The effects of ractopamine-HCl (optaflexx) on performance, carcass characteristics, and meat quality of finishing feedlot heifers

Beta-adrenergic agonists are commonly used in livestock production to accelerate growth by enhancing lean tissue gain. These compounds repartition nutrients away from fat deposition and toward protein accretion. Generally, increased growth is associated with ractopamine feeding, which improves feed conversion and increases body weight gain. However, little data exists on the effects of ractopamine-HCl on live performance or carcass characteristics of beef heifers. Data released from Elanco Animal Health indicate that differences between gender may exist in response to ractopamine, and therefore appropriate strategies for the administration of this compound must be defined for heifers independent of those for steers. The objective of this study was to determine the effects of Optaflexx, when fed for different dosages over different durations, on finishing heifer performance.


Introduction
Beta-adrenergic agonists are commonly used in livestock production to accelerate growth by enhancing lean tissue gain.These compounds repartition nutrients away from fat deposition and toward protein accretion.Generally, increased growth is associated with ractopamine feeding, which improves feed conversion and increases body weight gain.However, little data exists on the effects of ractopamine-HCl on live performance or carcass characteristics of beef heifers.Data released from Elanco Animal Health indicate that differences between gender may exist in response to ractopamine, and therefore appropriate strategies for the administration of this compound must be defined for heifers independent of those for steers.The objective of this study was to determine the effects of Optaflexx, when fed for different dosages over different durations, on finishing heifer performance.

Procedures
Non-implanted crossbred heifers (n= 281, 1,049 lb initial body weight) were fed diets based on steam-flaked corn and individually weighed on day 0. The animals were blocked by body weight into 10 weight blocks.Within each weight block, heifers were randomly allocated to 50 partially-covered, concretesurfaced pens (five to six animals/pen, 10 pens/treatment).Pens were then randomly assigned to five dietary treatments: no ractopamine (Control); 200 mg of ractopamine per heifer daily for 28 days prior to slaughter (200×28); 200 mg of ractopamine per heifer daily for 42 days prior to slaughter (200×42); 300 mg of ractopamine per heifer daily for 28 days prior to slaughter (300×28); and 100 mg for 14 days, 200 mg for 14 days, and 300 mg of ractopamine per heifer daily for the 14 days prior to slaughter (Step-up).
Pens of cattle were weighed using a platform scale on day 0 and immediately before being transported to a commercial abattoir for slaughter.All cattle were allowed ad libitum access to a common finishing diet.The entire daily ration was delivered at approximately 1:00 p.m. Dry matter intake, rate of gain, and feed efficiency were determined for each pen of cattle.Initial carcass weights were used for performance calculations and were estimated by multiplying initial live weight by an assumed dressing percentage of 62%.The dose and duration of Optaflexx feeding is summarized in Table 2.All values represented in tables were calculated based on the entire 42day period.b Formulated to provide 300 mg monensin, 90 mg tylosin, 0.5 mg melengestrol acetate and 0, 100, 200, or 300 mg ractopamine-HCl per head daily in a ground corn carrier.

Results and Discussion
Dry matter intake was lower for heifers fed the high dose of Optaflexx.In addition, carcass average daily gain and gain efficiency were improved in all animals fed Optaflexx when compared to the control treatment (P<0.05).Carcass gain efficiency was similar for those heifers fed Optaflexx at 200 and 300 mg/heifer daily.No differences among treatments existed for hot carcass weight; dressing percent; yield grade; marbling score; loin eye area; kidney, pelvic, and heart fat; incidence of liver abscess; or quality grade (P>0.19).
Duration of feeding Optaflexx appeared to have a greater impact than the dosage level.There was no advantage in feeding an escalating dose of Optaflexx when compared to continuous feeding of 200 mg/day.

Implications
In general, feeding Optaflexx to finishing feedlot heifers increased performance, and had relatively little impact on carcass characteristics.