Our objective was to determine the extent to which a stair-step compensatory nutrition regimen regulated the growth and subsequent lactation performance of developing beef heifers. Forty crossbred beef (Angus x Gelbvieh) heifer calves were assigned to either a control or stair-step compensatory nutrition regimen. Heifers fed the stair-step compensatory nutrition regimen were fed according to an alternating schedule beginning with a high energy diet (130% of the metabolizable energy recommended by the National Research Council) for 2 mo, followed by an energy restricted diet (70% of the recommended metabolizable energy) for 3 mo, and concluding with the high energy diet again for 2 mo. Thereafter, all heifers received the control diet. The average daily gain of heifers fed the stair-step compensatory nutrition regimen was 2-fold higher than that of control heifers during the final high energy phase. The RNA and protein contents in the mammary tissues of heifers fed the experimental regimen in early lactation were greater than those of control heifers. Lipid content was significantly reduced in the mammary tissues of heifers fed the experimental regimen. The expression of beta-casein in the mammary tissues of heifers fed the experimental regimen was higher than that of the control heifers. Heifers fed the stair-step regimen yielded 6% more milk than did those fed the control regimen. Total protein and casein in milk of heifers fed the experimental regimen were higher than those in milk of control heifers. These results indicate that the stair-step compensatory nutrition regimen enhanced mammary development and subsequent lactation performance.
Cundiff (1985) states: "It is estimated that today about 70 percent of the calves marketed from beef cattle breeds in the United States are crossbred and that between 50 and 60 percent of the cows are crossbred. This represents a ma jor shift to crossbreeding fro m straightbreeding which prevailed into the late 1 9~ in response to research demonstrating favorable effects of heterosis (e.g. see review by Cundiff, 1970." Research results have continued to affinn this trend of increased crossbreeding. For in stance, re earch ha s shown that a th ree-b reed crossbreeding program can increase productio n in terms of pounds of calf weaned per cow exposed to breeding by 23 percent (Cundiff et aI., 1974). Cundiff (1 984) provided further information on the productivity of FI cows. Clearly, this trend of increased marketing of finished crossbred cattle would not have continued if crossbreeding resulted in obvious detriments to the tenderness of the cooked meat or to other important carcass traits. However, objective information on the effect of crossbreeding on tenderness and other carcass traits was needed to asses the immediate and long-term ramifica tions of suppying meat from crossbred cattle to con sumers. If sexing of bull semen becomes possible, it will be important to know if steers and heifers are different with regard to carcass traits. Also of interest is the extent of the differences, if any, among different cooked muscles. Therefore the objectives of this study were to: 1) measure differences of tenderness among muscles, 2) assess the dif ferences between steer and heifers for various carcass characteristics, and 3) to estimate the effect that crossbreeding has o n carcass traits (Slanger et al., 1985). ExperiDlent~Procedure Carcass information was accumulated for eight years (197 1 through 1978) on approximately 475 steer and 125 heifer carcasses representing eight breeds and 62 combina tions of these eight breeds. All tbese cattle were raised, finished and slaughtered at North Dakota State University. The eight breeds were Angus. Hereford, Shorthorn, Charolais , Brown Swiss, Simmental, Galloway and Red Angus. The data were from straightbred, two-, three-and four-way crossbred cattle. However, straightbreds. F I' S Sfanger and Danielson are associate professors and Marchello is professor, Department of Animal and Range Sciences.
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