2000), Africa (Gerakis and Tsangarakis 1970; Belsky et al. 1989; Wezel et al. 2000), Australia (Tongway and Ludwig 1994; Facelli and Brock 2000), and South America (Rostagno et al. 1991; Mazzarino et al. 1991, 1998; Gutierrez et al. 1993). In the Jornada Basin, Schlesinger et al. (1996) used geostatistics to compare the scale of soil heterogeneity in arid habitats dominated by shrubs and in adjacent areas of arid grassland.