Literature Review Post-partum reproduction in suckling beef cows
Post-partum rebreeding in suckling beef cows has been discussed (Hammond, 1927 cited by Short et al., 1990; Wiltbank, 1991) and continues to be a vital subject for animal scientists (Goyache et al., 2005; Satrapa et al., 2010). Poor rebreeding is characterised by the non-appearance of oestrus (anoestrus) early in the post-partum period and extended interval to first service, which lengthens the calving interval above 365 days, reduces calf crop, and causes an economic loss to beef cattle producers (Short et al., 1990; Yavas and Walton, 2000; Quintans et al. 2010).
After parturition, uterine involution is generally completed within four to six weeks (Gier and Marion, 1968; Dobson et al., 2001; Zhang et al., 2010). Thereafter, the duration of post-partum anoestrus is governed by the recommencement of the hypothalamic pituitary ovarian (HPO) axis activity. Due to this fact, much attention has been given to the nature of the signal that controls pituitary secretion of LH and FSH, the response of the ovary to LH and FSH (Bryner et al., 1990; Martinez et al., 2005) and to the other ovarian effects that are independent of gonadotropins (Paranhos da Costa et al., 2006).
Several factors are implicated in the activation of the HPO axis activity during the post-partum period of suckling beef cows. These factors include suckling and nutrition (body energy reserves) as the major factors and many minor factors such as age, parity number, breed, individual genetic variation, presence of bull, diseases, twin births, dystocia, and retained placenta (Deutscher 1991; Grimard et al., 1995; Marongiu et al., 2002; Martinez et al., 2005; Álvarez-Rodrigez et al., 2010). The effects of body energy reserves, suckling and the related mechanisms that appear to influence post-partum reproduction of extensive beef cows are reviewed in sections 2.2 to 2.7
Body condition score
Maintenance of body energy reserves in bovines has been considered to be the basis of any reproductive management strategy (Houghton et al., 1990; Morrison et al., 1999; Flores et al., 2008). Body energy reserves were measured through weight loss, when energy reserves (fat) and protein reserves (muscle) are being depleted or, otherwise, through weight gain. Usually, weight gain or loss has been held as a good measure of well-being and productivity (Bishop et al., 1994; Butler, 2003). However, weighing cattle is laborious and rarely done by veterinarians, animal nutritionists or farmers. A more useful method in assessing the energy reserves is based on assessing individual BCS, based on visual observation and more accurately by palpation of back, ribs and rear quarters. There are two main scoring systems: (1) the American scoring system on a nine-point scale (Herd and Spratt, 1986) and (2) the Scottish scoring system on a five-point scale (Edmundson et al., 1989; Wiltbank, 1991). The Scottish body condition scoring system seems to have been adopted quite widely in the Southern African region, including Mozambique. Equivalence between these two scoring systems is presented in Table 2.1.
Other methods of assessing energy reserves in cattle
It is obvious that looking at the body weight of a cow does not give us an idea of existing energy reserves. Since an estimation of energy reserves represents an important tool for reproduction and production management, other techniques rather than BCS have been developed for cattle. Subcutaneous fat thickness at the longissimus dorsi (UFAT; 12th rib fat thickness) and rump fat thickness (RFAT) (Schroder and Staufembiel, 2006; Yokoo et al., 2008) have been mostly used in dairy cattle to estimate the body energy reserves. Although the measurement of RFAT requires the use of ultrasonographic examinations that are non-invasive practices, skills are, however, needed for manipulation. Its use in beef cattle is limited and apparently not studied under extensive conditions. Nonetheless, a recent publication regarding RFAT showed its high correlation with BCS (Ayres et al., 2009). These findings value the use of simple methods like BCS to estimate the body energy reserves. Due to this reason there is a probability that widespread use of RFAT in dairy and beef cattle may not take place.
Correlations between BCS and other direct methods of measuring energy reserves like energy balance, or indirect methods like circulating levels of hormones, leptin, were reported (Wathers et al., 2007; Murrieta et al., 2010). Recent results indicate that BCS remains the best indicator of nutritional status because of its high correlations with the related energy reserve indicators.
Effect of changes in body condition score on post-partum reproduction
The effects of BCS on postpartum rebreeding of beef cows (Bishop et al., 1994; Spitzer et al., 1995; Renquist et al., 2006; Flores et al., 2008) and dairy cows (Roche et al., 2009; Allbrahim et al., 2010) have been discussed. Variation in BCS of beef cows in the post-partum period has a number of practical implications for bovine reproduction, such as the association with the length of the post-partum interval to oestrus and ovarian activity (Bishop and Pfeiffer, 2008), and conception rates (Renquist et al., 2006; Roche et al., 2009).
It is well known that the Gonodatropin releasing hormone (GnRH) pulse generator system and the secretion of GnRH are inhibited by under nutrition (Randel, 1990; Wade and Schneider, 1996; Nqeno et al., 2010). Nevertheless, the physiology of nutrition illustrates that the oxidisable metabolic fuels are used for all physiological functions in the body and the excess is stored to be retrieved when a deficit occurs and, thus, to maintain production. Moreover, energy is partitioned by a priority to first maintain the life of the cow and then to propagate the species (Short et al., 1990). The approximate order of priority has being indicated to be (1) basal metabolism; (2) activity; (3) growth; (4) basic energy reserves; (5) pregnancy; (6) lactation; (7) additional energy reserves; (8) oestrus cycle and initiation of pregnancy; and (9) excess reserves (Short et al., 1990). Thus, reproduction takes place when basic physiological functions are satisfied in terms of energy.
The functioning of the hypothalamus-pituitary-ovarian axis when not energetically challenged is presented in Figure 2.1 (A), while when energetically challenged in Figure 2.1 (B). Figures 2.1 (A and B) illustrate that the prerequisite for the resumption of ovarian activity in the post-partum cows is an increased pulse frequency of episodic release of LH, which may occur in cows with moderate to good BCS (Bishop et al., 1994).
Effect of suckling on post-partum reproduction
Suckling is important for calf survival and forms the basis of high and sustainable income in beef cattle production systems. However, there is a concern that this exteroceptive stimulus prolongs the post-partum anoestrus, probably through a neural-mediated inhibition of LHRH or an inhibitory effect of RH on gonodatrophins or action at the ovary (Acosta et al., 1983; Convey et al., 1983; Pérez-Hernández et al., 2002). Notwithstanding the evidence that suckling might act in a chronic fashion to inhibit LH secretion through the post-partum period (Convey et al., 1983; Garcia-Winder et al., 1986; Crowe et al., 1998; Pérez-Hernández et al., 2002; Quintans et al., 2009), the true mechanism by which suckling extends the post-partum anoestrus is uncertain.
Suckling may occur six to nine times a day, with young calves suckling more frequently than older calves (Shimada et al., 1989; Stewart et al., 1993; Gazal et al., 1999; Das et al., 2000; Paranhos da Costa et al., 2006). Available reports indicate also that suckling frequency is high in first parity cows along with the short duration of the suckling meal (Stewart et al., 1993; Paranhos da Costa et al., 2006).
There is a breed-related difference in daily suckling frequency. Bos indicus cattle have a higher daily suckling frequency than Bos taurus and cross-breed cow-calf pairs (Das et al., 2000).
Although the precise mechanism by which suckling extends the post-partum anoestrus is uncertain, evidence exists that suckling frequency is the characteristic that correlates best with the anticipated onset of oestrus in the post-partum period (Radford et al., 1978; Shimada et al., 1989; Williams, 1990; Stewart et al., 1993; Lamb et al., 1999; Gazal et al., 1999; Marongiu et al., 2002; Álvarez-Rodriguez et al., 2010). In rodents, the inhibitory effect has been shown to be proportional to suckling intensity (Ford and Melamphy, 1973; Hammons et al., 1973) with comparable suggestion in ruminants (Quintans et al., 2009).
Effects of suckling on post-partum LH and FSH concentrations
Around parturition the hypothalamus-pituitary axis responds to a negative feedback effect of the placental and ovarian steroids by suppressing FSH release and accumulating this hormone in the anterior pituitary and depleting LH stores (Yavas and Walton 2000). Severe increase in FSH is observed after parturition followed by the emergence of first follicular waves of which the dominant follicles do not ovulate, leading to the development of second follicular waves in both suckling- and non-suckling beef cows (Murphy et al., 1990; Breuel et al., 1993; Crowe et al., 1998). There are similarities between suckling- and non-suckling beef cows in LH concentration in the anterior pituitary (Nett et al., 1988) and in the plasma (Radford et al., 1978, Walters et al., 1982) after 30 days post-partum. For Garcia-Winder et al. (1986), the plasma LH concentration in suckling beef cows is low irrespective of post-partum period. On the other hand, there is an indication of low LH pulse frequency in suckling cows after 30 days post-partum (Walters et al., 1982; Garcia-Winder, 1986; Quintans et al.; 2004) which is implicated in un-ovulation of dominant follicles emerged from second follicular waves (Williams et al., 1983; Acosta et al., 1983; Breuel et al., 1993; Crowe et al., 1998). Since reports suggest that the pattern of LH pulse frequency is crucial for oestrus to occur, the given data have reinforced the hypothesis that the suckling stimulus increases time to first ovulation by increasing the sensitivity of the hypothalamus to the negative feedback of estrogens during the post-partum period, resulting in decreased LH release.
The elimination of the suckling stimulus for 48 hr in Bos taurus beef cattle in intensive production systems increases serum LH concentration as well as pulse frequency and amplitude by 24 hr after calf removal, peaking by 48 hr, and then causes a decrease in LH concentration after the calf returns (Walters et al., 1982; Whisnant et al., 1985; Edwards; 1985). There is, however, a lack of information on the effects of temporary calf removal (12 hr or 48 hr) on Bos indicus cows in extensive production systems.
Effects of suckling on cortisol concentrations
Adrenocortical activity has been widely used as an indicator of social stress and, therefore, as an indicator of animal welfare (Milleder et al., 2003). Koolhaas et al., (1999) reportd that the difference in adrenocortical activity might express basic differences in physiology rather than in stress level. In addition, measuring glucocorticoid concentration in plasma has two methodological problems: (1) blood sampling itself causes an increase in glucocorticoid concentration; and (2) frequent sampling is necessary due to considerable fluctuations. Because reproductive performance is altered in cattle subjected to physiological stress (Dobson and Smith 2000), circulating cortisol levels have been measured together with plasma gonadotropins to analyse their relationship and to understand to which extent stress could impair reproduction (Echternkamp, 1984; Lyimo et al., 1999). Despite the fact that several studies on the correlation between cortisol and reproduction in cattle have been performed in dairy cattle and in Bos taurus beef cattle in intensive production systems, the influence of stress on gonadotrophin secretion and subsequent reproductive responses is dependent on the magnitude of the adrenal steroidogenic response and the animal’s adaptability to the stress.
During the post-partum period of suckling beef cows, cortisol correlates negatively with LH (Dunlap et al., 1981; Echternkamp, 1984). The increase in systemic cortisol of about 20-fold subsequent to intensive stress suppresses pulsatile LH release (Echternkamp, 1984). Unfortunately, very limited information is available on the effect of calf removal or restricted suckling on cortisol concentrations. Whisnant et al., (1985) studying hormonal changes during 48-hour calf removal reported that serum cortisol concentration did not differ between cows with removed calves as opposed to cows with suckling calves, but a transient elevation was notable in the calf-removed group from 9 to 12 hours after calf removal. Because LH concentration was greater in cows that have weaned than in suckling cows and cortisol pattern followed the above described trend, it was concluded that cortisol may not be a physiological inhibitor of LH secretion in the post-partum period of suckled beef cows.
List of Table
List of Figures
List of Abbreviations
1.1 Extensive beef cattle production
1.3 General objectives
1.4 Specific objectives
2. Literature Review
Post-partum reproduction in suckling beef cows
2.2 Body condition score
2.2.1 Other methods of assessing energy reserves in cattle
2.3 Effect of changes in body condition score on post-partum reproduction
2.4 Effect of suckling on post-partum reproduction
2.5 Effects of suckling on post-partum LH and FSH concentrations
2.6 Effects of suckling on cortisol concentrations
2.7 Strategies to reduce the suppressive effect of suckling on LH
3. Effects of post-partum BW, BCS, age and parity on ovarian steroids and metabolites during oestrus in Bos indicus cows in extensive production systems and the related effects on conception rates
3.2 Materials and methods
3.2.1 Study location
3.2.3 Experimental design
126.96.36.199 Principle of blood-sample collection to monitor hormonal changes
188.8.131.52 Oestrus synchronization
3.2.4 Hormonal assay
184.108.40.206 Test validation
220.127.116.11 Estradiol assay
18.104.22.168 Progesterone assay
22.214.171.124 Cortisol assay
126.96.36.199 Urea Nitrogen (BUN) assay
188.8.131.52 Creatinine assay
3.2.5 Statistical analysis
4. Effects of 12-hour calf withdrawal on conception rates and calf performance of Bos indicus beef cattle in extensive production systems
4.2 Materials and Methods
5. Effects of 48-hour calf removal on conception rates of Bos indicus cows and calfweaning weights in extensive production systems
5. 2 Materials and Methods
5.2.1 Blood sampling
5.2.2 Hormonal assay
5.2.3 Statistical analysis
5. 5 Discussion
6. General Discussion and Conclusions
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