You didn't put on extra weight overnight so it is equally unrealistic to take it off quickly. (bmi-calculator.net)
If he's neutered or very inactive, you would need to decrease his calories so he wouldn't put on extra weight. (pawster.com)
The other constraint on the live trade with China is regulations from Beijing requiring all cattle shipped from Australia to be killed within 14 days of arrival, meaning there is no time for putting on extra weight through a feedlot. (afr.com)