The Buck Starts Here  
Age and nutrition dramatically influence doe growth and productivity

By John J. Ozoga

Understandably, much of the attention concerning Quality Deer Management (QDM) is focused on the adult male segment of the herd. However, it’s important to recognize there is more to QDM than producing large-bodied bucks with huge antlers. The successful program first must produce healthy, productive does that raise large healthy fawns.

It’s equally important to understand that the adult sexes live separately most of the year and exhibit something referred to as “sex segregation.” The adult sexes not only have different social requirements, they also differ in their habitat preferences, seasonal nutrition requirements, growth rates and, yes, even their life expectancy

In a highly structured deer herd, where a fair number of bucks reach maturity, few males ever achieve the pinnacle role of herd sires. This is adaptive, because relatively few of the largest, strongest, and healthiest bucks sire most of the fawns.

On the other hand, most female whitetails produce fawns at a fairly young age and continue to do so throughout their life. This doesn’t mean does are equally productive. Age and nutrition interact to affect doe physical development and reproductive success, just as in the case of bucks. For does to realize their full growth and reproductive potential, they must attain a certain age and experience excellent nutrition in order to reach their maximum growth and reproductive potential.

At what age do female whitetails attain their full growth? Their peak body weight? How much do conception rates differ among doe age classes? Are some age classes prone to heavier newborn fawn losses than others? Do sex ratios of progeny vary according to doe age? Do some doe age classes produce superior offspring? How are all these variables influenced by nutrition?

Scientists don’t have all the answers, but research has  given us considerable insight into many of these questions.

The Cusino Study

Studies I conducted in the square-mile Cusino enclosure,  located in Upper Michigan, provided a special opportunity to follow the physical development of deer, to determine their growth rates, and to assess the reproductive success of females in the herd. Such measures were possible because we could live-trap and handle every deer in the herd each March, when deep snow forced deer to feed from live-traps. We also evaluated the importance of nutrition by comparing the performance of deer before ¾ versus after ¾ supplemental feeding.

At trap-out, each animal was marked for individual recognition, weighed, and measured. All does were X-rayed to see if they were pregnant, to determine how many fetuses they carried, and to calculate their breeding and fawning dates. Follow-up observations permitted us to determine fawn-rearing success on an individual doe basis. The annual trap-out provided  a complete census and the opportunity to regulate herd size and composition by releasing surplus animals outside the area.

Growth Rate of Well-fed Does

Assuming that the length of a deer’s hind foot is a good indicator of its skeletal size, well-nourished female whitetails at Cusino achieved about 88 percent of their adult skeletal size by the time they were seven months old. By the time they were yearlings (1-1/2 years old) they had accomplished about 98 percent of their skeletal growth. All achieved maximum skeletal size by the time they were 2-1/2 years old.

Female chest girth measurements lagged slightly behind foot length, suggesting that does continued to add body mass another year or two. Seven-month-old individuals attained 82 percent of their adult girth size, while yearlings reached 92 percent. Does attained 97 percent of their maximum girth size when 3-1/2 years old, but did not reach their maximum until 5-1/2 years old. Since larger girth and heavier body weights go together, does with larger girth measurements may be in superior physical condition.

Surprisingly, the supplementally fed fawns continued to grow during winter. However, smaller fawns gained proportionately more weight than larger specimens. For example, doe fawns weighing less than 74 pounds in November gained an additional six percent by March, compared to a slight loss in weight for heavier individuals.

Seven-month-old female fawns, yearlings, and 2-1/2 year-olds reached about 52 percent, 83 percent, and 94 percent of their respective adult live body weights. Does 3-1/2 years of age averaged 142 pounds in weight, which was about 98 percent of their adult maximum. When 4-1/2 years old, they achieved their maximum body size, averaging between 145 and 151 pounds, depending upon their exact age.

Despite fairly tight breeding and birthing seasons, in  addition to supplemental nutrition, we observed considerable variation in body weights among individuals within the same age class. The average weight for seven-month-old fawns, for example, was 75 pounds; but individuals ranged from 55 to 106 pounds. Yearling does averaged 120 pounds, but ranged from 94 to 156 pounds. Some does 3-1/2 years of age, and older, were as small as 128 pounds, whereas others were as large as 191 pounds.

Since these weights were recorded in March, adult deer had lost weight over winter. They weighed about 8 to 10 percent more at peak weight in autumn.  Some mature does achieved peak weights of more than 200 pounds and would have dressed-out in excess of 150 pounds. Even some of the doe fawns would have had dressed weights of around 80 pounds.

Keep in mind, these deer were of the large northern borealis subspecies. Even when equally well nourished, southern subspecies will not attain such large size.

These data demonstrate estimating doe age based on body size is just about impossible, even when nutrition is controlled at a very high level.

Poor Nutrition Stunts Growth

We did not commence supplementally feeding the Cusino enclosure herd until January 1972. Prior to that, beginning in 1952, the deer were on their own and survived solely on natural forage. In the old days, we did not make hind foot and chest girth measurements, but each animal was of known-age and weighed annually in March.

The enclosure herd was obviously malnourished prior to supplemental feeding, as evidenced by rather dramatic improvements in their physical size following diet supplementation. The better diet also chronologically advanced their physical maturity.

Supplementally fed does averaged 18 to 22 percent heavier weights than naturally nourished does of similar age. For example, pre-feeding doe fawns averaged only 60 pounds, yearlings 96 pounds, 2-1/2 year olds 106 pounds, and 3-1/2 year olds 116 pounds.

Whereas well-fed does achieved 94 percent of their adult weight by 2-1/2 years of age, malnourished individuals did not accomplish such growth until they were 3-1/2 years old. The under-fed individuals also reached their mature body weight ¾ about 121 pounds, ¾ when 4-1/2 years old.

It’s important to recognize, however, even older malnourished does never achieved the massive body proportions of well-fed young adult does.

This relationship between diet and doe body growth has been demonstrated in other studies. For example, in the Adirondacks of New York, where the range was of poor quality, dressed weights for doe fawns, yearlings, and adult does averaged 55 pounds, 84 pounds, and 93 pounds, respectively. At the same time, excellent range on New York's Seneca Army Depot produced doe fawns dressing-out at 74 pounds, yearlings at 97 pounds, and older does at 103 pounds. Likewise, in West Virginia, yearling does from poor range in the East region dressed-out at 72 pounds, compared to 91 pounds for those from better range in the West region.

These studies demonstrate that poor nutrition -- primarily during the spring, summer, and autumn periods -- slows doe body growth and delays the onset of maturity. When wellfed, female whitetails accomplish nearly all of their skeletal growth by 18 months of age, and achieve about 94 percent of their maximum body size by 2-1/2 years of age. On the other hand, poorly fed individuals never accomplish their potential body size. And as we’ll discuss later, their reproductive performance also suffers.  

As noted in Quality Whitetails: The Why and How of Quality Deer Management, “Although mature body weight is a good indicator of herd health, it is far more revealing to examine dressed weights of fawns and yearlings. Each geographic area appears to have different optimum weights for these age classes...It is important to know what average dressed weights for healthy fawns and yearlings from your areas should be.”

Soil Fertility and Doe Size

A number of investigators have shown that soil fertility influences deer body mass. And some found positive correlations between specific soil nutrients and deer body size.

Recently, Mississippi State University researchers Bronson Strickland and Stephen Demarais reported their findings from  an extensive study done in Mississippi. They presented physical data for 247,051 deer harvested from five regions of the state that exhibit distinctly different soils, topography, and climate.

Strickland and Demarais found deer from the fertile western delta region of the state to be the largest for all age classes. Deer from the loess, upper coastal plain, lower coastal plain, and coastal flatwoods soils regions were progressively smaller for each sex and age class.

Female body weights increased through 4-1/2 years on the more fertile soils, as they did in the Cusino enclosure. However, does living on the less fertile soils exhibited a slower rate of growth, and their body weights peaked at 3-1/2 years of age.

According to Strickland and Demarais, “Energetic costs associated with reproduction in the lower fertility regions may have precluded the additional year of body mass growth. Another explanation is that females from fertile regions were able to deposit more subcutaneous fat in spite of the energetic costs of reproduction. Body mass increases under this alternative would be due to better body condition associated with greater fat deposits and not a greater rate of muscle or skeletal grow, per se.”

Unfortunately, the investigators did not record skeletal measurements, such as hind foot length.

Productivity

A female whitetail’s reproductive potential increases with age and is determined by her physical condition, which can vary annually depending upon nutrition.

To achieve puberty and breed at 6 to 8 months of age, northern fawns must weigh in excess of 80 pounds; Southern subspecies must weigh more than 70 pounds. Even so, not all animals reaching such proportions breed and reproduce.

Despite their large body size, for example, few supplementally-fed doe fawns in the square-mile Cusino enclosure bred. In fact, only three of more than 200 doe fawns examined during a 19-year period were pregnant. This suggests that factors other than nutrition and physical size suppressed reproduction among these northern Michigan fawns.

Pregnancy rates for all adult does in the Cusino enclosure increased with supplemental feeding. Fetal rates for yearlings doubled with improved diet, increased 50 percent among 2-1/2 year-old does, and 21 percent among older individuals. Prior to diet supplementation, yearling does carried an average of only 0.65 fetuses per doe, because more than one-third of them failed to breed and none carried twins. With improved nutrition, yearlings averaged 1.50 fetuses per doe.  About 97 percent of them bred and many conceived twins. The greater twinning rate for yearling does doubtless was due to their better growth and physical condition under supplemental nutrition. Nevertheless, those weighing less than 68 pounds as fawns, or below an estimated 127 pounds when bred, rarely conceived twins. In contrast, 44 percent of the yearling does reaching a heavier weight produced twins.

Normally, a doe is past her prime by age eight, and her productivity declines. However, that was not the case for well-fed Cusino enclosure does. As a group, these older does often conceived triplets and had the highest fetal rates: 2.20 fetuses per doe.

            These data rather clearly demonstrate the relationship  between good nutrition, accelerated growth, and maximum productivity among whitetail does. In this study, improved nutrition accounted for an 18 to 22 percent increase in doe body weights, and resulted in a 21 to 100 percent improvement in productivity, depending upon the age class.

Older Does Produce Superior Offspring

A fawn’s body size at weaning age will depend upon its age and nutrition. Late-born fawns may not attain optimal size even if well-nourished during early life. Likewise, fawns born on schedule will not achieve their full growth potential if they are under nourished. But even when most fawns were born about the same time, and the mother’s nutrition was optimal, older, more experienced mothers in the Cusino enclosure consistently raised superior fawns.

We were able to determine the mothers of 264 enclosure fawns. On average, the smallest female fawns (71 pounds) and the smallest male fawns (81 pounds) were raised by 2-year-old does. Differences in the size of female fawns raised by does in older age-classes were not particularly striking, ranging from 2 to 8 pounds heavier. Although 3-year-old does, as a group, also raised fairly small male fawns (82 pounds), the male fawns raised by older does averaged 5 to 15 pounds heavier.

This also suggests there is adaptive logic involved in  the tendency for older does to produce a preponderance of buck fawns. That is, these large sons would stand a better chance of one day becoming large dominant bucks, sire many offspring, and perpetuate their mother’s genes.

Conclusions

Although the weight of mature does is a good indicator of herd health, better gauges are the skeletal size and weight of fawns and yearlings. However, young age classes tend to have different optimal weights and measurements in each geographic region. Therefore, to evaluate the effects of management strategies, it's important to know average weights and measurements for healthy fawns and yearlings in your area.

Also, harvested animals should be accurately aged and care must be exercised when combining data for various doe age classes. Does do not achieve their full growth potential until age 3-1/2 or 4-1/2, and might not exhibit their highest level of productivity until later in life.

When comparing changes in body weights and skeletal measurements, it’s best to group does into at least four categories: fawns, yearlings, 2-year-olds and 3-years-plus. This should minimize bias and provide a good indicator of management effectiveness.

When malnourished does are restored to optimal nutrition, their growth rates will improve. They will also increase in size and achieve sexual maturity sooner, and therefore be more successful mothers. Even small increases in body size can yield substantial improvements in productivity, especially among young does. So, as deer herds are brought into better nutritional balance, antlerless deer harvest rates must increase to accommodate the rise in reproductive success.