At insemination, the semen is deposited in the body of the cow’s uterus and uterine contractions take it to the tips of both uterine horns in a matter of minutes (we know this from dye test work at abattoirs). Once it arrives at the uterine tip, the sperm forms a stable reservoir at the junction of the uterine tip and the fallopian tube where it waits for ovulation to occur by attaching to the uterine wall. The fallopian tube connects the uterus to the ovary and is where fertilisation takes place.
The egg is released into the fallopian tube at ovulation, along with the follicular fluid which flows into the uterus where it “activates” the sperm reservoir through a process known as “capacitation”. Capacitated sperm swim extremely fast and enter the fallopian tube in a race to fertilise the egg. If insemination occurs after ovulation, the sperm arrive at the tip of the uterus to find that follicular fluid is already present and are activated immediately, so the race to fertilise the egg begins much sooner than insemination before ovulation.
Whatever the timing of AI, the AI dose reaches the uterine tip in short order post insemination where all sperm are subsequently activated at the same time. This means that all male and female sperm cells in the AI dose have an equal opportunity to fertilise the egg regardless of the timing of AI, so on average the sex ratio can only ever be 50:50.