Fixed-time artificial insemination (FTAI) lets a beef herd bring in the genetics of a sought-after bull without buying him, but it never gets every cow in calf. Producers therefore still need back-up bulls for natural mating, and Alastair Rayner, writing in Beef Central's weekly genetics review, argues that they should plan those bulls around a realistic insemination result rather than the best outcome.
The first step is to measure. A good final pregnancy rate can hide a disappointing AI result, so producers should count pregnancies to insemination separately from those to natural service. Because the two may be only weeks apart, that needs early pregnancy testing by an experienced operator who can age the foetus.
The count also decides how many bulls are needed. In a group of 100 females with 55% pregnant to AI, about 45 remain to be served, so using the same number of bulls as for 100 naturally joined females may provide more bull capacity than required. The difficulty is that the AI result is usually unknown when the bulls go in, and testing first lengthens joining and loses the tighter calving that FTAI offers.
Rayner points to research published in Translational Animal Science in 2021 from a Virginia Tech project led by Dr Claire Timlin. It analysed records of 14,868 females on 17 Virginia farms over 13 breeding seasons from 2010 to 2017, mostly Angus × Simmental cattle, with mature bulls that had passed breeding soundness tests entering ten days after FTAI and staying 70 days. Pregnancy to FTAI averaged 54.9%; on average there was one bull per 31 females, though only about 14 non-pregnant females per bull needed service.
Bull numbers explained little of the variation in pregnancy at first return to heat, except in autumn groups served by a single bull, where pregnancy fell as the number of open females per bull rose. The team proposed one bull per 50 females enrolled in FTAI, about 25 needing service per bull if half conceive to AI. Rayner notes this came from herd records rather than a controlled trial, and that paddock size, terrain and bull age may differ in Australian conditions.
Returns after synchronisation also bunch together, so the bull team faces a heavy short-term workload, and a sound bull can still be injured during joining. If 55% conceive to AI, a large share of the calf crop is still sired by back-up bulls, so Rayner says they should be chosen for the same breeding goals, structure, temperament, calving ease and EBVs, as the AI sire.
Alastair Rayner is strategic account manager for southern Australia with Vytelle and principal of RaynerAg.
Source: Beef Central
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