From BIF Guidelines Wiki

Weight ratios are often used to compare animals' genetic merit within contemporary groups when Expected Progeny Differences are not available. Caution should be used in interpreting these values as they are generally very low accuracy estimates of genetic merit. Ratios should be calculated separately for bulls, steers, and heifers. Ratios should be calculated using adjusted weights. For example, a weaning weight ratio should use 205-day adjusted weaning weight and also use values adjusted for age of dam.

A weight ratio is generally calculated as,

Weight ratios show each animal's percentage deviation from the average of its contemporaries. For weight ratios to be of value, contemporaries should be herd mates, similar in age, and raised under the same management and environmental conditions.