Proofs for bulls do change as they accumulate more data

Why do proofs change and does the extent of change vary depending on the type of proof for the bull? To answer these questions ICBF examined bull proofs from 1901 AI sires born over the 20-year period 1995 to 2014. A range of proof types and breeds were compared in the analysis, including proofs from young Genomically Selected (GS) bulls, proofs from bulls that were daughter proven in Ireland (DP-IRE), and proofs from bulls that were daughter proven bulls internationally (DP-Int). In addition, three different breeds were included in the analysis; the Black & White breed (i.e., Holstein & Friesian), the Jersey breed and the Red breeds (i.e., Montbelliarde, Norwegian Red etc).

Initial proofs from Spring 2015 were taken for the 1901 sires and compared against their latest proofs from Spring 2020. Results are presented in Table 1.

During this period, an additional 500k record were added to the bulls proofs, with the majority of these being added to the young GS bulls, whose number of daughter records went from zero in 2015 to 282k in 2020 (Table 1). As a result, proofs for all bulls changed, but changed relatively more for the young GS bulls. For example, the average EBI of the young GS bulls dropped by €30.3, compared to €9.6 for the bulls that were daughter proven in Ireland.

Table 1. Comparison of EBI proofs (Spring 2015 versus Spring 2020) for dairy AI sires.

Knowledge of this outcome (that GS bulls drop relatively more) could convince you to not use GS bulls in your breeding program this Spring. But would this be the correct decision? The answer to that question is a definite no, as the average EBI of the young GS bulls was still €103 ahead of B&W daughter proven internationally bulls and €157 ahead of the daughter proven in Ireland bulls, based on latest EBI proofs.
Looking at the ranking of top bulls confirms that the top bull listings (based on Spring 2020 EBI proofs) is still dominated by young GS bulls (Figure 1), with 16 of the top 20 bulls being categorised as GS bulls in Spring 2015.

Figure 1. Comparison of 2015 and 2020 EBI proofs for B&W genomic bulls, compared to all other bulls in the analysis.

These results confirm completely the outcomes from our first Mythbusters article (Mythbuster#1), which indicated that herds using a higher % of young GS bulls in their breeding program were more profitable (in terms of milk solids and female fertility performance), than herds using a higher % of daughter proven bulls or herds using stock bulls.
As a result of these two pieces of independent validation work, the breeding advice for farmers this Spring is clear and unequivocal; use teams of high EBI AI bulls equally in your breeding program this Spring. Invariably these will be young GS bulls.