An experiment was conducted to determine the effects of high vs low body condition scores (BCS) produced by restricted feeding on reproductive characteristics, hormonal secretion, and leptin concentrations in mares during the autumnal transition and winter anovulatory period. Mares with BCS of 6.5 to 8.0 were maintained on pasture and/or grass hay, and starting in September, were full fed or restricted to produce BCS of 7.5 to 8.5 (high) or 3.0 to 3.5 (low) by December. All but one mare with high BCS continued to ovulate or have follicular activity during the winter, whereas mares with low BCS went reproductively quiescent. Plasma leptin concentrations varied widely before the onset of restriction, even though all mares were in good body condition. During the experiment, leptin concentrations gradually decreased (P < 0.0001) over time in both groups, but were higher (P < 0.009) in mares with high vs low BCS after 6 wk of restriction, regardless of initial concentration. No differences (P > 0.1) between groups were detected for plasma concentrations of LH, FSH, TSH, GH, glucose, or insulin in samples collected weekly; in contrast, plasma prolactin concentrations were higher (P < 0.02) in mares with high BCS, but also decreased over time (P < 0.008). Plasma IGF-I concentrations tended (P = 0.1) to be greater in mares with high vs low BCS. The prolactin response to sulpiride injection on January 7 did not differ (P > 0.1) between groups. During 12 h of frequent blood sampling on January 12, LH concentrations were higher (P < 0.0001), whereas GH concentrations (P < 0.0001) and response to secretagogue (EP51389; P < 0.03) were lower in mares with high BCS. On January 19, the LH response to GnRH was higher (P < 0.02) in mares with high BCS; the prolactin response to TRH also was higher (P < 0.01) in mares with high BCS. In conclusion, nutrient restriction resulting in low BCS in mares resulted in a profound seasonal anovulatory period that was accompanied by lower leptin, IGF-I, and prolactin concentrations. All but one mare with high BCS continued to cycle throughout the winter or had significant follicular activity on the ovaries. Although leptin concentrations on average are very low in mares with low BCS and higher in well-fed mares, there is a wide variation in concentrations among well-fed mares, indicating that some other factor(s) may determine leptin concentrations under conditions of high BCS.
Previous observations from this laboratory indicated that horses with high BCS could have resting plasma leptin concentrations ranging from low (1 to 5 ng/mL) to very high (10 to 50 ng/mL). To study the possible interactions of leptin secretion with other endocrine systems, BCS and plasma leptin concentrations were measured on 36 mares and 18 geldings. From mares and geldings that had a mean BCS of at least 7.5, five with the lowest (low leptin) and five with the highest (high leptin) leptin concentrations were selected. Jugular blood samples were collected twice daily for 3 d from the 20 selected horses to determine average resting hormone concentrations. Over the next 12 d, glucose infusion, injection of thyrotropin-releasing hormone (TRH), exercise, and dexamethasone treatment were used to perturb various hormonal systems. By design, horses selected for high leptin had greater (P < 0.0001) leptin concentrations than horses selected for low leptin (14.1 vs. 2.8 +/- 0.92 ng/mL, respectively). In addition, mares had greater (P = 0.008) leptin concentrations than geldings. Horses selected for high leptin had lower (P = 0.027) concentrations of GH but higher (P = 0.0005) concentrations of insulin and thriiodothyronine (T3) than those selected for low leptin. Mares had greater (P = 0.0006) concentrations of cortisol than geldings. There was no difference (P > 0.10) in concentrations of IGF-1, prolactin, or thyroid-stimulating hormone (TSH). Horses selected for high leptin had a greater (P = 0.0365) insulin response to i.v. glucose infusion than horses selected for low leptin. Mares had a greater (P = 0.0006) TSH response and tended (P = 0.088) to have a greater prolactin response to TRH than geldings; the T3 response was greater (P = 0.047) in horses selected for high leptin. The leptin (P = 0.0057), insulin (P < 0.0001), and glucose (P = 0.0063) responses to dexamethasone were greater in horses selected for high leptin than in those selected for low leptin. In addition, mares had a greater (P < 0.0001) glucose response to dexamethasone than geldings. Cortisol concentrations were decreased (P = 0.029) by dexamethasone equally in all groups. In conclusion, differences in insulin, T3, and GH associated with high vs. low leptin concentrations indicate a likely interaction of these systems with leptin secretion in horses and serve as a starting point for future study of the cause-and-effect nature of the interactions.
Thirty-eight Angus-cross beef cows were used to evaluate differences in DMI, residual feed intake (RFI), and endocrine markers on the basis of cow size and RFI ranking during 2 stages of production. Cows housed in individual pens (2.2 × 9.1 m) were fed, over a 70-d feeding period, 30% Bermuda grass hay and 70% ryegrass baleage diet during lactation (LACT) and a 100% ryegrass hay diet during postweaning (NOLACT). Individual daily feed intake, BW, and BCS were recorded, and hip height was used to determine frame score (FS). Feed intake was used to calculate RFI for each cow, and cow was the experimental unit. Blood samples were obtained on d 0 and 70 and were analyzed for glucose, insulin, leptin, triiodothyronine (T3), and thyroxine (T4). Cows were assigned to a light (LIT) or heavy (HEV) BW groups on the basis of mean BW at the beginning of the LACT period. On the basis of RFI values for each feeding period, cows were placed into a negative (NEG; RFI < 0.00) or positive (POS; RFI > 0.00) RFI group and into a low (LOW; ≤0.2 SD mean RFI), medium (MED; within ±0.19 SD), or high (HI; ≥0.2 SD mean RFI) RFI group. During LACT, DMI was 4.8% greater (P = 0.03) and FS was greater (P < 0.01; 6.4 and 5.5 ± 0.16) for the HEV compared with LIT BW cows. No RFI by day interaction or RFI group main effect occurred for endocrine markers during LACT; however, a negative relationship (P = 0.04) existed between BW group and combined T3 data, and a positive relationship (P = 0.04) existed between RFI and combined insulin data. For both LACT and NOLACT, RFI was similar (P > 0.05) among BW groups; however, DMI was 6.5% and 8.9% greater (P < 0.01) for POS compared with NEG RFI in the LACT and NOLACT periods. In LACT, DMI was greater (P < 0.01) for HI and MED RFI compared with LOW RFI, and in NOLACT, DMI was greater (P < 0.01) for the HI compared with MED and LOW RFI cows and MED compared with LOW RFI cows. During NOLACT, DMI was 8.9% greater (P < 0.01) for the HEV (12.4 ± 0.22 kg) compared with LIT (11.3 ± 0.19 kg) BW cows. Change in BCS was greater (P ≤ 0.03) in higher RFI cows in both RFI groups only in the NOLACT period. Differences in T3 and T4 on d 0 and 70 were 25% and 15% greater (P ≤ 0.04) for the LIT BW group compared with the HEV BW group. A negative correlation existed (P ≤ 0.04) between BW group and T3 and T4, as well as leptin and RFI (P = 0.03). Although cow BW was independent of RFI and T3 and T4 levels tended to be greater in lighter BW cows, DMI was consistently greater for cows with heavier BW and higher RFIvalues.
Thirty-two Suffolk wether lambs were fed for 84 d in a 2 x 2 factorial experiment using two levels of dietary protein (9.0 to 12.1% CP, low protein, LP; or 12.8 to 14.4% CP, high protein, HP) and supplemental Cr (none, C; or 400 ppb Cr as chromium tripicolinate, Cr). At 14- to 21-d intervals, lambs were weighed, and jugular blood samples were collected. Mean ADG and carcass weight (P > .10) did not differ. In lambs fed HP, Cr reduced liver weight and increased kidney weight (P < .01). Lambs fed HP had elevated plasma urea N (PUN; P < .01) and albumin (P < .04). During an i.v. epinephrine challenge on d 43, plasma cortisol declined in lambs fed Cr (Cr x time, P < .03) and in lambs fed LP (CP x time, P < .001). An i.v. glucose tolerance test conducted 3 h later showed that supplemental Cr decreased glucose clearance rate in lambs fed HP (CP x Cr, P < .10) but not in lambs fed LP. On d 62, PUN was increased in lambs fed HP (P < .001) between 0 and 3 h postprandial, and there was a Cr x CP interaction (P < .04). Postprandial plasma NEFA declined with Cr vs C (Cr x time, P < .07) and with HP vs LP (CP x time, P < .10). By d 66, lambs fed Cr had an elevated (P < .03) blood platelet and fibrinogen content. Chromium increased erythrocyte count in lambs fed HP (Cr x CP, P < .08), and isolated peripheral lymphocytes had greater blastogenic response to 4 microg/mL of phytohemagglutinin (Cr x CP, P < .001). The lymphocyte response to pokeweed mitogen (.2 microg/mL) was reduced in lambs fed Cr (P < .10). In the present experiment, Cr supplementation had minimal and inconsistent effects on production and metabolic criteria of lambs.
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