Authored by: Angela Gonella, UF/IFAS North Florida Research and Education Center, and Mario Binelli, UF/IFAS Department of Animal Sciences
Beef cattle ranchers need to know about a wide variety of topics: forages, breeds, herd health, vaccination protocols, fence construction, tractors, squeeze chutes, markets, records, taxes, etc… and reproductive physiology is yet another of them! Understanding some basic reproductive physiology can help producers make better decisions about replacement heifers, bull fertility, the breeding season, nutrition, reproductive technologies, and reproductive management. So, to help you catch up on some of the most important concepts, we have decided to put together a series of short articles for the Panhandle Ag e-News covering practical topics in reproductive physiology.
This is the first of what we hope will be a series of short articles designed to explain reproductive physiology in a simple, practical way. Today’s topic: puberty in the female.
What is puberty?
Puberty is the acquisition of reproductive competence. In simple terms, it is the transition from a sexually immature animal to one with an active reproductive system that can ovulate and become pregnant. However, remember one important concept: puberty is a process, not a single event. The reproductive system does not suddenly “turn on” one morning. Instead, several physiological changes occur gradually until the brain, ovaries, and reproductive tract begin working together in a coordinated manner for the ultimate purpose of generating a calf.
A major player in this process is the hypothalamus, a small region of the brain that controls many important body functions, including reproduction. The hypothalamus releases a hormone called gonadotropin-releasing hormone, or GnRH (Figure 1). If you have been around the cattle business for a while, you may recognize the name GnRH. We routinely administer GnRH as part of estrus synchronization and timed artificial insemination protocols. However, we didn’t invent GnRH for synchronization programs. The animal naturally produces it. GnRH travels from the hypothalamus to the pituitary gland, another small structure located at the base of the brain. In response to GnRH, the pituitary releases two important reproductive hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH act on the ovaries, stimulating follicular development, estradiol production, and eventually ovulation. Before puberty, ovarian follicles already grow and produce estradiol, but the brain is highly sensitive to estradiol’s negative feedback. In other words, relatively small amounts of estradiol are enough to suppress GnRH and LH secretion. As the heifer approaches puberty, this sensitivity decreases. GnRH and LH pulses become more frequent, follicular development progresses, estradiol concentrations increase, and eventually enough estradiol is produced to trigger an LH surge and induce the first ovulation.
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When does a heifer reach puberty?
Because puberty is a gradual process, how do we determine when has it occurred? In research, we commonly define puberty based on the evidence that the heifer has ovulated for the first time. We can determine this by measuring progesterone in blood or using ultrasonography to identify a corpus luteum (CL) on the ovary. The presence of a CL tells us that ovulation has occurred and that the heifer has started ovarian cyclicity. But there is an important difference between being physiologically pubertal and being ready to breed.
Some heifers may reach puberty quite young, particularly breeds or genetic lines selected for early sexual maturity. However, a heifer can ovulate without having reached an appropriate body size and developmental stage for breeding. Breeding a heifer too early or at inadequate body weight can create management problems. She still needs nutrients for her own growth while simultaneously supporting pregnancy and, later, lactation. If her nutritional requirements are not met, subsequent reproductive performance can be compromised. For this reason, the practical goal is not simply for heifers to reach puberty. The goal is for them to reach puberty before the start of their first breeding season, with enough time to establish normal estrous cycles before they are expected to be bred.
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Puberty, metabolism, and growth
One of the most important factors affecting puberty is body growth. The reproductive system is metabolically expensive. Biologically, reproduction becomes a priority only when the animal has sufficient resources to support it. Therefore, the brain continuously receives information about the animal’s nutritional and metabolic status. This is when hormones such as leptin become important. Leptin is produced primarily by adipose tissue and tells the brain about the animal’s energy reserves. It is one of several metabolic signals that tell the brain whether the animal has sufficient energy reserves and an adequate nutritional status to support reproductive function. Other metabolic signals, including insulin, glucose, and insulin-like growth factor 1 (IGF-1), also interact with the reproductive axis. This explains something cattle producers have observed for generations: heifers that grow too slowly tend to reach puberty later.
Age matters, but body weight and nutritional development matter just as much (Figure 2). Traditionally, replacement-heifer programs have targeted a percentage of expected mature body weight by the beginning of the breeding season. The exact target can vary with breed type, genetics, ranch environment, feed costs, and management strategy, but the basic biological principle remains the same: inadequate growth can delay puberty.
At the same time, the goal is not simply to make heifers as heavy as possible. The goal is to provide heifers with an appropriate nutritional environment for development, allowing them to reach puberty and become pregnant early without excessive development costs.
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Puberty and the social environment
Nutrition and genetics are not the only factors influencing puberty. The animal’s social environment can also affect reproductive development. One interesting example is the bull effect. Exposure of prepubertal heifers to mature bulls can, under some circumstances, accelerate the onset of puberty. Bulls provide visual, behavioral, and olfactory signals that can influence females’ neuroendocrine system. Pheromonal and other sensory cues may ultimately alter GnRH and LH secretion. However, bull exposure is not a substitute for a good heifer development program. A heifer that is significantly underweight or nutritionally restricted will not suddenly become reproductively mature simply because a bull is standing on the other side of the fence. Think of social cues as one more signal entering the reproductive system, associated with nutrition, genetics, age, season, and other environmental factors. This is another good example of why puberty should be understood as a process controlled by multiple signals rather than by a single hormone or event.
Environmental factors can also influence the onset of puberty. One concern is exposure to external factors such as heat-stress. Heat stress delays puberty attainment in beef heifers by disrupting the communication between hypothalamus, pituitary and the ovary. Heat stress can also reduce dry matter intake and compromise weight gain. Therefore, several studies have shown that heifers under elevated heat stress can show a delay in puberty. Mitigation strategies such as artificial shade, sprinklers or fans can help to mitigate this problem.
Why should a cattle producer care about puberty?
Puberty determines whether your replacement heifers are ready when the breeding season begins. A heifer that has not reached puberty by the beginning of the breeding season has fewer opportunities to conceive during a limited breeding window. In contrast, heifers that have cycled before breeding will have more opportunities to conceive early. And getting heifers pregnant early matters!!
Heifers that conceive early in their first breeding season generally calve earlier in the calving season. This provides more time between calving and the next breeding season, a particularly important advantage for first-calf heifers, which are simultaneously dealing with lactation, continued growth, and recovery from calving. So, when developing replacement heifers, don’t think about only one question: Are they big enough to breed? Also consider whether they have reached puberty and started cycling before the breeding season. That distinction is important.
A practical way to evaluate whether replacement heifers are approaching puberty is to determine their reproductive tract scores (RTS). Using transrectal palpation and ultrasonography (Figure 3), the reproductive tract is typically scored from 1 to 5 based on uterine development and ovarian structures. Heifers with scores of 1–2 are generally considered immature, a score of 3 indicates that puberty is approaching, and scores of 4–5 indicate heifers that are pubertal or cycling. RTS can therefore be a useful tool before the breeding season to evaluate whether a group of heifers is developing appropriately and identify animals that may not yet be reproductively ready.
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Take-home message
Puberty in beef heifers is not simply a calendar day or a particular body weight. It is a biological process influenced by genetics, growth, nutrition, metabolic signals, the brain, the ovaries, and the environment. At the center of this process is the activation of the hypothalamic-pituitary-ovarian axis: GnRH stimulates FSH and LH secretion, and ovarian follicles develop and produce estradiol. Eventually, the heifer ovulates for the first time and begins normal estrous cyclicity. For producers, the practical message is simpler: develop replacement heifers so puberty occurs before, not during, their first breeding season. Because when the breeding season starts, you want them ready to breed.
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