Developmental and reproductive characteristics of beef heifers classified by number of estrous cycles experienced by start of first breeding

The Impacts of Supplemental Protein during Development on Amino Acid Concentrations in the Uterus and Pregnancy Outcomes of Angus Heifers

Replacement heifer development is one of the most critical components in beef production. The composition of the ideal uterine environment could maximize fertility and reproductive efficiency. Our hypothesis was that protein supplementation would affect the uterine environment of beef heifers without inhibiting development or reproduction. To test the effects of dietary supplementation on these outcomes, a randomized complete block design with repeated measures was implemented. Angus heifers (n = 60) were blocked by body weight (BW) and randomly assigned to one of three supplemental protein treatment groups (10% (CON), 20% (P20), and 40% (P40)). Mixed model ANOVAs were used to determine whether protein supplementation treatments, time, and the interaction or protein supplementation, semen exposure, and the interaction influenced uterine luminal fluid (ULF) and pregnancy outcomes. Amino acids (AAs) were impacted (p < 0.001), specifically, the essential AAs: Arg, Iso, Leu, Val, His, Lys, Met, Phe, Trp. Protein supplementation influenced multiple AAs post-insemination: Arg (p = 0.03), CC (p = 0.05), 1-MH (p = 0.001), and Orn (p = 0.03). In conclusion, protein supplementation did not affect the reproductive development via puberty attainment or the timing of conception even with alterations in growth. However, uterine AA concentrations did change throughout development and protein supplementation influenced ULF d 14 post-insemination, which may affect the conception rates.

Attainment and maintenance of pubertal cyclicity may predict reproductive longevity in beef heifers†

We hypothesized the manner that heifers achieve puberty may indicate their future reproductive longevity. Heifers with discontinued or delayed cyclicity during puberty attainment may have irregular reproductive cycles, anovulation, and infertility in their first breeding season contributing to a shorter reproductive lifespan. Therefore, plasma progesterone (P4) was measured from weaning to breeding on 611 heifers born 2012-2017 and four pubertal classifications were identified: (1) Early; P4 ≥ 1 ng/ml < March 12 with continued cyclicity, (2) Typical; P4 ≥ 1 ng/ml ≥ March 12 with continued cyclicity, (3) Start-Stop; P4 ≥ 1 ng/ml but discontinued cyclicity, and (4) Non-Cycling; no P4 ≥ 1 ng/ml. Historical herd records indicated that 25% of heifers achieved puberty prior to March 12th in the 10 years prior to the study. Start-Stop and Non-Cycling yearling heifers were lighter indicating reduced growth and reproductive maturity traits compared with Early/Typical heifers. In addition, Non-Cycling/Start-Stop heifers were less responsive to prostaglandin F2 alpha (PGF2α) to initiate estrous behavior and ovulation to be artificially inseminated. Non-Cycling heifers had fewer reproductive tract score-5 and reduced numbers of calves born in the first 21-days-of-calving during their first breeding season. Within the Start-Stop classification, 50% of heifers reinitiated cyclicity with growth traits and reproductive parameters that were similar to heifers in the Early/Typical classification while those that remained non-cyclic were more similar to heifers in the Non-Cycling group. Thus, heifers with discontinued cyclicity or no cyclicity during puberty attainment had delayed reproductive maturity resulting in subfertility and potentially a shorter reproductive lifespan.

Beef heifer fertility: importance of management practices and technological advancements

The development of replacement heifers is at the core of cow-calf beef production systems. In 2020, the USDA, National Agricultural Statistics Service reported 5.771 million beef heifers, 500 pounds and over, are under development for cow replacement. A compilation of data from several studies indicate that between 85% and 95% of these heifers will become pregnant in their first breeding season. Several thousands of heifers being raised for replacement may not deliver a calf on their first breeding season and result in economic losses to cow-calf producers. Many management procedures have been developed to maximize the reproductive potential of beef heifers. Such approaches include, but are not limited to the following: nutritional management for controlled weight gain, identification of reproductive maturity by physiological and morphological indicators, and the implementation of an estrous synchronization program. The implementation of management strategies has important positive impact(s) on the reproductive efficiency of heifers. There are limitations, however, because some heifers deemed ready to enter their first breeding season do not become pregnant. In parallel, genetic selection for fertility-related traits in beef heifers have not promoted major genetic gains on this particular area, most likely due to low heritability of female fertility traits in cattle. Technologies such as antral follicle counting, DNA genotyping and RNA profiling are being investigated as a means to aid in the identification of heifers of low fertility potential. To date, many polymorphisms have been associated with heifer fertility, but no DNA markers have been identified across herds. Antral follicle count is an indication of the ovarian reserve and is an indicator of the reproductive health of a heifer. We have been working on the identification of transcriptome profiles in heifers associated with pregnancy outcome. Our current investigations integrating protein-coding transcript abundance and artificial intelligence have identified the potential for bloodborne transcript abundance to be used as indicators of fertility potential in beef heifers. In summary, there is an ongoing pressure for reducing costs and increasing efficiency in cow-calf production systems, and new technologies can help reduce the long-standing limitations in beef heifer fertility.

Age at puberty and pregnancy rate in beef heifer genotypes with contrasting nutritional intake from 8 to 13 months of age

The aim of this study was to examine the effect of plane of nutrition between 8 and 13 months of age on reproductive performance of heifers of early (EM; n = 154) or late (LM; n = 155) maturing beef breeds and with dairy (dairy-bred, n = 154) or beef (beef-bred, n = 155) dams. Heifers were fed to have an average daily gain (ADG) of 0.50 kg (MOD) or >1.00 kg (HI) for a 141- and 150-day indoor winter period. Subsequently, heifers grazed pasture, and a 12 week breeding programme was implemented. Compared to heifers fed the MOD intake diet, heifers fed the HI intake diet were younger (P < 0.001) and had greater bodyweights (P < 0.001) at puberty but did not have a greater 6- (P =  0.41) or 12- (P =  0.32) week pregnancy rate. Dairy-bred heifers were of a similar age (P = 0.55) but had a lesser bodyweight (P <  0.001) at puberty and had a greater 6- (P <  0.05) and 12- (P < 0.01) week pregnancy rate compared to beef-bred heifers. Compared to LM heifers, EM heifers were younger (P < 0.001), had a lesser bodyweight (P < 0.01) at puberty and had a greater 6-week (P < 0.01) but not 12-week (P =  0.96) pregnancy rate. Enhanced nutrition resulted in a younger age at puberty but had no effect on 12-week pregnancy rate. Dam but not sire breed affected 12-week pregnancy rate.

Impacts of postweaning growth rate of replacement beef heifers on their reproductive development and productivity as primiparous cows1

This experiment evaluated the effects of postweaning body weight (BW) gain of replacement beef heifers on their reproductive development and productivity as primiparous cows. Seventy-two Angus × Hereford heifers were ranked on day -6 of experiment (17 d after weaning) by age and BW (218 ± 1.6 d of age and 234 ± 3 kg of BW), and assigned to receive 1 of 3 supplementation programs from days 0 to 182: 1) no supplementation to maintain limited BW gain (LGAIN), 2) supplementation to promote moderate BW gain (MGAIN), or 3) supplementation to promote elevated BW gain (HGAIN). Heifers were maintained in 2 pastures (36 heifers/pasture, 12 heifers/treatment in each pasture) with free-choice alfalfa-grass hay, and supplements were offered individually 6 d per week. Heifer shrunk BW was recorded on days -6 and 183 for average daily gain (ADG) calculation. Blood samples were collected for puberty evaluation via plasma progesterone weekly from days 0 to 182. On day 183, heifers were combined into a single group and received the same nutritional management until the end of the experimental period (day 718). From days 183 to 253, heifers were assigned to a fixed-time artificial insemination program combined with natural service. Average daily gain from days 0 to 182 was greater (P < 0.01) in HGAIN vs. MGAIN and LGAIN (0.78, 0.60, and 0.37 kg/d, respectively; SEM = 0.02), and greater (P < 0.01) in MGAIN vs. LGAIN heifers. Puberty attainment by the beginning of the breeding season was also greater in HGAIN vs. MGAIN and LGAIN (87.5%, 62.5%, and 56.5%, respectively; SEM = 7.1) but similar (P = 0.68) between MGAIN vs. LGAIN heifers. A treatment × day interaction was detected (P < 0.01) for calving rate, as HGAIN heifers calved earlier compared with MGAIN and LGAIN heifers. Ten heifers per treatment were assessed for milk production via weigh-suckle-weigh at 56.8 ± 1.5 d postpartum, followed by milk sample collection 24 h later. No treatment differences were detected (P ≥ 0.16) for milk yield and composition. However, mRNA expression of GLUT1 in milk fat globules was less (P ≤ 0.02) in LGAIN vs. MGAIN and HGAIN heifers, and expression of GLUT8 mRNA was also less (P = 0.04) in LGAIN vs. HGAIN heifers. No treatment differences were detected (P ≥ 0.44) for offspring weaning BW. Collectively, results from this experiment indicate that HGAIN hastened the reproductive development of replacement heifers, without negatively affecting their milk productivity and offspring weaning weight as primiparous cows.