HR Assignment 代写 various systems of cattle production around the world

There are various systems of cattle production around the world. In some countries, the cattle production is dual-purpose, with cows being used to provide milk and meat. However, for the most countries, cattle production may be divided into two sectors: (1) dairy production and (2) meat production (Acker & Cunningham 1991). In continental Europe and the developing countries, the same cattle are used as a source of both meat and milk. In contrast, in the countries of the developed world such as the UK, Australia, New Zealand, the USA, and Canada meat production and milk production have been separated (Ball & Peters, 2004).

In the UK, the proportion of herds with more than 100 cows is increasing rapidly (Ball & Peters, 2004). The whole milk pricing system, which historically based on the percentage of butterfat, tends to change as more people are drinking low-fat and skim milk (Acker & Cunningham 1991) but still butterfat has been appreciated as the most valuable content of whole milk. The economy for dairy farmers has been also changed with declining milk prices and higher cost for feed. This is the main reason behind the continuous trend of fewer numbers of dairy farmers in the country, and increasing numbers of cows per farm, which means that the farmer spends less time per cow (Ball & Peters, 2004). In the UK, the population of the cattle decreased after the BSE epidemic and the subsequent outbreak of foot-and mouth disease. Those cows that remain need to reproduce even more efficiently to provide a good choice of replacement stock, adapted in the new market trend for products with lower content of fat (in meat and in milk).

The main objective of the dairy based production systems is to produce milk as economically as possible (Lucy, 2001; Dobson et al., 2007). In intensive dairying systems animal breeding aims to maximize milk yield, although milk composition and other factors like fertility and health are becoming increasingly important (Pryce et al, 1998; Ball & Peters, 2004; Oltenacu & Algers, 2005). In such production systems, most male calves are a by-product and are sold or reared for beef production. Semen of bulls selected as high genetic merit sires are used in artificial insemination (AI) schemes aiming to transfer the genetic progress in the herd (Rook & Thomas, 1983; Ball & Peters, 2004). The increased milk production have been mostly achieved due to steady genetic progress in the commercial herds based on continuous importation of genes from USA and Canada and widespread use of high genetic merit sires in AI schemes (Garnsworthy & Webb, 1999) .

Milk yield and milk composition are related to genetic, nutritional and environmental factors (Rook & Thomas, 1983). On a short-term basis, the efficiency of nutrient use for milk production is primarily dependent on the milk production level. As milk yield increases, a lower proportion of total feed intake is used for maintenance of the cow (Chilliard, 1992). In generally, in high yielding dairy cows milk production and reproductive performance are highly negatively correlated (Macmillan et al., 1996; Royal et al., 2000; Stevenson, 2001; Opsomer et al., 2006).

The last decades, the genetic improvement in dairy cows has led to a dramatic increase in milk yield, which has been tied up with an inherent decline in reproductive performance (Butler, 2000; Royal et al., 2000; Evans et al., 2006; Garnsworthy et al., 2008a). Probably, many other reasons are at least in the same degree accountable for this decrease in reproduction performance such as nutrition, management and poor expression or detection of estrus (Garnsworthy & Webb, 1999; Lucy, 2001; Dobson et al., 2007).

On the whole, nutritional factors interact with reproductive performance with prominent impacts on livestock productivity and viability (Butler and Smith, 1989; O'Callaghan and Boland, 1999; Garnsworthy & Webb, 1999). At present, these interactions are yet on study but it seems that the relationship between nutrition and reproduction is dynamic, complex, not well understood and responses are often quite variable and inconsistent (Gordon 1996; Boland & Lonergan, 2003). The reproductive efficiency of the postpartum cow is a critical element determining overall biological and productive efficiency. Thus, failure of cows to resume cyclicity after calving is a critical point that in general influencing the economical profitability of the cattle industry (Roberts et al., 1997; Stott et al., 1999; Evans et al, 2006; Garnsworthy et al., 2008a), especially in the intensive milk producing systems.

In general, high producing dairy cows experiences a metabolic stress involving NEB in early lactation (Bauman & Currie, 1980). The main characteristics of this hard condition are loss of body weight and mobilization of body fat stores because feed intake cannot support the energy required for milk yield and maintenance (Beam & Butler, 1999; Butler, 2000; De Vries & Veerkamp, 2000). Energy requirements to support follicle growth and ovulation are negligible (less than 3 MJ ME/ day) compared with requirements for maintenance and production (60-250 MJ ME / day in a lactating dairy cow). However, in the case of lactating dairy cows, inadequate nutrition in the short term, or as a consequence of a prolonged depletion of body reserves during early lactation, can have significant detrimental effects on resumption of ovarian activity postpartum, conception rate and infertility (Boland & Lonergan, 2003).

Body condition score (BCS) has been considered an indirect measure of nutritional status (Macmillan et al., 1996; Garnsworthy, 2006). It is now well known that body condition of cow, especially at certain points of its annual productive cycle impacts directly on the fertility (Butler 2003; Garnsworthy, 2006). This is true for a variety of important fertility indices among them the calving interval (CI) (Lucy 2001; Oltenacu & Algers, 2005; Dobson et al., 2007). Body condition at calving is believed to affect reproductive performance through its effects on tissue mobilization in early lactation and on uterine involution. Moreover, BCS is strongly related to condition loss and negative energy balance (NEB), which influences circulating metabolic hormones and metabolites (Boland & Lonergan, 2003). Recent reports indicate that average target BCS is lower now than it was in the 1980s. This is because cows selected for higher milk yield over the past thirty years are genetically thinner, so they tend to mobilise body fat. Consequently, cows of high genetic merit are likely to experience deeper and more prolonged negative energy balance in early lactation (Garnsworthy, 2006).

Season of calving impacts on the fertility due to indirect effects of seasonal differences in nutrient quality and by direct effects of photoperiod (Savio et al., 1990). Many wild species of Bovidae are seasonally breeding and changes in daily photoperiod consider of being the stimulus for onset or termination of ovarian activity (Gordon, 1996). Thus, it is possible a similar inherent mechanism based on the annual pattern of photoperiod to be present in the domestic cow and that predisposes towards calving during the late spring to early summer, the optimal time for food supply in the wild cattle species (Gordon, 1996; Ball & Peters, 2004).

Nowadays, the declines in fertility of high yielding dairy cow has been reported widely and has been become a scourge for the intensive dairying systems (Butler, 2000; Royal et al., 2000; Lucy, 2001; Evans et al., 2006). IGF-I was the first hormone being implicated in the pathology of this epidemic. Negative energy balance (NEB) influences IGF-I concentrations in plasma of dairy cows (Ronge et al., 1988; Spicer et al., 1990; Beam and Butler, 1998). Many other factors such as diet, BCS at calving and season of calving may affect the circulating levels of IGF-I through their impacts on the NEB (Beam and Butler, 1998; McGuire et al., 1995; McGuire et al., 1998). IGF-I is found in high concentrations in follicular fluid, particularly in the dominant follicle. There is a large body of evidence suggesting the existence of a local intra-ovarian IGF system complete with ligands, receptors and binding proteins (Webb et al., 1999). Ideally, IGF-I was the foremost hormonal molecule linked nutritional, condition and seasonal effects with NEB and reproductive performance.

The discovery of Leptin has been considerably changed the scientific views about adipose tissue, now fat depots are considered not only as an active regulator of body weight but also an endocrine organ (Allen et al. 2007; Vernon, 2005; Marieb & Hoehn, 2007). Leptin positively correlated with BCS and its circulating levels affected by photoperiod. The NEB of postpartum cows causes a sustained reduction in plasma Leptin which may be partially responsible for decreasing DMI (Chilliard et al. 2005). Moreover, Leptin has been found to modulate nutrient transfer and partitioning by interaction with other hormones including insulin, glucagon, glucocorticoids, growth hormone, IGF-I, cytokines and thyroid hormones. Also, Leptin partially controls gonadotropin secretion through its hypothalamic/pituitary actions, but circulating or locally produced Leptin may also provide direct modulation of ovarian function (Hill, 2004; Garnsworthy et al., 2008a). The possibility of dietary, condition and photoperiodic modulation of circulating Leptin aligned with the numerous effects of this molecule in cow reproductive physiology set up Leptin in a prominent place in the field of seeking for interactions between nutrition and reproduction.

Adiponectin is another candidate molecule may be implicated on the wide field of condition to reproduction interactions (Mitchell et al., 2005). Fat mass may exert direct negative feedback on Adiponectin secretion (Gordon et al., 2007) and a negative correlation between circulating Adiponectin and fat mass has been reported in many animal models and human (Arita et al., 1999; Kadowaki and Yamauchi, 2005; Jacobi et al., 2004). In the pig ovary adiponectin receptors are weakly expressed and Adiponectin may be involved in the control of reproductive function in animals through the AMPK signaling cascade (Scaramuzzi et al. 2010). Also, Adiponectin expressed insulin-sensitizing effects and involved in regulation of insulin resistance and glucose homeostasis (Antuna-Puente et al., 2008).

Insulin is a key hormone in endocrine control which facilitates the movement of glucose across cell membranes and thereby regulating the concentration of blood glucose (Guyton & Hall, 2006). Glucose is the principal source of energy for life processes of the mammalian cell (Kaneko, 1997). In postpartum cows, decreased insulin reduces the glucose uptake by insulin-dependent organs such as adipose and muscle tissue and increases the glucose availability for the insulin-independent mammary gland (Bossaert et al., 2009). Decreased insulin (McGuire et al., 1995) and increased GH (Rhoads et al., 2004) concentrations also promote adipose tissue mobilization and elevated levels of circulating NEFA which favors mammary gland uptake and milk yield. During early lactation, glucagon concentrations increase relative to the dry period to stimulate lipolysis and gluconeogenesis to provide the body with the required energy to support the high milk production (De Boer et al., 1985). Genetic selection of dairy cows for high milk production was linked with a longer interval from parturition to first ovulation, high plasma concentrations of GH and BOHB, and low plasma concentrations of glucose and insulin (Garnsworthy et al., 2008a; Gutierrez et al., 2006). Feeding cow diets to increase circulating insulin concentrations for the first 50 DIM increased the proportion of dairy cows ovulating during this period (Gong et al., 2002). Control of glucose homeostasis through dietary modulation of circulating Insulin has been a key nutritional strategy to alleviate postpartum dairy cow from the detrimental effects of NEB and improve reproductive performance (Garnsworthy et al., 2009).

The last decade the infertility of high yielding cows became the first scientific focus for many research groups around the world looking up the possibility to partially reverse this situation. Because the genetic makeup of the contemporary high yielding cow is given, and is very difficult to change (it needs lots of time and investments to alter) much attention has been paid on the interaction among nutritional, condition and seasonal stimulus with reproductive performance. The Fertility Group, in Division of Animal Sciences, at University of Nottingham in collaboration with other researchers the last years steadily confronts with the problem. The main output of this entire try is the development of a well-cut nutritional strategy which possibly helps the cows to improve the fertility without any loss for the farmers due to depressed milk production (Gong et al., 2002; Adamiak et al. 2005, 2006; Fouladi-Nashta et al. 2005, 2007; Garnsworthy et al., 2008ab, 2008c, 2008d, 2009). All the experiments in this thesis were based upon this nutritional strategy (Garnsworthy et al., 2009) which took advantage of dietary induced increases (or decreases) of insulin at certain points of postpartum cycle of dairy cow.

The overall objective of this thesis is to study the effects of different dietary, condition and seasonal challenges on metabolic hormone profiles, circulating adipokines, and lactational and reproductive performance in dairy cows. A special emphasis was directed on assessing metabolic and hormonal status, and the mobilisation of tissue reserves. More specifically the objectives were to:

examine interactions between nutrition and reproduction.

investigate if there is an interaction between adipokines and pancreatic hormones in dairy cows and elucidate basic concepts of nutrient partitioning mechanism.

explore glucose homeostasis and homeorhesis in dairy cows.

clarify if different dietary (HS vs. HF), condition (FAT vs. OBESE) and seasonal challenges have an impact on the profile of circulating metabolic hormones, adipokines, metabolites, NEB, and milk yield contents.

seek for relations among circulating adipokines, circulating pancreatic hormones, blood metabolites, milk contents, different diets, different body composition, milk yield, and energy balance.

assess liver response on different stimulus and metabolites in relation with milk yield and reproductive performance.

scrutinize dietary, condition and seasonal impacts on reproductive and lactational performance in dairy cows.

Finally, the major scope of these experiments is to find evidence for the underlying mechanism which is possibly incorporated and coordinated by the interaction of pancreatic hormones, adipokines, and blood metabolites. This mechanism is mentioned to be implicated not only for the level of nutrient partitioning among the different tissues of the organism and milk but also for reproductive success.

本篇ESSAY 是 ENLUNWEN.COM 业余作者阿诚的作品，并非我们PHD大师专家级作品