There’s an ecological theory, the Trivers-Willard hypothesis, that predicts (among other things) that mothers in poor body condition will produce more female than male offspring, and vice versa. The logic behind the theory is that since males require a higher plane of nutrition to develop into large, strong, competitive individuals, it would be advantageous for the species to favor production of male offspring when conditions are ideal.
How they accomplish this isn’t clear, but the levels of many hormones, particularly those produced by fat cells, are known to fluctuate depending on body condition and feeding. This could be the signal that determines whether uterine conditions are most favorable for males or females.
The link between maternal body condition and production of relatively more male or female offspring has been confirmed in several species. Researchers from the Ecology Group, Institute of Natural Resources, Institute of Veterinary, Animal and Biomedical Sciences of Massey University in New Zealand investigated this hypothesis in horses by following a group of wild horses over several pregnancies.
They found that mares in poor body condition did indeed produce more fillies than colts, and that when the same mares had produced colts and fillies over several pregnancies they had been in significantly poorer body condition at conception when they had fillies. The reverse was also true, that mares in better condition produced a higher proportion of colts. Breeding your mare when she’s in good body condition won’t guarantee you get a colt, but it may increase the odds. (But, yes, it’s still the sperm that ultimately results in the sex of the offspring. Biology hasn’t changed a bit.)