Abstract
BackgroundArtificial insemination is a preferred breeding method for beef heifers as it advances the genetic background, produces a predictive and profitable calving season, and extends the heifer’s reproductive life span. As reproductive efficiency in heifers is key for the success of beef cattle production systems, following artificial insemination, heifers are exposed to a bull for the remainder of the breeding season. Altogether, up to 95% of heifers might become pregnant in their first breeding season. Heifers that do not become pregnant at the end of the breeding season represent an irreparable economical loss. Additionally, heifers conceiving late in the breeding season to natural service, although acceptable, poses serious losses to producers. To minimize losses due to reproductive failure, different phenotypic parameters can be assessed and utilized as selection tools. Here, we tested the hypothesis that in a group of pre-selected heifers, records of weaning weight, age at weaning, age at artificial insemination, and age of dam differ among heifers of varied reproductive outcomes during the first breeding season.ResultsNone of the parameters tested presented predictive ability to discriminate the heifers based on the response variable (‘pregnant to artificial insemination’, ‘pregnant to natural service’, ‘not pregnant’). Heifers categorized with body condition score = 6 and reproductive tract score ≥ 4 had the greatest proportion of pregnancy to artificial insemination (49% and 44%, respectively). Furthermore, it was notable that heifers presenting body condition score = 6 and reproductive tract score = 5 presented the greatest pregnancy rate at end of the breeding season (89%). Heifers younger than 368 d at the start of the breeding season did not become pregnant to artificial insemination. Those young heifers had 12.5% chance to become pregnant in their first breeding season, compared to 87.5% if the heifers were older than 368 days.ConclusionOur results suggest that beef heifers with body condition score = 6 and reproductive tract score ≥ 4 are more likely to become pregnant to artificial insemination. Careful assessment should be undertaken when developing replacement heifers that will not reach 12 months of age by the beginning of the breeding season.
Highlights
Artificial insemination is a preferred breeding method for beef heifers as it advances the genetic background, produces a predictive and profitable calving season, and extends the heifer’s reproductive life span
935 records indicated a weaning weight of 278.0 ± 35.5 kg, an adjusted weaning weight of 266.2 ± 30.4 kg, and an age at weaning of 227.2 ± 32.6 d for all heifers The 259 records obtained for heifers with pregnancy data demonstrated a weaning weight of 294.7 ± 38.9 kg, an adjusted weaning weight of 278.7 ± 26.6 kg, and an age at weaning of 229.2 ± 34.3 d (Table 1, Additional file 1: Figure S2)
At the time of artificial insemination (AI), the heifers included in our pregnancy outcome analysis averaged 418.7 ± 22.6 days of age (Table 1, Additional file 1: Figure S2)
Summary
Artificial insemination is a preferred breeding method for beef heifers as it advances the genetic background, produces a predictive and profitable calving season, and extends the heifer’s reproductive life span. Pregnancy rates range from 53% to 95% under natural service [2,3,4,5,6,7,8,9,10] and are reduced to 48% to 69% when artificial insemination (AI) is the only breeding strategy utilized [2,3,4,5,6,7,8,9, 11, 12]. The selection and management of replacement heifers to obtain greater reproductive success within their first breeding season is of great importance to beef cattle production systems [14]. Producers can implement a progestin based estrous synchronization protocol [9, 10] for induction of cyclicity in peripubertal heifers
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