Materials and Methods
Seventy Angus X Simmental crossbred calves (initial BW 366.5 ± 9.3 lb) were used to determine the effect of age at feedlot entry and castration on growth, performance, and carcass characteristics (3 X 2 factorial arrangement). Cattle were allotted to treatments based on sire, birth date, and birth weight. At 82 days of age (5-26-99), calves in the steer group were castrated. Calves were placed in the OARDC feedlot in Wooster, OH at 110 days of age (EW), 202 days of age (NW), or 371 days of age (Y). Calves in the NW and Y group remained with their dams and grazed in southern Ohio on mixed pastures of orchard grass, Kentucky bluegrass, clover, and tall fescue until 188 days of age. From 188 days of age until feedlot entry, NW cattle grazed on orchard grass pastures in Wooster, OH. Cattle that did not enter the feedlot until 371 days of age grazed on orchard grass pastures and were offered a 26.06% CP supplement, orchard grass hay, whole shelled corn, and corn silage to achieve a gain of 1.45 lb/day until feedlot entry. Four weeks before weaning time, and upon arrival in Wooster, all calves were vaccinated for protection against IBR, PI-3, Haemophilus somnus, Pasteurella, and 7-way clostridial bacterin (Quadraplex, Somnugen 2P, and Dybelon, respectively; Bio-ceutic, St. Joseph, MO) and de-wormed with Ivomec pour-on (Merck, Rahway, NJ). Early-weaned steers were re-vaccinated when NW steers were placed in the feedlot. Research protocols regarding animal care followed guidelines recommended in the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (Consortium, 1988).
Early-weaned, NW, and Y steers were implanted with Synovex-S (10 mg estradiol benzoate, and 100 mg progesterone; Fort Dodge Animal Health, Overland Park, KS) at 167, 202, and 291 days of age, respectively, and with Revalor-S (24 mg estradiol and 120 mg trenbolone acetate; Hoechst-Roussel Agri. Vet. Co., Overland Park, KS) at 264, 291, and 384 days of age, respectively. Revalor-S was given when calves were estimated to be 100 days from slaughter.
All calves were penned individually in a totally enclosed feedlot barn. Pen construction consisted of metal gates and a slatted concrete floor. Pens were 2.6 X 1.5 m, giving each animal 3.9 m2 of space. Cattle were fed a 70% concentrate, 18.1% CP receiving diet (Table 1) for the first 14 days after feedlot entry. Initially, all calves were fed 10 lb of DM and intake was gradually increased during the 14-day receiving period. A 70% concentrate, 14.3% CP intermediate diet, was fed to EW calves from 124 to 202 days of age. From 202, 216, and 385 days of age, respectively, EW, NW, and Y calves were fed an 85% concentrate, 14.1% CP finishing diet. Cattle were offered feed ad libitum once daily, beginning at 0800 hours, and feed refusals were recorded daily for each animal. Initial and final weights were determined, using the average of weights taken before feeding on two consecutive days. Interim weights were taken every 28 days, before feeding. Average daily gain, DMI, and feed efficiency (gain/feed) were determined for each 28 day period, as well as for the entire trial. Feed samples were collected every 7 days throughout the trial and analyzed for DM according to the procedures of Goering and Van Soest (1970). Monthly composites of feed were analyzed for N content using a LECO 2000 N analyzer (LECO Corporation, St. Joseph, MI). Health status of the steers was recorded daily. Rectal temperatures were measured in animals with decreased feed intakes, or in those with severe nasal mucous drainage and rapid or labored breathing. Any animal with a rectal temperature > 103.0oF, taken before feeding in the morning, was treated with antibiotics (Micotil, Elanco, Indianapolis IN; Nuflor, Schering Plough, Union NJ) according to label instructions. Antibiotic treatment continued until rectal temperature was below 103.0oF.
At each feedlot entry date, cattle were scanned between the 12th and 13th rib using a PIE 200 ultrasound machine (Classic Ultrasound Equipment, Classic Medical Supply, Tequesta, FL) to determine backfat depth and longissimus muscle area. Calves were also measured at the hip at each feedlot entry date to determine hip height. Calves were removed from the trial on an individual basis when they reached a pre-determined backfat thickness of 0.5 inches. Calves were required to be a minimum weight of 1100 lbs when slaughtered. Hot carcass weight, backfat depth, percentage of kidney, pelvic and heart fat, longissimus muscle area, and USDA quality and yield grades were determined by qualified OSU personnel at 48 hours after harvest.
Data were analyzed using the GLM procedures of SAS (1996) for a randomized design with a 3 X 2 factorial arrangement of treatments. The model included effects due to age at feedlot entry, castration and the age at feedlot entry by castration interaction. Residual mean square was the error term and animal was the experimental unit.