Management Strategies for Beef Heifer Development

Four experiments were conducted to determine the impact of development systems, trace mineral source, and trace mineral supplementation on beef heifers. Experiment 1 utilized 300 Angus-based, spring-born heifers to evaluate postweaning heifer development systems on gain, reproductive performance, and feed efficiency as a pregnant heifer. Heifers were blocked by BW and randomly assigned to graze corn residue, upland range, or were fed 1 of 2 diets in a drylot differing in energy levels: high or low. Heifer development system did not impact AI or final pregnancy rates. Development cost per pregnant heifer was not different among treatments. Furthermore, pregnant heifer feed efficiency was not impacted by development system. In Experiment 2, March-born and May-born crossbred heifers were stratified by BW and randomly assigned to 1 of 2 postweaning treatments from mid-January to mid-April: (1) ad libitum meadow hay and 1.64 kg/d of a 32% CP supplement (HAY) or (2) grazed meadow and 0.41 kg/d of supplement (MDW). March and May-born HAY heifers experienced greater ADG during the treatment period. During the summer period, however, MDW heifers had greater ADG than HAY heifers, likely due to compensatory gain. Heifer development system did not impact pregnancy rate in the March or May replacement heifers; however, March-born heifer pregnancy rate was greater than May-born. The lower pregnancy rate in May heifers may be due to declining forage quality during the late-summer breeding season. In Experiment 3, heifers were synchronized with a 14-d CIDR-prostaglandin F2α protocol and either injected with a trace mineral or received no injection at CIDR insertion. Prior to synchronization, heifers were range developed and offered free-choice mineral. Mineral status prior to mineral treatment did not differ among heifers. The proportion of heifers pregnant within the first 21 d and 33 d of the breeding season was not different nor was overall pregnancy rates. In summary, injectable trace mineral at CIDR insertion 33 d before artificial insemination did not influence reproductive performance in heifers with adequate trace mineral status. In a final study, beef heifers previously managed on 3 separate development systems were stratified by previous development treatment and BW and allocated into 1 of 8 pens per yr. Pens were randomly assigned to 1 of 2 mineral sources, hydroxy (HD) or sulfate (CON). Mineral status was analyzed via two liver biopsies prior to and following the 68-d mineral treatment. Heifer BW, ADG, and reproductive performance was not different in heifers receiving either mineral source treatments. Mineral source treatment did not affect final Mn or Zn concentrations. Liver Cu concentrations were greater for CON than HD heifers at the end of the trace mineral trial; however, all heifers maintained adequate status throughout the study. The difference in Cu status may be due to ruminally insoluble hydroxy Cu allowing thiomolybdate absorption, thus reducing hepatic Cu stores and resulting in decreased Cu status. Advisors: Rick N. Funston and Andrea S. Cup