For Simmental and Simmental influenced cattle, we can see from this illustration what is weighted the most in computing the numbers.
API: All Purpose Index — Evaluates sires for use on the entire cow herd (bred to both Angus first-calf heifers and mature cows) with the portion of their daughters required to maintain herd size retained and the remaining heifers and steers put on feed and sold grade and yield.
Taken from Understanding ASA’s Indexes
All EPDs, with the exception of tenderness,are taken into consideration in this index.
Though increasing growth is invariably a good thing in terminal sires, its strong association with mature size makes it less desirable in replacement female sires, as increasing mature size increases cowherd maintenance requirements. Keep in mind that the positive benefits of increased growth in sires’ steers and cull females are accounted for in the API.
Nevertheless, the index is telling us that the extra cost of maintaining larger cows outweighs the benefit of increased growth in other areas of the system. Even so, the API is evidently finding sires with more carcass weight than would be expected given their growth potential. (The top 25 average in the 95 and 90th percentiles for weaning and yearling weight, while reaching the 70th percentile for carcass weight.)
From the top 25, it is evident that more weight is placed on marbling than yield grade. This is likely because there is no financial incentive to improving yield grade unless there is a problem (i.e. yield grades reach 4); for the most part, due to Simmentals superior yielding ability, SimAngus half-bloods see few discounts for yield grade.
TI. The TI is designed for evaluating sires’ economic merit in situations where they are bred to mature Angus cows and all offspring are placed in the feedlot and sold grade and yield. Consequently, maternal traits such as milk, stay-ability and maternal calving ease are not considered in the index