Progesterone (P4) is a cholesterol-derived hormone essential for regulating the uterine environment, supporting conceptus development, and maintaining pregnancy in cattle (Wiltbank et al., 2014; Couto et al., 2019). Its key functions include follicular modulation, early embryonic support, suppression of estrus and ovulation through negative hypothalamic feedback, and the preparation and maintenance of the endometrium during gestation (Clemente et al., 2009; Lonergan et al., 2016). P4 is synthesized by luteal cells of the corpus luteum and plays a central role regulating ovarian function and establishing pregnancy through its effects on gonadotropin secretion and uterine function (Rigoglio et al., 2013; Wiltbank et al., 2014; Graaff and Grimard, 2018).

In beef cows, although fertilization rates approach 90%, embryonic losses during the first three weeks of gestation may reach 30-40% (Martins et al., 2019a). Exogenous increases in P4 during early diestrus enhance conceptus elongation and interferon-τ production, which are essential for maternal recognition of pregnancy (Wiltbank et al., 2014; Couto et al., 2019; Martins et al., 2019b). Conversely, insufficient P4 during this period compromises embryonic survival and overall reproductive success (Wiltbank et al., 2014). However, it is important to note that most studies demonstrating these effects were conducted in dairy cattle, whose metabolic and endocrine characteristics differ from those of beef cattle.

Several progesterone supplementation strategies have been evaluated, including intravaginal devices (CIDR^®), subcutaneous implants (Crestar^®), long-acting intramuscular injections (P4LA^®), and oral progestins such as melengestrol acetate (MGA^®), all of which have shown the potential to improve pregnancy rates and reduce pregnancy losses (Carter et al., 2008; Pugliesi et al., 2016; Yan et al., 2016; Couto et al., 2019, 2022; Martins et al., 2019a). However, data regarding the economic impact of these interventions in Nellore herds remain scarce.

Progesterone regulates endometrial receptivity, conceptus elongation, and early embryo-maternal interactions during the early stages of gestation (Clemente et al., 2009; Wiltbank et al., 2014; Lonergan et al., 2016). There is evidence that elevated progesterone concentrations during early diestrus can improve embryonic development and enhance uterine support (Carter et al., 2008; Martins et al., 2019b). Since early embryonic and placental development is closely linked to subsequent fetal growth, progesterone supplementation after timed artificial insemination (TAI) may not only favor pregnancy establishment but also contribute to improved subsequent calf performance.

Given this gap, we hypothesized that P4LA supplementation in Nellore cows subjected to TAI would improve conception rates, reduce pregnancy losses, and enhance economic outcomes. Therefore, the objectives of this study were to: i) evaluate the effects of P4LA on conception rates and pregnancy losses; ii) assess its impact on the cost per pregnancy; and iii) determine its influence on calf weaning weight.

Cows receiving P4LA had higher conception rates than the control group, both at day 30 (58.3% vs. 50.0%; P = 0.04) and day 60 post-TAI (52.7% vs. 42.3%; P = 0.02; Table 2). A tendency toward reduced pregnancy loss was also observed in the supplemented group (9.4% vs. 15.4%; P = 0.07; Table 2).

Table 2

Parameter	P4LA	Control	P-value
Conception rate at 30 days	58.3% (127/218)	50.0% (117/234)	0.04
Conception rate at 60 days	52.7% (115/218)	42.3% (99/234)	0.02
Pregnancy loss	9.4% (12/127)	15.4% (18/117)	0.07

P4LA - long-acting injectable progesterone.

The CPC was lower in P4LA-supplemented cows, being 10% lower at 30 days and 18% lower at 60 days than in control cows, indicating that achieving pregnancy was more cost-effective when P4LA was used (Table 3).

Table 3

Parameter	P4LA	Control
CPC at 30 days (US$)	26.94	29.65
CPC at 60 days (US$)	29.65	35.08

P4LA - long-acting injectable progesterone.

Calves from supplemented cows had a higher weaning weight (215 vs. 203 kg; P = 0.01). Consequently, productivity per treated cow was higher in the P4LA group (95.7 vs. 72.2 kg of weaned calf), leading to greater gross income per cow (US$ 213 vs. US$ 160; Table 4). Considering conception costs, the net profitability from calf sales was 46% higher in the P4LA group, demonstrating the economic feasibility of this supplementation strategy.

Table 4

Parameter	P4LA	Control
Kg of weaned calf per cow	95.7	72.2
Gross income per cow (US$)	213	160
Net profitability (US$)	183.63	125.83
Total net profitability (US$)	40,031.34	29,444.22

P4LA - long-acting injectable progesterone.

The partial budget analysis indicated that progesterone supplementation (P4LA) resulted in an additional 23.5 kg of weaned calf per exposed cow, corresponding to an increased return of US$ 52.38/cow (at a mean selling price of US$ 2.23/kg; Table 5).

Table 5

Category	Item	Value (US$/cow)
Increased returns	23.5 kg additional calf × US$ 2.23/kg	+52.38
Decreased costs	–	0
Increased costs	P4LA	–0.88
Decreased returns	–	0
Net benefit		+51.50

P4LA - long-acting injectable progesterone.

The hypothesis that P4LA supplementation in Nellore cows subjected to TAI would improve conception rates, reduce pregnancy losses, reduce cost per conception, and increase the kilograms of weaned calf produced per treated cow was partially supported by the results, as pregnancy loss showed only a statistical trend. Although the influence of progesterone on reproductive processes in cattle has been extensively investigated, few studies have simultaneously evaluated its biological and economic impacts within an integrated production context. Therefore, our findings contribute new evidence regarding both reproductive efficiency and system-level productivity in Bos indicus herds.

The P4LA administration on Day 7 after TAI increased conception rates at both early (D30) and late (D60) pregnancy diagnoses, while pregnancy loss showed only a statistical trend toward reduction. This pattern suggests that the primary effect of P4LA supplementation was associated with improved pregnancy establishment rather than a clearly demonstrated enhancement of subsequent embryonic maintenance. Progesterone plays a central role in promoting uterine receptivity and supporting early embryonic development by modulating endometrial secretory activity and embryo-maternal signaling (Robinson et al., 2006). Therefore, increased luteal support during the early post-insemination period may have optimized the uterine environment during the critical window of maternal recognition of pregnancy. Although the numerical reduction in pregnancy loss is biologically plausible, given that insufficient luteal function has been linked to early embryonic mortality, this effect was not statistically significant in the present study. Similar improvements in pregnancy outcomes following progesterone supplementation have been reported in Bos indicus females receiving embryos (Neto et al., 2024), reinforcing the importance of adequate progesterone exposure during early gestation. However, environmental, nutritional, and cow-related factors may modulate luteal function and embryonic survival, potentially explaining the modest magnitude of the observed effect on pregnancy maintenance.

The economic analysis demonstrated that P4LA supplementation reduced cost per conception at both pregnancy diagnosis time points, reflecting the biological advantage of higher pregnancy rates. This reduction indicates improved reproductive efficiency because total reproductive costs were distributed across a greater number of successful pregnancies. In addition, the partial budget analysis evaluated the incremental economic impact of supplementation, showing that supplemented cows weaned an additional 23.5 kg of calf per exposed cow, corresponding to an increased return of US$ 52.38/cow at prevailing market prices. After accounting for the additional supplementation cost of US$ 0.88/cow, the net incremental benefit reached US$ 51.50 per treated cow. Together, these analyses indicate that P4LA supplementation improved reproductive efficiency while generating a positive economic return.

The improvement in offspring performance further highlights the system-wide productivity gains associated with enhanced reproductive success. Calves from supplemented cows were heavier at weaning, and the total kilograms of weaned calf produced per treated cow were substantially greater in the P4LA group. This response may be associated with enhanced embryonic and placental development, likely due to increased progesterone concentrations during early gestation. Progesterone plays a central role in regulating endometrial function and conceptus elongation during the early luteal phase (Wiltbank et al., 2014; Lonergan et al., 2016) by promoting a uterine environment that supports placental establishment and fetal development. Although placental development and fetal programming were not directly assessed in this study, improved uterine support during this critical period may have contributed to the differences in calf growth. Calves from both treatment groups were born during the same calving season, making it unlikely that seasonal factors influenced the observed differences in weaning weight.

These findings have broader implications for the profitability and sustainability of beef production systems. Improved reproductive efficiency increases calf crop percentage, reduces reproductive waste, and enhances uniformity of weaning groups. Additionally, by increasing the kilograms of weaned calf produced per cow, overall production efficiency is enhanced, potentially improving productivity per unit of output (Baruselli et al., 2025). Thus, P4LA supplementation represents a management strategy capable of simultaneously improving biological performance and overall production efficiency under commercial conditions.

Some limitations should be acknowledged. The study was conducted in a single commercial herd, and progesterone concentrations were not directly measured, which limits the ability to fully characterize the endocrine responses underlying the observed effects. Future studies should validate these findings across different environments, genetic groups, and management systems while incorporating broader economic analyses, including feed, health, and environmental components, to more comprehensively assess long-term system sustainability.

In conclusion, supplementation with long-acting progesterone on Day 7 after TAI improved reproductive performance and increased the kilograms of weaned calf produced per cow, resulting in higher economic returns in Nellore beef cows. Although P4LA increased the cost per treated cow, the improvement in pregnancy outcomes reduced the cost per conception and generated a positive net return under the conditions of this study.