Seasonal regulation of prolactin secretion in hypophyseal stalk transected beef calves

Effects of selected endophyte and tall fescue cultivar combinations on steer grazing performance, indicators of fescue toxicosis, feedlot performance, and carcass traits

Five tall fescue [Lolium arundinaceum (Schreb.)] pastures [wild-type 'Kentucky-31' with 78.0% of plants infected with ergot alkaloid-producing endophyte Neotyphodium coenophialum (KY31 E+), 'Jesup' AR542 endophyte-infected contaminated with 30.3% tall fescue containing ergot alkaloid producing-endophyte (Jesup AR542 E+); 'GA-186' AR584 endophyte-infected contaminated with 11.8% tall fescue containing ergot alkaloid producing-endophyte (AGRFA 140); 'PDF' AR584 endophyte-infected contaminated with 5.5% tall fescue containing ergot alkaloid producing-endophyte (AGRFA 144); and 'KYFA 9301' AR584 endophyte-infected contaminated with 10.0% tall fescue containing ergot alkaloid producing-endophyte (AGRFA 150)] were compared for steer growth performance, toxicity, feedlot performance, and carcass traits. Steers (mean initial BW=322 kg) grazed pastures for 84 d in spring and 56 d in autumn for 2 yr. Steers were shipped after grazing in Prairie, MS, to Macedonia, IA, for finishing. Mean herbage mass was not different (P=0.15) among pastures. Posttreatment (d 28+) serum prolactin concentrations were depressed (P=0.013) on KY31 E+. Steers grazing KY31 E+ had greater (P<0.01) posttreatment rectal temperatures during spring. Spring hair coat scores were greatest (P<0.01) on KY31 E+ at d 56 and 84. Steer ADG was least (P<0.01) on KY31 E+ in spring and depressed (P=0.014) on KY31 E+ and Jesup AR542 E+ in autumn. Spring grazing ADG was greater (P=0.049) on AGRFA 150 than Jesup AR542 E+ and AGRFA 140. No BW differences (P=0.09) among pastures were seen at reimplant during feedlot finishing. Pasture had no effect on ADG after reimplant (P=0.68), days on feed (P=0.56), or final BW (P=0.55). Exposure to fescue toxicosis did not affect (P≥0.19) carcass traits. Hair coat price discounts applied for spring-grazed steers on KY-31 E+ affected (P<0.01) initial steer monetary values. There were no pasture differences for finishing costs (P≥0.61) or final carcass value (P=0.59). Elite tall fescue cultivar and novel endophyte combinations improve growth performance of grazing calves over KY31 E+. Producers whose calves graze KY-31 E+ tall fescue should consider retaining ownership of these cattle through feedlot finishing to avoid market discounts and capture value from compensatory BW gains during finishing.

Use of nonergot alkaloid-producing endophytes for alleviating tall fescue toxicosis in stocker cattle

Grazing studies were conducted to determine cattle growth performance, evaluate toxicosis, and compare grazing behavior in stocker cattle grazing nonergot alkaloid-producing endophyte-infected (AR542 or AR502), endophyte-free (E-), or wild-type toxic endophyte-infected (E+) Jesup, Georgia-5, and Kentucky-31 tall fescue. Replicated 0.81-ha tall fescue paddocks were established at the Central Georgia Branch Station at Eatonton and the Northwest Georgia Branch Station at Calhoun during October 1998 and were stocked with beef cattle for autumn and spring periods from fall 1999 through spring 2002. Mean ergot alkaloid concentrations were higher (P < 0.01) on E+ pastures than the other treatments at both locations. At Calhoun and Eatonton, post-treatment serum prolactin concentrations were decreased (P < 0.01) on E+ compared with AR542, AR502, and E- tall fescue. Cattle on AR542, AR502, and E- pastures had lower (P < 0.05) post-treatment rectal temperatures than cattle grazing E+ tall fescue during spring at Eatonton and Calhoun. Calf ADG was higher (P < 0.05) on AR542, AR502, and E- as compared with E+ tall fescue during autumn and spring grazing at Eatonton, and at Calhoun, cattle on E+ pastures had lower (P < 0.05) ADG in both autumn and spring. Gain/hectare was higher (P < 0.05) on AR542, AR502, and E- than on E+ during autumn at Eatonton and during spring at both locations. In autumn at Calhoun, gain/hectare was greater (P < 0.05) on AR502 and E- compared with E+ tall fescue. During April, May, and June, cattle grazing E+ pastures at Eatonton spent more (P < 0.01) time idling, more (P < 0.01) time standing, and used more (P < 0.01) water than cattle on AR542 and E- tall fescue. Daily prehensions and biting rate were each higher (P < 0.01) on AR542 and E- tall fescue than E+ tall fescue in both grazing seasons. There were no differences among pasture treatments for bite size in either spring (P = 0.50) or autumn (P = 0.34). Steers grazing E+ pastures had lower DMI than steers grazing AR542 and E- pastures during spring (P < 0.10) and lower DMI than steers grazing E- pastures during autumn (P < 0.05). Daily steer water usage was decreased (P < 0.10) in E+ pastures compared with AR542 and E- pastures during late fall. These results indicate that nonergot alkaloid-producing endophyte technology is a promising option for alleviating tall fescue toxicosis in stocker cattle.

Metabolic cues for puberty onset in free grazing Holstein heifers naturally infected with nematodes

Leptin is a new plausible candidate for the molecular link between nutritional status and the reproductive axis. In previous studies we described that continuous natural nematode infections in heifers retarded growth and delayed the onset of puberty, and that the insulin-like growth factor I (IGF-I) was involved. In the present study we monitored the leptin levels during development in heifers naturally parasitized versus those chronically treated with ivermectin and we investigated whether growth hormone (GH) accounted for the differences in IGF-I previously noted. Insulin levels were also measured. Prolactin hormone was recorded as an indicator of immune system activation. We found a direct correlation between leptin and body weight during development and a prepubertal surge of the hormone 2 weeks before the first progesterone peak that indicates the onset of puberty. This suggests that leptin may act as a signal for this event. Insulin did not vary during growth and prepuberty. On the other hand, GH as not responsible for diminished IGF-I levels in parasitized animals as levels were similar in both groups. The GH levels were high at birth and then diminished rapidly and remained constant during development and puberty. The last hormone studied, prolactin, followed seasonal changes of sunlight duration and presented sporadic bursts in infected animals. These were related to high nematode infection and are probably involved in the immune response of the host.

Secretion of dopamine and norepinephrine in hypophyseal portal blood and prolactin in peripheral blood of Holstein cattle

The objective was to test the hypothesis that dopamine regulates prolactin (PRL) secretion by determining acute changes in catecholamine concentrations in hypophyseal portal blood of cattle, and their relation to peripheral blood concentration of PRL in hypophyseal stalk-transected (HST) and sham-operated controls (SOC). Holstein heifers (606 +/- 21 kg BW; mean +/- SE) were subjected to neurosurgery for 8 h to collect hypophyseal portal blood with a stainless steel cannula designed with a cuff placed under the pituitary stalk and peripheral blood via a jugular vein catheter. PRL plasma concentration was measured by radioimmunoassay, and dopamine and norepinephrine in portal plasma by radioenzymatic assay. During anesthesia before HST or SOC, PRL plasma concentration ranged from 20-40 ng/ml throughout 255 min. PRL abruptly increased and remained above 90 ng/ml after HST compared with a steady decrease to <20 ng/ml in SOC heifers throughout 440 min. Within 5 min after severing the hypophyseal stalk, dopamine in portal blood (>8 ng/ml) was significantly increased (P < 0.05) compared with peripheral blood (<2 ng/ml). Norepinephrine concentration in portal blood was significantly greater (P < 0.05) than in peripheral blood during the first 60 min. The sustained high PRL level in peripheral plasma after severing the hypophyseal stalk stimulated hypothalamic dopamine secretion from hypophyseal portal vessels during the prolonged period of blood collection. Norepinephrine concentration in these cattle was greater in hypophyseal portal than in peripheral blood, implicating both an important hypothalamic source of the catecholamine as well as an adrenal gland contribution during anesthesia.

Long-term growth after hypophyseal stalk transection and hypophysectomy of beef calves

Hypothalamic hormones regulate episodic and basal secretion of hormones from the anterior pituitary gland that affect metabolism and growth in cattle. This study focused on long-term growth in young calves subjected to hypophysectomy (HYPOX), hypophyseal stalk transection (HST), and sham operation control (SOC). Cross-bred (Hereford x Aberdeen Angus) and Hereford, and Aberdeen Angus calves were HYPOX (n = 5), HST (n = 5), or SOC (n = 8) at 146 +/- 2 days of age, whereas another group was HST (n = 5) or SOC (n = 7) at 273 +/- 5 days of age. Body weight was determined every 21 days from birth to 1008 days of age. Anterior vena cava blood was withdrawn at 4-day intervals from day 64-360 for RIA of GH, TSH, T4, T3, and LH, and at 20-min intervals for 480 min to determine episodic hormone secretion. Daily feed intake was determined in HST and SOC calves during an 80-day period. Birth weight averaged 35 +/- 1 kg (+/- SE) and was 142 +/- 4 kg at 126 days and 208 +/- 8 kg at 252 days before surgery. From day 146-1008, growth was arrested (P < 0.001) in HYPOX (0.06 +/- 0.01 kg/day) compared with SOC (0.50 +/- 0.04 kg/day) calves. Growth continued but at a significantly lower rate (P < 0.05) in calves HST at 146 days (0.32 +/- 0.07 kg/day) and 273 days (0.32 +/- 0.06 kg/day) compared with SOC (0.50 +/- 0.09 kg/day). Growth continued to be impaired to 1008 days, but more so in those HST at 146 days (432 +/- 43 kg BW) than 273 days (472 +/- 5 kg BW) and less (P < 0.05) than SOC (586 +/- 37 kg BW). Daily feed intake was consistently less (P < 0.05) in HST compared with SOC calves. Although episodic GH secretion was abolished and peripheral serum GH concentration remained consistently lower in HST (2.4 ng/ml) than SOC (5.5 ng/ml; P < 0.01), the calves continued to grow throughout 1008 days. Peripheral serum TSH concentration was less (P < 0.05) HST compared with SOC calves. There was an abrupt decrease (P < 0.001) in serum T4 (4-fold) and T3 (3-fold) concentration after surgery that remained to 360 days in HST compared with SOC calves. At the time calves were killed, pituitary gland weight was markedly reduced (P < 0.001) in HST (0.18 +/- 0.01 g/100 kg BW) compared with SOC (0.54 +/- 0.03 g/100 kg BW). Histological examination of pituitary glands from HST calves indicated the persistence of secretory GH and TSH cells in the same areas of the adenohypophysis as SOC calves. Coronal sections of the gland stained with performic acid-Alcian blue-periodic acid-Schiff-orange G, revealed GH and TSH secreting cells in HST calves similar to controls. These results indicate that long-term growth continues, but at a slower rate, after hypophyseal stalk transection of immature calves in spite of complete abolition of episodic GH secretion and consistently decreased basal secretion of GH, TSH, T4, and T3 compared with sham-operated animals. Growth was abolished after hypophysectomy of immature calves in which circulating GH and TSH was undetectable.