Responses to an estrogenic growth promoter in beef steers fed varying nutritional regimens

We investigated the influence of DM and(or) energy intake and dietary CP le vels on the performance and nitrogen (N) retention of beef steers with and without growth promoter implants. In Exp. 1, four implanted (Synovex-S, 200 mg of progesterone plus 20 mg of estradiol benzoate) Angus steers and four Angus steers that were not implanted were assigned to concurrent 4 x 4 Lat in squares. Initial BW averaged 296 kg. Each square consisted of moderate a nd moderately high DM intake treatments (4 and 6 kg/d) and low and adequate CP intake treatments (450 and 600 g/d) in a 2 x 2 factorial arrangement, P eriods were 2 wk of adaptation, 5 wk of growth, and 1 wk of balance collect ion. Experiment 2 consisted of two replicates of 32 Hereford steers each (i nitial BW 324 kg). Each replicate was a 4 x 2 factorial in which steers wer e individually fed for 63 d. All steers had ad libitum access to a 60% corn -based concentrate diet containing either 7.9, 10.0, 12.1, or 14.6% CP (DM basis), and steers were either implanted or not implanted with Synovex-S. E xperiment 3 was similar to Exp. 2 except that all steers (initial BW 315 kg ) received a low-protein diet (7.6% CP) with calculated energy densities of either 1.86, 2.04, 2.22, or 2.42 Mcal ME/kg DM, and steers were limited to an equalized DM intake of 9.5 kg daily. In Exp. 1, gains for the low CP, m oderate and moderately high DM intakes and the adequate CP, moderate and mo derately high DM intakes were 240, 555, 208, and 730 gld, respectively, for steers not implanted and 333, 643, 488, and 988 gld, respectively, for imp lanted steers (SEM = 102 g/d). Respective values for retained N were .13, . 18, .16, and .26 g/kg BW.75 and .13, .15, .22, and .29 g/kg BW.75 (SEM = .0 4 g/kg BW.75). Implant response was greater (CP x implant, P < .01) for bot h gain and retained N when adequate CP compared to low CP diets were fed. F or Exp. 2, the lowest CP diet reduced ADG (.97 vs 1.27 kg/d) and efficiency of gain (100 vs 120 g gain/kg DM). Synovex-S was less effective in improvi ng efficiency for the lowest protein diet than for the other diets (11.7 vs 20.2%). During Exp. 3, neither Synovex-S nor dietary energy influenced gai n and efficiency. We concluded that adequate dietary protein is necessary t o optimize the response to estrogenic growth promoters and that the low res ponse under inadequate protein and energy intake is not improved by increas ing the energy density of the diet.