We do not have data with feeding MGA to steers because it is not approved for use in steers as you have mentioned. We are not aware of work in steers done in other countries. However, the data available on feeding MGA to heifers would suggest some increase in marbling. MGA is generally fed to feedlot heifers to prevent cycling (estrus). See Macken et al., Professional Animal Scientist 19:159-170, published in 2003. This was commercial research trials with implants and feeding MGA. In this study with five pens and 290 heifers per treatment fed 150 days, feeding MGA increased marbling score from 5.19 to 5.47 where 5.0 is small 0 or the cutoff for choice and select carcasses. On a percentage basis, the percentage of heifers grading choice or above was increased from 55.2% to 67.1%. These heifers tended to gain slightly more weight, but clearly feeding MGA had an impact on carcass quality. Terry Mader at the Northeast Station at the University of Nebraska has done research in this area as well. In a study in the 1999 Nebraska Beef Report (pp. 51 to 54), different implant systems were compared, and some comparisons were done with and without feeding MGA. In this study, within implant systems, feeding MGA increased marbling score from 5.06 to 5.20 or from 4.95 to 5.32 using the same scale as before. Choice percentages were increased, and carcasses grading Select were decreased 5 to 8% units as well. It is not clear what this mechanism is. One might speculate that if heifers are not cycling, then performance and grade would be improved. However, this would not hold true for steers. But feeding MGA to steers and seeing an improvement in carcass quality is not surprising.

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