From puppyhood into adulthood, a dog's coat evolves to a genetically predetermined length. For most breeds it will stay at this length, while other breeds experience continued growth and require regular haircuts. Few scientific studies have established exact times for regrowth.
Puppy Fur Growth
Puppies go through many changes, including shedding their puppy fur. Puppies are born with a coat of fur, but that coat is gradually replaced with an adult coat. This begins when a puppy is anywhere from 3 to 6 months old, and can continue throughout puppyhood, which can last for over a year. Puppy fur falls out more quickly than adult fur grows, so during this stage, a puppy's fur may appear patchy and shaggy.
Once a dog has replaced his puppy coat with an adult coat, it goes through a growth cycle. The first cycle is the anagen phase, which is the active growth phase. During this phase the coat reaches its full length, determined by genetics and breed. The next phase, known as the transient catagen phase, signals the end of growth. That phase is a lead-in to the resting, non-growth phase known as the telogen phase.
Cycles Established, Other Info Uncertain
Most dog breeds have a telogen-predominant growth cycle. This means that once their coat is fully grown, most of the hairs stay at that length instead of beginning the anogen phase again. Exceptions include breeds that need regular haircuts, like poodles, Yorkshire terriers and others usually referred to as having hair rather than fur. Although these cycles are known, the University of Tennessee–Knoxville College of Veterinary Medicine notes very little information is established regarding what controls hair growth cycles and rates.
NIH Study Helps Clarify
To help clarify growth rates, the National Institutes of Health's National Center for Biotechnology Information did a study to determine regrowth rates after clipping fur for surgery, and whether season influenced growth rates. They recruited and clipped 11 Labrador retrievers throughout the year, and found their coats grew back to full length between 13.6 and 15.4 weeks. They also found the season did not influence how long it took for the coats to grow back to their pre-clipped length.
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