Few things affect farm financial viability more than getting cows in-calf early. Mating for a short time seems like being a great idea after “slogging through the required workload” of spring time. Anything to make things easier. Unfortunately empty cows have a significant reduction asset value. The best method to lower empty rates is to mate for longer. 

Heat detection is an issue. The best farms only detect 90%, most farms only 80%. Consider 100 cow farm with 85% heat detection. 85 cows will be submitted for AI in the first 3 weeks of mating. With 50% conception only 43 will get in calf. Of the remaining 57 with a 85% heat detection and 50 % conception only 25 will get in calf in the second 3 weeks of mating. Leaving 32 cows empty. Would you mate for 6 weeks of AI and accept 32% empty rate? Mating for 74 days will ensure all cows are calved prior to the start of mating, this will also maximise your financial asset value.