Weight, or newtons of force due to gravity, is calculated using the formula: W=mg. Here, W is the weight in newtons (N), m is mass (in kilograms) and g is the acceleration due to gravity (which, on Earth, is always \(9.8 m/s^2\)). Solving for mass, then, requires that you divide weight (1,200 N) by gravity (\(9.8 m/s^2\)). This gives you a mass of about 122 kg.
Therefore, if 1200 newtons of force are acting on a cow on a trailer, the mass of the cow would be approximately 122 kilograms.