Investigating the impact of plasma prolactin on maternal behaviour, calf health and bull fertility in beef cattle

In beef production systems, successfully raising and nourishing a calf from birth to weaning is dependent on the success of a cows mothering ability. Key aspects of maternal success in the cow include calf delivery, successful passive transfer of immunity, adequate supply of nutrients for growth, and protection from overt danger and predation. In other mammals, the onset of maternal behaviour at the time of parturition is ultimately driven by a shift in a number of key hormones (Geburt et al., 2015; Poindron, 2005). Whilst some aspects of maternal behaviour in livestock may be learned and improved throughout subsequent generations, hormonal cues and possible alterations in ratios and deficiencies have been shown to have a significant impact in certain instances (Broad et al., 1993; Dwyer et al., 1999; Kendrick and Keverne, 1991). Previously recognised hormonal cues in other livestock species may not be fully applicable to the bovine, and attempts at recognising physiological biomarkers for maternal behaviour have proved unsuccessful Other challenges, in extensive livestock settings, include both the applied nature and objectivity in measuring behavioural traits. Recent investments in automated devices such as GPS collars and automated proximity loggers have shown promise for remote monitoring of livestock, particularly for recording contact between animal pairs. The hormone prolactin has been shown in other species to be a crucial component to the hormonal cascade that initiates mothering instinct (Larsen and Grattan, 2012; Sairenji et al., 2017). Over a series of pilot experiments, this thesis aimed to explore both the applied nature of behavioural monitoring of maternal behavioural traits and the influence of the hormone prolactin on maternal behaviour, calf performance and bull fertility in beef cattle.
The first research objective, presented in Chapter 2, investigated the relationship of periparturient plasma prolactin levels and maternal behaviour of beef cows. Contact measured by ultra-high frequency (UHF) proximity logger collars between cow-calf pairs and application of a ‘response to calf handling’ maternal behaviour score (MBS) were selected as the proxies to measure behaviour. Prolactin was measured in plasma using a competitive double antibody ELISA kit (AssayGenie) across the periparturient period. Previous studies in other species indicated that periparturient prolactin concentrations may be positively correlated with cow-calf contact, but in fact the opposite was observed. A negative correlation between prolactin concentrations both pre- and post-calving and cow-calf contact measured by logger pairs was found. Further, no significant relationship was observed between MBS or contact data measured by proximity loggers. A strong significant positive correlation between cow-calf contact in the first 24, 48 and 72 hours with the full 12 day suite of logger data was also identified. Shortfalls encountered in this experiment include the chosen ELISA kit, which has a short read-range of up to 50ng/mL and thus unable to reliably identify individual animals with prolactin concentrations above this threshold.
The second research objective, presented in Chapter 3, investigated the relationship between both of these chosen behavioural proxies (logger data and MBS) and periparturient plasma prolactin concentrations with calf health and performance. No statistically significant correlation between either contact data measured by loggers or MBS and calf performance was found. Trends were observed when weaning weights and Average Daily Gain (ADG) were grouped based on MBS, although the sample population was likely insufficient to find a relationship of statistical significance.
The third research objective, presented in Chapter 4, focused on characterising the seasonal plasma prolactin profile of beef bulls reared under southern Australian conditions, as well as investigating its relationship with bull breeding soundness evaluation parameters. Plasma prolactin concentrations were found to significantly differ between summer and winter months. However, there was no statistically significant correlation between plasma prolactin and the reproductive parameters used in this study.
In conclusion, results indicate that at least, in cattle, cow-calf contact measured by proximity loggers is negatively correlated with MBS and periparturient prolactin, as well as calf performance measures such as ADG and weaning weights. No relationship between cow-calf contact measured by the loggers and MBS was found, which may indicate that measuring behavioural traits in livestock can be complex. Statistically significant differences in seasonal prolactin profiles throughout the year of beef bulls were observed, albeit different to what is previously described in the literature and suggests that the innate episodic and circadian rhythms of endocrine hormones need to be further considered in experimental design for future research.