|Abstract:||Chromium has been shown to have several positive effects on animal growth and insulin function. Currently, there is only 1 FDA approved source of Cr to be fed to cattle, chromium-propionate (Cr-prop). We hypothesized that feeding Cr-prop to cows during gestation would increase dam insulin sensitivity and glucose metabolism in gestation and increase subsequent milk production, thereby increasing weaning weights of calves from dams supplemented with Cr-prop compared to those not supplemented with Cr-prop. Furthermore, we hypothesized that feeding Cr-prop to steers in the feedlot would increase growth performance, increase insulin sensitivity, and improve marbling in the carcasses of steers fed Cr-prop when compared to steers not fed Cr-prop. To test these hypotheses, 2 experiments were conducted.
In experiment 1, the objectives were to determine the effects of supplementing Cr-prop through mid- and late gestation on beef cow BW and BCS, milk production, and progeny development, pre-weaning. Spring-calving, Angus-cross cows (n = 66) were fed 1 of 2 supplements through mid- and late-gestation: (1) 1.81 kg corn/hd·d-1 as fed (Control), or (2) 1.81 kg corn fortified with 3 mg Cr/hd·d-1 as fed (CrP). Chromium supplementation did not affect (P ≥ 0.24) pre- or postpartum cow BW, BCS, or BW and BCS change. There was an interaction (P < 0.01) of d in gestation and CrP supplementation for plasma glucose concentrations only. During mid-gestation, plasma glucose concentrations in cows fed CrP decreased 0.282 mmol/L compared to cows fed Control; however, by late-gestation, glucose concentrations in cows fed CrP increased 0.321 mmol/L compared to cows fed Control. Supplementation with CrP did not affect (P ≥ 0.58) mean insulin or insulin:glucose; however, insulin and glucose concentrations were reduced (P <0.01) as d in gestation increased, regardless of CrP supplementation. There was no effect of dam treatment on calf BW at birth (P = 0.40) or weaning (P = 0.56). Chromium supplementation did not affect (P ≥ 0.20) 24-h milk weight in mid- or late lactation. Milk composition was not affected (P ≥ 0.25) by CrP at mid-lactation. In late lactation, cows fed CrP had greater (P = 0.01) milk urea nitrogen compared to cows fed Control, with no other differences observed (P ≥ 0.23). In this experiment, supplemental CrP did not affect cow BW, BCS, milk production, or calf BW at birth or weaning. Furthermore, cow insulin sensitivity did not change; however, concentrations of both glucose and insulin decreased as days in gestation increased, regardless of CrP supplementation. The steer progeny from these supplemented dams were used in a second experiment to determine the effects of CrP during the finishing period.
In experiment 2, objectives were to test the effects of feeding Cr-prop to finishing steers in the feedlot on growth performance, insulin sensitivity, and carcass characteristics. Angus-cross steers (n = 34) from the dams fed in experiment 1 were stratified by BW and dam treatment and assigned to 1 of 2 treatments: (1) no supplemental Cr (Cont), or (2) 3 mg supplemental Cr/hd·d-1 (CrP). Both supplements, Cont and CrP, were delivered via 0.454 kg ground corn top dressed on the basal diet. There was no effect (P ≥ 0.45) of CrP on ADG, DMI, G:F, or final BW. However, steers fed CrP had greater (P = 0.10) days on feed (DOF) than steers fed Cont. There were no effects (P ≥ 0.41) of CrP on HCW, back fat, or KPH. Steers fed CrP had increased (P = 0.01) dressing percentage (DP) and tended to have increased LM area (P = 0.15), decreased marbling scores (P = 0.11), and intramuscular fat (P = 0.11) compared to steers fed Cont. There were no differences (P ≥ 0.25) in Quality or Yield Grade distributions. A glucose tolerance test (GTT) was conducted early in the finishing phase (21 DOF) and later in the finishing phase (98 DOF). There was a Feedlot Treatment (FT) × Time × DOF interaction (P = 0.08) for glucose concentrations. There were no other interactions (P ≥ 0.21) for glucose or insulin concentrations over time. There was no FT × DOF (P ≥ 0.21) for insulin area under the curve (iAUC), insulin:glucose, insulin or glucose baseline, or peak insulin or glucose concentrations. A FT × DOF interaction was observed for glucose area under the curve (gAUC; P = 0.01), glucose clearance rate (k; P = 0.02), and glucose half-life (T½; P = 0.07); however, DOF seemed to have the greatest effect on GTT. At 98 DOF, all steers had increased (P < 0.01) peak glucose and insulin, k, iAUC, insulin:glucose, and baseline insulin, when compared to 21 DOF, regardless of treatment, but, gAUC and T1/2 decreased (P < 0.01). While steers fed CrP tended (P = 0.11) to have increased baseline glucose concentrations compared to steers fed Cont, CrP supplementation did not affect (P ≥ 0.17) other measures of glucose or insulin. Results of this experiment indicate that, contrary to our hypothesis, CrP increased LM area and DP, but decreased marbling scores and intramuscular fat, with no effect on growth performance. With increased DOF, all steers became more insulin resistant, indicated by the use of more insulin to clear less glucose; and these effects were not mitigated by CrP supplementation.
Although the effectiveness of Cr supplementation has been proven in nonruminant animals and diabetic humans, the efficacy of Cr supplementation to gestating cows and feedlot finishing cattle remains to be determined. Factors limiting research in this area are many; however, the largest challenges may be determining Cr levels in the basal diet and the FDA limit on Cr inclusions in the diet. Future research with increasing Cr concentrations and greater animal numbers may be warranted based off of these preliminary findings.