With the September Yearling Sales coming around the corner, one of the more common beliefs often quoted by yearling buyers is that a horse with a large girth has "plenty of heart room", thus by inference a large heart.
Separating out the misconception that a large heart is necessarily a good thing (its not, it is dependent on the size of the horse), is there truly a relationship between girth circumference and heart size?
We decided to dig up some data on 850 horses ranging from yearlings to older horses to see what the relationship might be. The girth circumference was measured in centimeters and ranged from 170cm to 190 cm. Heart size is commonly defined by the calculation of Left Ventricular Mass which is defined by the equation
LV Mass (g) = 1.04 x [(LVIDd + LVFWd + IVSd)3 – LVIDd3]-13.6
Where, LVIDd = Left Ventricular Internal Diameter in diastole, IVSd = Interventricular Septal in diastole and LVFWd = Left Ventricular Free Wall Thickness in diastole. LV Mass varied from a yearling filly with a very small LV Mass of just over 1900 to a stallion just retired from racing with an LV Mass just shy of 5100 g.
From the above diagram you can see that the linear relationship between girth circumference and LVMass for all types of racehorses is low to moderate at best (0.2545). But what if we look at yearlings alone? Again, we took 800 yearlings, ranging from 395 to 524 days of age in three separate countries and their girth circumference and LV Mass measurements.
As you can see from above for yearlings the relationship of girth circumference to LV Mass is even weaker at just 0.014. Essentially there is no relationship when it comes to girth circumference and LV Mass in yearlings but as the horses age and mature there is a stronger relationship between the measurements.
What the data does show is if you want to know how large a yearlings heart is, judging it by girth circumference isn't the way to do it.