Free: Contests & Raffles.
I all but exhausted that source(Google Scholar) on Blacktails quite awhile ago. I eventually decided it was worthwhile to read Whitetail research due to the sheer volume of material on that species. Photoperiod does appear to be the initiating factor in the initiation of the estrus cycle and most years the rut tends to progress along with predicable timing. From all my reading combined with annual trailcram study of buck activity in a single location, I now believe that there must be other factors that affect the rate at which the rut progresses in an individual year, and possibly the success rate in breeding. Something as simple as the quantity of Springtime rains, which affect the quantity and quality of Summer and Autumn browse has been shown to have a strong correlation to the number of fawns a doe will carry in gestation. It seems plausible that something as simple as that one factor alone may also affect the point at which a doe is physically ready to carry a fetus. There could be other unknowns that play a part as well.Yes, you are correct that nutritional constraints and fat/lean biomass ratio will affect at what age and when a doe or cow will become impregnated. Several studies investigating the histological (microscopic) studies of killed Blacktail doe ovaries allowed the determination of the number of estrus cycles the deer had undergone during season. The results indicate that the majority of Blacktail doe are successfully impregnated on their SECOND estrus cycle. Other related studies found that the estrus cycles were approximately 21 days apart (19 - 23 range) Also, some unknown percentage of Blacktail doe will undergo an "early estrus", occurring sometimes as early as October 12th or thereabouts, and the number of doe entering this first estrus cycle gradually grows as the month progresses. This "early estrus" seems to really be just the first unsuccessful estrus of the season. I’m unable to find any studies that show the first (early) estrus cycle being in October for blacktail deer. The blacktail studies that I found have the first estrus cycle starting as early as the beginning of November. The length of blacktail deer estrous cycle can be anywhere from 22-29 days with a gestation period of 203-210 days. I’m wondering if you are thinking about how most blacktails don’t become impregnated during the first ovulation which is different than first estrous cycle? The interval between the first and second ovulations average 8-9 days. The short interval between ovulations, the high proportion of does conceiving second ovulation, the short conception period for does and the regularity in the timing of breeding (photoperiod) seasons are adaptations to restrict optimal period for the birth of fawns. Two reasons would be for optimal time of year with spring green-up so fawns can grow and put on as much weight and fat/lean body mass as possible before winter and predator swamping which will allow a certain percentage of fawns to survive to around 79 days (give or take) when survival stabilizes.Considering the fact that the "peak of the rut" (the point as which most doe are successfully bred/impregnated) occurs, in most cases, sometime during the second week of November (second estrus cycle), one could guess that by just counting backwards 21 days from this peak of the rut point, one could estimate when the majority of doe are experiencing their first cycle, with the expectation that some bucks are responding to the sudden availability of hot does. We might also expect that this specific time might provide better hunting opportunities compared to say a week prior or after that predicted date. While I have no scientific studies to cite supporting better hunting on suspected BT doe first estrus cycle dates, it seems to be very clear that when in the woods over the course of the general season (15 - 31 Oct), hunter shooting activity seems to have very specific peaks that seem to correspond well with these ideas. Typically, early morning shooting activity by hunters appears to be very heavy (at least here in SW WA) sometime around the 20 - 22 October period in many years, which corresponds well with these estimated dates for first estrus and subsequent peak of the rut. If the dates of a weekend, (when most hunters would be expected to be off work and in the woods) does not fall on within a day or so of these dates, there seems to be far less rifle fire occurring. Similarly, member postings on this forum showing photos of mature BT bucks suddenly appearing in abnormal places (schoolyards etc.) during daylight hours often begin to be posted seem to show a sudden onset in mid-October. every year. One must infer that they are suddenly giving way to their normal secretive lifestyles for but one reason only. Hot doe! The confounding point is that predicting what will happen in a single year seems impossible until either trailcams or hunter rifle fire indicates an increase in buck activity during a period. It may be happening on the date of the season opener or as late as 10 or more days later than that. Once that date is established though, it is possible to estimate the timing of peak of the rut, and subsequent estrus cycles to follow for those doe that remain unbred. (four cents worth)