Feeding zilpaterol hydrochloride is associated with decreased dry matter intake shortly after initiation of feeding dependent on season and previous intake

A database of daily feed deliveries for steers and heifers fed at 3 commercial feedyards in Kansas between January 1, 2010, and January 31, 2012 (n = 1,515 pens), was used to investigate the prevalence and extent of changes in DMI after initiation of feeding zilpaterol hydrochloride (ZIL) at 8.3 mg/kg (DM) for 20 d. Season affected the percentage of pens experiencing a decrease in DMI post-ZIL (P < 0.01), but there were significant (P < 0.01) season × sex, season × feedyard, season × pre-ZIL DMI, season × days post-ZIL, and season × period post-ZIL interactions. Average DMI decreased within 1 d after initiation of ZIL feeding in all seasons; however, this initial decrease was greater (P < 0.01) in the summer (-0.30 kg) and winter (-0.27 kg) than in the spring (-0.05 kg) or fall (-0.06 kg). The decrease in DMI averaged across all days post-ZIL was greater in summer than during other seasons for both steers and heifers, and the change in intake was greater in steers than heifers in all seasons but fall. Size of intake change within each season varied by feedyard and by season. The percentage of pens that had a large DMI decrease (≥ 0.9 kg/d) was greatest during the summer (33%), and the percentage of pens with no decrease was the least (15%); during the fall, 34% of pens had no DMI decrease and only 8% of pens had a large decrease in DMI. Intake before ZIL initiation affected size and prevalence of DMI decrease; with increasing pre-ZIL DMI, the percentage of pens with a decrease increased from 62% for pens with pre-ZIL DMI of less than 7.7 kg/d to 82% for pens consuming greater than 10.5 kg/d pre-ZIL (P < 0.01). Of those pens with greater than 10.5 kg/d pre-ZIL DMI, 27% had DMI decrease of greater than 1.4 kg/d compared to only 3% for pens consuming <8.7 kg/d pre-ZIL. The average dosage of ZIL consumed per animal with an average DMI of 7.3, 8.2, 9.1, 10.0, and 10.9 kg/d was calculated to be 61, 68, 76, 83, and 91 mg/animal daily, which may be related to the differences in DMI decrease. Pre-ZIL DMI contributed to DMI decrease during ZIL administration, but the increased occurrence and size of DMI decrease during the summer may indicate an additional physiological mechanism.