Grullo is commonly used to refer to a spectrum of coloration from light grey to dark grey that is the result of color dilution genetics that reduce the strength of black coloration and to a lesser degree, reduces the strength of red coloration. Here, we are including just our cattle that are true black base coat coloration, diluted to a consistent grey or silver coloration.
There are multiple genetic triggers that cause dilution of color in cattle. The most documented in Longhorns is the dun color genetics. There are at least two other dilution factors in other breeds of cattle. We theorize that there is at least one dilution factor in Longhorns besides the documented dun dilution genetics.
We are not color genetics experts. Most of our color genetics information comes from Double Helix Longhorns' Color page, UC Davis, and Neogen.