Comparison of organic vs. inorganic trace minerals on rate and efficiency of gain and conception rates in beef heifers
Whitehurst, William Ashley III
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Objectives of this experiment were to compare rate and efficiency of gain, and conception rates of yearling heifers supplemented with Cu, Zn and Mn as either methionine chelated trace mineral (CTM) or the same minerals in SO 4 form. The experimental design utilized 3 ranches, each having 2 replications per treatment with pen as the experimental unit for ADG, DMI and G:F. Heifer was the experimental unit for pregnancy rates. Ranch 1 contained 498 Angus heifers, ranch 2, 236 Red Angus cross heifers, and ranch 3, 1,742 Angus cross heifers. All heifers were fed silage based diets that contained approximately 13.5% CP, 64% TDN (DM basis) and had low levels of SO 4, Mo or Fe in feed or H 2O. Diets contained 24 ppm Cu, 70 ppm Zn and 64 ppm Mn. Supplements were fed for 181 d (Ranch 1), 149 d (Ranch 2) and 151 d (Ranch 3) prior to breeding. Heifers were weighed at trial initiation (BW 270 kg ± 2.8), end of drylot feeding, at breeding and at pregnancy diagnosis. Ranch 1 heifers were bred by AI followed by natural service (45 d breeding), Ranch 2 heifers were bred by natural service (50 d breeding) and Ranch 3 heifers were bred by AI once. Pregnancy was determined via ultrasound. Ranch effects were significant (P < 0.001) for gain, ADG, G:F and overall pregnancy rate, but not for conception in the first 21 d. No ranch x treatment interactions were detected for any measurements (P e 0.47) and no differences (P e 0.46) were detected between treatments for total gain, ADG, G:F or the number of heifers that conceived during the first 21 d on Ranches 1 or 2. Conception rate increased (P = 0.03) for CTM heifers from ranch 3 with one AI breeding. Across ranches, conception rates during the first 21 d of breeding did not differ (P = 0.12) between treatments but overall pregnancy rate was greater (P = 0.05) for heifers supplemented with CTM. Under the conditions of this experiment results suggest that supplementation with CTM contributed to greater pregnancy rates in heifers.