Response of beef cows offered a chlortetracycline fortified mineral and either strip or continuous stocked to stockpiled fescue

Abstract Text: Food and Drug Administration proposed changes to the USA feed law addresses judicious use of medically important antimicrobials.  Cattle producers grazing Kentucky 31 tall fescue (Festuca arundinacea) routinely feed mineral fortified with chlortetracycline (CTC).  Available CTC fortified minerals often contain  3.08 g/kg CTC (350 mg CTC/113.4 g mineral consumed).  Fescue management may include fall stockpiling for deferred grazing with forage being allocated using either continuous or strip stocking.  This project evaluated the effect of CTC delivered in a free choice mineral supplement with either strip stocking (STRIP) or continuous stocking (CONT) on stockpiled fescue.  The study design was a 2x2 factorial with 3, 2.4 ha pasture replications per treatment combination.  The same mineral package was used for both the no CTC (CTC-) and CTC (CTC+) supplements.  The CTC+ mineral contained 3.08g/kg CTC.  Ninety-six pregnant Bos taurus cows were randomly allocated to the 12 pastures. STRIP pasture was allocated at 0.04 ha/d in 4 and 3 d strips. Grazing occurred from Dec 4 to Jan 29. The STRIP:CTC- had the least, per cow mineral intake (7.4 ± 0.43 g/d), differing (P < 0.1) only from CONT:CTC- and STRIP:CTC+.  All other mineral intakes were similar (8.5 ± 0.43 g).  Initial rising plate estimated forage allowance was 2,513 ± 129.2 kg/ha.  Forage utilization was not affected by either grazing method or CTC addition and averaged 47 ± 3.7%.  The numerical difference in forage utilization was 44% (CONT) and 49% (STRIP).  CTC+ resulted in a 22 kg greater BW change (P< 0.05) from the initial to interim weigh date but not interim to final (7kg less than CTC-).  Final BW and BCS were not affected by grazing management or CTC+.  External body temperature and thermocirculatory index (TCI, based on rectal, skin, and ambient temperature) at the rump and ear differed between STRIP and CONT (P< 0.05), but was not affected by CTC+.  CTC+ had a greater skin temperature between the dew claw (P< 0.1) at the interim but not final weigh date.  CTC+ had no effect on TCI.   In conclusion, grazing method did not significantly affect forage utilization, BW, or BCS but resulted in different skin temperature responses.  Adding CTC to the mineral primarily resulted in an initial BW gain response without affecting final BW and BCS.  Feeding CTC to cattle grazing stockpiled fescue did not result in a sustained benefit for performance or body temperature.

Keywords: chlortetracycline, fescue, strip grazing, strip stocking