PS4-4. Association Between Number of Permanent Incisors, Bone Ossification, and Days of Age in Dairy Cows.

Abstract Number of permanent incisors (PI) present at slaughter is used to differentiate “young” from “old” carcasses. Cattle exhibiting three or more PI are classified as 30 months of age or older and are subject to specified risk material (SRM) removal requirements, restricted fabrication options, and diminished export eligibility. Bone ossification (BO), evaluated by USDA graders, is also used to estimate carcass physiological maturity and determine quality grade. However, published relationships between PI, BO, and true chronological age (DOA) remain limited. Sixty dairy cow carcasses (57 Holstein, 3 Jersey) with ear tags displaying legible birth dates were selected following exsanguination and evaluated for number of PI. Subsequent to carcass splitting, BO in the sacral, lumbar, and thoracic vertebrae was determined (A50 = 50; B50 = 150; C50 = 250; D50 = 350; E50 = 450) by one trained observer. Correlations and polynomial regression models were calculated to characterize relationships among PI, PI pairs, BO, and DOA. Permanent incisor count (r = 0.811), pairs of PI (r = 0.795), and BO (r = 0.793) were strongly associated (P < 0.01) with DOA. Moreover, count of PI (r = 0.697) and pairs of PI (r = 0.689) were moderately associated (P < 0.01) with BO. Polynomial models were developed to estimate DOA using individual PI count (DOA = 22.086 x PI2 - 52.703 x PI + 802.24; R2 = 0.69), pairs of PI (DOA = 18.879 x PI pairs2 - 28.715 x PI pairs + 754.62; R2 = 0.66), and BO (DOA = 0.0108 x BO2 - 1.7161 x BO + 996.94; R2 = 0.70). Bone ossification was also estimated from count of PI (BO = 6.382 x PI2 -28.197 x PI + 138.37; R2 = 0.53) and pairs of PI (BO = 6.3333 x PI pairs2 x 29.4 + PI pairs + 140.67; R2 = 0.53). Mean age and ossification increased consistently as PI increased. Cattle with two PI were the youngest (771 ± 51.7 DOA) and least physiologically mature (107.3 ± 56.3 BO), whereas those with eight PI were the oldest animals (1735 DOA ± 511.6) and the most ossified (310.7 ± 110.1). Notably, 31.3% of cattle exhibiting three or four PI were younger than 913 DOA (30 months), demonstrating misclassification risk when dentition is used to determine regulatory age status. In this cattle population, the third PI erupted at approximately 843 DOA, and by 913 DOA, these cattle exhibited an average of approximately 3.73 PI. Revising age-dentition relationships established nearly half-a-century ago could improve current industry and regulatory decision-making regarding age verification, SRM protocols, and carcass maturity assessment.