No abstract

The working hypothesis was that dietary fats differing in fatty acid composition would differentially influence ovarian follicular growth. Cows (n = 27) were fed isoenergetic, isonitrogenous, and isofibrous diets containing no added fat (control; CT, n = 7) or diets supplemented with fats containing primarily saturated (SAT, n = 7), polyunsaturated (PU, n = 7), or highly polyunsaturated (HPU, n = 6) fatty acids. Coincident changes in serum lipid metabolites, insulin, and GH and the concentration of IGF-I in large and medium-sized follicles also were examined. Body weights and body condition scores remained similar for all groups throughout the study. Polyunsaturated fat increased (diet x day, P = .06) the number of medium-sized follicles on d 5 through 9 of a synchronized estrous cycle within 3 wk of onset of feeding and maximized (P < .001) this to a fourfold difference at ovariectomy after 7 wk. Fats with predominantly SAT and HPU tended (P < .10) to produce these effects after 7 wk. All fat-supplemented diets increased serum concentrations of total cholesterol (P < .05), GH (P < .05), and follicular fluid IGF-I in large follicles (P < .065) compared to CT but differentially influenced serum concentrations of insulin. Polyunsaturated fat stimulated a marked increase (P < .001) in serum insulin relative to controls within 3 wk, whereas SAT and HPU increased (P < .05) serum insulin only after 6 to 7 wk. We conclude that consumption of PU fatty acids stimulates a greater rate of ovarian follicular growth in cattle compared to CT, AT, and HPU. Future research should investigate the potential role of insulin in mediating PU effects on follicular growth.

Objectives of the current studies were to characterize the pattern of GnRH secretion in the cerebrospinal fluid of the bovine third ventricle, determine its correspondence with the tonic and surge release of LH in ovariectomized cows, and examine the dynamics of GnRH pulse generator activity in response to known modulators of LH release (suckling; neuropeptide Y [NPY]). In ovariectomized cows, both tonic release patterns and estradiol-induced surges of GnRH and LH were highly correlated (0.95; p < 0.01). Collectively, LH pulses at the baseline began coincident with (84%) or within one sampling point after (100%) the onset of a GnRH pulse, and all estradiol-induced LH surges were accompanied by corresponding surges of GnRH. A 500- microg dose of NPY caused immediate cessation of LH pulses and lowered (p < 0.001) plasma concentrations of LH for at least 4 h. This corresponded with declines (p < 0.05) in both GnRH pulse amplitude and frequency, but GnRH pulses were completely inhibited for only 1.5-3 h. In intact, anestrous cows, GnRH pulse frequency did not differ before and 48-54 h after weaning on Day 18 postpartum, but concentrations of GnRH (p < 0.05) and amplitudes of GnRH pulses (4 of 7 cows) increased in association with weaning and heightened secretion of LH. We conclude that the study of GnRH secretory dynamics in third-ventricle CSF provides a reasonable approach for examining the activity and regulation of the hypothalamic pulse generator in adult cattle. However, data generated using this approach must be interpreted in their broadest context. Although strong neurally mediated inhibitors of LH pulsatility (suckling; NPY) had robust effects on one or more GnRH secretory characteristics in CSF, only high doses of NPY briefly abolished GnRH pulses. This implies that the GnRH signal received at the hypophyseal portal vessels under these conditions may differ quantitatively or qualitatively from those in CSF, and theoretically would be undetectable or below a biologically effective threshold when LH pulses are absent.

Over the last 20 yr, global positioning system (GPS) collars have greatly enhanced livestock grazing behavior research. Practices designed to improve livestock grazing distribution can now be accurately and cost effectively monitored with GPS tracking. For example, cattle use of feed supplement placed in areas far from water and on steep slopes can be measured with GPS tracking and corresponding impacts on distribution patterns estimated. Ongoing research has identified genetic markers that are associated with cattle spatial movement patterns. If the results can be validated, genetic selection for grazing distribution may become feasible. Tracking collars have become easier to develop and construct, making them significantly less expensive, which will likely increase their use in livestock grazing management research. Some research questions can be designed so that dependent variables are measured by spatial movements of livestock, and in such cases, GPS tracking is a practical tool for conducting studies on extensive and rugged rangeland pastures. Similarly, accelerometers are changing our ability to monitor livestock behavior. Today, accelerometers are sensitive and can record movements at fine temporal scales for periods of weeks to months. The combination of GPS tracking and accelerometers appears to be useful tools for identifying changes in livestock behavior that are associated with livestock diseases and other welfare concerns. Recent technological advancements may make real-time or near real-time tracking on rangelands feasible and cost-effective. This would allow development of applications that could remotely monitor livestock well-being on extensive rangeland and notify ranchers when animals require treatment or other management.