AAVS_13_11_2498-2507

INTRODUCTION

Reproductive efficiency is a cornerstone of profitability in beef cattle systems across Indonesia, where seasonal fluctuations and structural constraints in feed supply frequently limit animal performance (Agus and Mastuti-Widi, 2018). In many smallholder settings, cattle are maintained on crop residues and other low-quality forages that rarely meet the nutritional requirements for maintenance and reproduction, predisposing herds to extended calving intervals and reduced pregnancy rates (Hariyono et al., 2025; Indrawirawan et al., 2022; Warman et al., 2023). Among available feeds, rice straw is an abundant by-product of rice-based cropping systems that dominate the Indonesian landscape, and it remains a common basal roughage despite its poor nutritive value (Shah et al., 2025; Tiro et al., 2023). High lignin and silica contents, low nitrogen concentration, and limited digestibility suppress voluntary intake, constraining energy and protein supply and thereby compromising metabolic status and reproductive function (Hoerbe et al., 2020; Sonjaya et al., 2020; Wahyono et al., 2021).

Multiple interventions have been proposed to improve the use of rice straw, including chemical treatment with urea and strategic protein supplementation (Sarnklong et al., 2010). For many smallholders, the most feasible and scalable approach is to provide protein-based concentrates alongside rice straw to raise dietary nitrogen and fermentable substrates (Abo-Donia et al., 2022; Puastuti et al., 2024). Protein supplementation has consistently increased intake, enhanced digestibility, and improved production metrics even when the basal forage is of low quality (Quang et al., 2015). At the same time, higher dietary protein can elevate urea nitrogen, and its association with fertility is context dependent, underscoring the need to evaluate protein strategies alongside energy balance and mineral status under practical field conditions (Grussing and Meyer, 2016; Rajala-Schultz et al., 2001).

Beyond macronutrients, trace minerals are indispensable to reproductive physiology because they support antioxidant defenses, endocrine regulation, and immune competence (Anam et al., 2021). Selenium is particularly important due to its roles in selenoprotein-mediated redox control, thyroid hormone metabolism, and cellular immunity, all of which bear on ovarian function and conceptus survival (Mehdi and Dufrasne, 2016; Ullah et al., 2020). Selenium uptake can be impaired by dietary nitrates, sulfates, and antinutritional factors, which increases the risk of deficiency in tropical systems that rely on crop residues (Cámara-Martos, 2024). Low selenium status has been associated with reproductive disorders that compromise fertility and herd-level efficiency (Mehdi and Dufrasne, 2016). Empirical studies have reported that selenium supplementation can reduce postpartum disorders and improve conception, with several studies suggesting that organic sources offer advantages due to their superior bioavailability compared to inorganic salts (Khalili et al., 2019; Wang et al., 2025). Reviews further document the positive effects of selenium on immune function and feed utilization, which may indirectly support reproduction when animals are managed on low-quality forages (Saeed et al., 2024; Ullah et al., 2020).

Despite the recognized importance of both dietary protein–energy balance and selenium in cattle reproduction, few studies have evaluated their combined supplementation strategy under rice straw-based feeding systems typical of tropical regions. Therefore, this study investigated the effects of supplementing a selenium-enriched high-protein concentrate providing both improved nutritional quality and selenium fortification on blood metabolites, hematological indices, reproductive hormones, and pregnancy rate in beef cows. The findings highlight the synergistic relationship between enhanced protein–energy nutrition and selenium enrichment, offering practical implications for improving reproductive efficiency and productivity in smallholder beef cattle systems.

MATERIALS AND METHODS

Experimental design, animals, and diet

The experiment was conducted at Pasir Tengah Farm, West Java, Indonesia, under tropical environmental conditions. All procedures involving animals were reviewed and approved by the Institutional Animal Care and Use Committee of Universitas Gadjah Mada. A total of 60 multiparous Brahman crossbred cows were used in this experiment. The animals were randomly assigned into two dietary treatment groups, each consisting of 30 cows and considered as replicates. Two dietary treatments were implemented as follows: Control (CON): ad libitum rice straw plus 3 kg/head/day of basal concentrate; Supplemented (SUP): ad libitum rice straw plus 2 kg/head/day of basal concentrate, 1 kg/head/day high-protein concentrate enriched with organic selenium (0.30 ppm). The organic selenium used was in the form of selenomethionine-chelated amino acid.

Prior to the initiation of the feeding trial, all cows were evaluated for body weight, age, and body condition score (BCS) to ensure homogeneity among treatment groups. The average body weight, age, and BCS were 384.53 ± 57.44 kg, 3.6 ± 0.4 years, and 2.75 ± 0.23 for the CON group, and 404.30 ± 65.79 kg, 3.5 ± 0.5 years, and 2.78 ± 0.21 for the SUP group, respectively. The BCS was assessed using a 1–5 scale, where 1 represented an emaciated animal and 5 represented an obese one. These results indicate that both groups were comparable in terms of energy reserves and physiological status at the beginning of the experiment, confirming that the randomization process successfully produced balanced treatment groups.

The basal concentrate was commercially sourced from PT Widodo Makmur Perkasa, Indonesia. The high-protein concentrate supplement consisted of corn grain, corn gluten meal, pollard, soybean meal, and wheat flour, and was manufactured by PT Agromix Lestari, Indonesia. Selenium was added as selenomethionine and incorporated by uniform mixing to ensure consistent intake across animals. All diets were provided on an as-fed basis, and the detailed chemical composition of the feeds is presented in Table 1. Fresh, clean water was provided ad libitum throughout the experimental period. The trial lasted four months, coinciding with the period required for estrus synchronization, insemination, and pregnancy diagnosis.

Table 1: Chemical composition of the basal concentrate, high-protein concentrate, and rice straw.

Items1 (%)	Rice straw	Basal concentrate	High-protein concentrate
Dry matter	61.76	87.50	88.79
Organic matter	76.65	91.63	93.06
Crude protein	5.25	12.46	19.42
Ether extract	0.75	3.24	4.57
Crude fiber	28.36	23.87	17.55
NFE	42.29	52.06	51.52
TDN	43.75	40.92	54.57

1Values expressed as a percentage of the dry matter: NFE: nitrogen-free extract; TDN: total digestible nutrient.

Estrous synchronization

Estrous synchronization was initiated during the third week of the feeding trial to align estrus cycles across cows, a procedure that improves the efficiency of artificial insemination (AI) programs (Anam et al., 2021). Prior to synchronization, transrectal palpation was performed to verify the presence and functional status of the corpus luteum. Each cow received a single intramuscular injection of 25 mg prostaglandin F2α (PGF2α; Lutalyse™, Dinoprost-T, Zoetis Inc., USA). Behavioral and physiological signs of estrus were monitored closely for up to 72 hours post-injection. Indicators of estrus included restlessness, frequent vocalizations, vulvar edema and warmth, hyperemia of vaginal mucosa, clear cervical mucus discharge, and mounting behavior. These signs were recorded to ensure accurate estrus detection, which is critical for the timing of insemination.

Artificial insemination and pregnancy diagnosis

Cows exhibiting estrus signs were inseminated with frozen–thawed semen from Belgian Blue bulls (BBG Scrl, Ciney, Belgium). Semen straws were thawed in sterile water at 37°C for 30 seconds prior to use, in accordance with international standards for AI protocols (Nisa et al., 2022). Insemination was carried out 10–12 hours after estrus detection to maximize conception rates. Pregnancy diagnosis was achieved at 90 days post-insemination through transrectal palpation, which remains a reliable method for detecting early pregnancy in cattle (Leigh et al., 2019). Pregnancy rate was expressed as the percentage of inseminated cows confirmed pregnant in each group.

Dietary sampling and blood collection

Feed intake for each group (rice straw and concentrate) was recorded daily to monitor diet adherence and nutrient supply. Representative feed samples were collected every three weeks for chemical analysis. All feed samples were analyzed for proximate composition following AOAC (2005) procedures to determine dry matter (DM), crude protein (CP), crude fiber (CF), ether extract (EE), and organic matter (OM) contents.

Blood samples (10 ml) were collected from the caudal vein of each cow in the morning, prior to feeding, at 90 days post-AI to ensure a baseline metabolic state for hormonal and biochemical analyses (Anam et al., 2021). Samples were obtained using sterile BD Vacutainer® needles and immediately transferred into serum separator tubes. After clotting at room temperature for 15 minutes, the tubes were centrifuged at 2000 rpm for 20 minutes, and the serum was stored at −20 °C pending analysis. Serum metabolites, including total protein, cholesterol, triglycerides, creatinine, uric acid, and blood urea nitrogen (BUN), as well as macrominerals (calcium and phosphorus), were analyzed using DiaSys Diagnostic Systems (Holzheim, Germany). Selenium concentrations in serum was quantified using an Atomic Absorption Spectrophotometer (AAS).

For hematological analysis, 3 ml of whole blood was collected into K3 EDTA tubes (Valucab®, OneMed, Indonesia) and maintained at ~4 °C until analysis within 24 hours. Hematological parameters measured included red blood cell (RBC), haemoglobin concentration, hematocrit, and white blood cell (WBC) counts, mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), and leukocyte differentials (granulocytes, lymphocytes, monocytes). Hormonal analyses were conducted using bovine-specific enzyme-linked immunosorbent assay (ELISA) kits, specifically progesterone (DRG EIA-1561, DRG International, USA) and estradiol (DRG EIA-2693, DRG International, USA), to evaluate reproductive endocrinology and correlate with pregnancy outcomes.

Statistical analysis

All data were statistically analyzed using IBM SPSS Statistics version 26 (IBM Corp., Armonk, NY, USA). The Shapiro–Wilk test was employed to verify normality of data distribution, consistent with recommendations for small to medium sample sizes. Differences between treatment groups (CON vs. SUP) were assessed using independent-samples t-tests. Statistical significance was declared at P < 0.05. Effect size was calculated using Cohen’s d to provide a standardized measure of treatment impact, categorized as small (≥0.2), medium (≥0.5), or large (≥0.8) (Cohen, 2013).

RESULTS AND DISCUSSION

Nutrient intake

The analysis of nutrient intake demonstrated that supplementation with selenium-enriched high-protein concentrate influenced nutrient consumption patterns in Brahman crossbred cows, despite the absence of significant effects on DM and OM intake (P>0.05). As presented in Table 2, the average DM intake did not differ significantly

Table 2: Nutrient intake (kg/head/day) of Brahman crossbred cows fed a rice straw-based diet with or without selenium-enriched high-protein concentrate supplementation.

Nutrient intake	Treatment		SEM	P-value	95% Confidence Interval		Effect size
Nutrient intake	CON	SUP	SEM	P-value	Lower	Upper	Effect size
Dry matter	17.43	17.31	0.15	0.691	-0.730	0.485	-0.122
Organic matter	13.76	13.66	0.12	0.690	-0.560	0.372	-0.094
Crude protein	2.49b	2.98a	0.03	<0.001	0.395	0.590	0.493
Crude fiber	7.48a	6.35b	0.08	<0.001	-1.406	-0.869	-1.137
Ether extract	0.56b	0.58a	0.01	0.013	0.005	0.046	0.026
NFE	13.05	13.39	0.12	0.173	-0.151	0.832	0.341
TDN	12.11b	14.28a	0.15	<0.001	1.683	2.667	2.175

abSuperscript letters on the same row denote statistically significant differences at a significance level of P<0.05. CON: control; SUP: supplemented. NFE: nitrogen-free extract; TDN: total digestible nutrient. SEM: standard error of the mean.

between treatments, indicating that total forage consumption remained relatively constant across groups. This finding aligns with previous observations in ruminants fed basal diets of poor-quality roughages, where overall DM intake tends to plateau due to physical limitations imposed by high-fiber forages rather than by concentrate supplementation (Buxton and Redfearn, 1997; Azmi et al., 2021). Similarly, OM intake also remained unchanged, suggesting that the bulk intake capacity of cows fed ad libitum rice straw may not be easily altered by nutrient supplementation.

Although DM and OM intakes were unaffected, marked differences were recorded in nutrient partitioning. CP intake was significantly higher in the SUP group compared to the CON (2.98 vs. 2.49 kg/day; P<0.001). This increase directly reflects the additional 1 kg/day of high-protein concentrate provided in the SUP diet, demonstrating that supplementation strategies can successfully enhance protein supply without suppressing basal forage intake. Increased CP intake is critical under tropical feeding systems, where rice straw is notoriously deficient in nitrogen (Sarnklong et al., 2010). Inadequate protein supply limits microbial growth and rumen fermentation, which in turn restricts feed digestibility and animal productivity (Palmonari et al., 2024). By providing additional rumen-degradable protein and amino acids, supplementation can stimulate microbial protein synthesis and improve overall energy utilization (Quang et al., 2015).

Interestingly, CF intake was significantly lower in SUP cows (6.35 vs. 7.48 kg/day; P<0.001), reflecting a substitution effect whereby cows consuming more protein concentrate relied less on fibrous rice straw. This reduction in CF intake may improve digestive efficiency, as excessive fiber intake often reduces digestibility due to high lignin and silica content of rice straw (Oladosu et al., 2016). Similar substitution effects have been documented when cattle are offered concentrate supplements alongside poor-quality roughage, with higher concentrate intake displacing fibrous components in the diet (Norrapoke and Pongjongmit, 2025). From a physiological perspective, lower fiber intake combined with improved protein supply may enhance the rumen microbial ecology, particularly that of cellulolytic and amylolytic bacteria, resulting in more efficient digestion and fermentation (Hackmann and Firkins, 2015).

EE intake was also slightly but significantly higher in SUP animals (P<0.05). Although the absolute difference was small, increased dietary fat intake can contribute to a higher energy density, which is particularly valuable in supporting reproductive functions that require an elevated energy supply (Palmquist and Jenkins, 2017). Importantly, TDN intake was significantly greater in SUP cows (P<0.001). This improvement is crucial because reproductive success is closely linked to the availability of digestible energy (Cardoso et al., 2020).

Blood profile

The hematological profile presented in Table 3 revealed several important differences between treatment groups. While total WBC counts were not significantly altered (P>0.05), lymphocyte counts were significantly higher in SUP cows (P<0.05). The rise in lymphocyte proportion likely reflects improved nutritional status and immune competence supported by the higher protein and energy intake in the SUP group. Adequate protein nutrition enhances leukocyte proliferation and antibody synthesis, while selenium contributes indirectly by protecting immune cells from oxidative injury through its role in selenoenzyme systems such as glutathione peroxidase (Avery and Hoffmann, 2018; Mehdi and Dufrasne, 2016). This synergistic effect between improved nutrient intake and selenium-mediated antioxidant defense may explain the increased lymphocyte counts observed, consistent with reports that combined nutritional and antioxidant enhancement improves immune responsiveness in ruminants (Zheng et al., 2022).

Table 3: Hematological parameters of Brahman crossbred cows fed a rice straw-based diet with or without selenium-enriched high-protein concentrate supplementation.

Items	Treatment		SEM	P-value	95% Confidence Interval		Effect Size
Items	CON	SUP	SEM	P-value	Lower	Upper	Effect Size
WBC, 103/µl	4.40	6.55	0.68	0.117	-0.611	4.911	2.150
Lymphocytes, 103/µl	1.86b	3.05a	0.29	0.035	0.099	2.276	1.188
Monocytes, 103/µl	0.38	0.50	0.06	0.306	-0.127	0.377	0.125
Granulocyte, 103/µl	2.05	3.23	0.43	0.179	-0.606	2.956	1.175
As % of the total
Lymphocytes, %	47.78	46.30	2.60	0.787	-12.987	10.037	-1.475
Monocytes, %	8.59	7.89	0.35	0.336	-2.206	0.806	-0.700
Granulocyte, %	43.64	39.91	3.48	0.610	-19.023	11.573	-3.725
RBC, 106/µl	4.80b	6.25a	0.30	0.010	0.404	2.488	1.446
Haemoglobin, g/dl	6.85b	9.05a	0.43	0.006	0.749	3.651	2.200
PLT, 103/µl	284.63	319.00	74.14	0.826	-294.217	362.967	34.375
Hematocrit, %	21.88b	28.54a	1.41	0.012	1.714	11.611	6.663
MCV, fL	45.38	45.75	1.37	0.897	-5.702	6.452	0.375
MCH, pg	14.35	14.51	0.38	0.839	-1.519	1.844	0.163
MCHC, g/dl	31.83	31.83	0.26	1.000	-1.142	1.142	0.000

abSuperscript letters on the same row denote statistically significant differences at a significance level of P<0.05. CON: control; SUP: supplemented. WBC: white blood cell; RBC: red blood cell; PLT: platelets; MCV: mean corpuscular volume; MCH: mean corpuscular hemoglobin; MCHC: mean corpuscular hemoglobin concentration. SEM: standard error of the mean.

RBC indices also differed significantly between treatments. RBC counts were higher in SUP cows (6.25 vs. 4.80 ×10⁶/µl), accompanied by greater haemoglobin concentration (9.05 vs. 6.85 g/dl) and hematocrit (28.54 vs. 21.88%) (P<0.05). These findings indicate enhanced erythropoietic activity and oxygen-carrying capacity as a result of improved dietary protein and energy supply, which provide the essential substrates for haemoglobin synthesis and RBC formation (Revskij et al., 2019; Schei et al., 2005). Selenium may have played a complementary role by maintaining erythrocyte membrane integrity and protecting newly formed cells from oxidative hemolysis rather than directly stimulating their production (Mehdi and Dufrasne, 2016; Shahidin et al., 2025). Improved erythropoiesis and antioxidant stability together enhance tissue oxygenation, which supports reproductive processes such as follicular development, luteal function, and embryonic survival (Hagita et al., 2025; Zivot et al., 2018).

Serum biochemical and mineral profiles, presented in Table 4, show that most metabolites including triglycerides, cholesterol, total protein, uric acid, creatinine, calcium, and phosphorus did not differ significantly between groups (P>0.05). However, two critical differences were observed. The supplemented cows exhibited a higher circulating BUN than controls (P<0.05); (Table 4). This outcome is biologically consistent with the diet composition, as the SUP group consumed more CP and TDN, which would elevate ruminal ammonia production and hepatic urea synthesis (Xia et al., 2018). Under rice-straw based systems that are typically nitrogen-deficient, a moderate rise in BUN following protein supplementation generally indicates greater nitrogen turnover and urea recycling (Gunun et al., 2013). Previous studies have reported that higher dietary protein levels increase BUN concentrations, as observed in Hanwoo steers and swamp buffaloes (Chanthakhoun et al., 2012; Oh et al., 2024). However, BUN is not solely determined by dietary protein intake; individual beef cows may exhibit inherent regulatory capacity to maintain relatively stable BUN concentrations despite fluctuations in nitrogen supply, suggesting animal-specific adaptation in nitrogen metabolism (Tshuma et al., 2019).

From a reproductive standpoint, interpretation of elevated BUN must be made cautiously. In dairy cattle, excessive urea concentrations have been associated with alterations in the uterine environment, reduced embryo survival, and decreased conception rates when dietary energy is inadequate or when protein–energy synchrony is poor (Butler, 2003; Elrod and Butler, 1993; Santos et al., 2009). In beef cattle, however, the relationship is less consistent, and moderate BUN elevations under adequate energy supply do not necessarily impair fertility (Rajala-Schultz et al., 2001; Santos et al., 2009). The BUN concentration observed in the present study (18.8 mg/dl) is close to the upper physiological range reported for beef cows when sufficient dietary energy supports efficient ammonia utilization (Kohn et al., 2005; Reynolds and Kristensen, 2008).

Table 4: Serum biochemical and mineral profile of Brahman crossbred cows fed a rice straw-based diet with or without selenium-enriched high-protein concentrate supplementation.

Items	Treatment		SEM	P-value	95% Confidence interval		Effect size
Items	CON	SUP	SEM	P-value	Lower	Upper	Effect size
Triglycerides, mg/dl	90.87	74.58	15.20	0.616	-86.369	53.803	-16.283
Cholesterol, mg/dl	289.18	301.52	16.04	0.720	-62.107	86.773	12.333
Total protein, g/dl	8.11	8.42	0.49	0.763	-1.947	2.577	0.315
BUN, mg/dl	15.56b	18.80a	0.77	0.026	0.485	5.998	3.242
Uric acid, mg/dl	6.11	5.19	1.45	0.768	-7.657	5.820	-0.918
Creatinine, mg/dl	2.36	2.73	0.33	0.601	-1.166	1.913	0.373
Calcium, mg/dl	11.20	10.85	0.40	0.677	-2.198	1.488	-0.355
Phosphor, mg/dl	3.53	3.31	0.23	0.658	-1.284	0.847	-0.218
Selenium (mcg/100ml)	6.63b	9.48a	0.69	0.030	0.332	5.402	2.867

abSuperscript letters on the same row denote statistically significant differences at a significance level of P<0.05. CON: control; SUP: supplemented. BUN: blood urea nitrogen. SEM: standard error of the mean.

Table 5: Reproductive hormones of Brahman crossbred cows fed a rice straw-based diet with or without selenium-enriched high-protein concentrate supplementation.

Items	Treatment		SEM	P-value	95% Confidence interval		Effect size
Items	CON	SUP	SEM	P-value	Lower	Upper	Effect size
Estrogen, pg/ml	75.08	77.19	11.00	0.929	-49.299	53.515	2.108
Progesterone, ng/ml	4.57b	10.76a	1.25	0.006	2.268	10.105	6.187

abSuperscript letters on the same row denote statistically significant differences at a significance level of P<0.05. CON: control; SUP: supplemented. SEM: standard error of the mean.

Although the degradability characteristics of the protein sources were not determined, the high-protein concentrate likely contained both rumen-degradable and undegradable fractions. Hence, part of the BUN elevation could also reflect incomplete nitrogen–energy synchrony rather than being directly caused by excessive protein intake, underscoring the need for future studies to quantify protein degradability and ruminal ammonia dynamics.

Serum selenium concentrations were significantly higher in SUP cows (P<0.05). This confirms that selenium in the high-protein concentrate was bioavailable and effectively absorbed, consistent with prior reports indicating efficient selenium transfer into blood following dietary supplementation (Ullah et al., 2020). Elevated selenium status is directly linked to improved antioxidant capacity, reduced oxidative damage in reproductive tissues, and enhanced luteal activity (Anam et al., 2024; Surai et al., 2019).

Hormonal profile and pregnancy rate

The reproductive hormone outcomes are summarized in Table 5. Serum estrogen concentrations did not differ significantly between the control and supplemented groups (P>0.05), indicating that selenium-enriched high-protein supplementation did not measurably alter ovarian estrogenic activity during the study period. In cattle, circulating estradiol is largely determined by the stage of follicular development and granulosa-cell aromatase activity (Beg and Ginther, 2006). Nutritional inputs typically act indirectly through metabolic–endocrine pathways and over several days, so short-term dietary changes do not necessarily produce marked shifts in estradiol output (Scaramuzzi et al., 2006).

By contrast, serum progesterone concentrations were significantly elevated in SUP cows (P<0.05). Progesterone is indispensable for establishing and maintaining pregnancy, as it regulates endometrial receptivity, supports embryo implantation, and suppresses estrous cycles (Spencer et al., 2016). The higher progesterone levels observed in supplemented cows suggest enhanced corpus luteum (CL) function. Selenium may contribute to CL activity by reducing oxidative stress in luteal cells, which are highly susceptible to reactive oxygen species during steroidogenesis (Kamada et al., 2014). Moreover, improved protein and energy status, as reflected by higher CP and TDN intakes, likely provided the metabolic precursors necessary for progesterone synthesis. Similar findings have been reported in beef cattle where adequate nutrition and selenium supplementation enhanced luteal function and progesterone output (Mehdi and Dufrasne, 2016; Palomares et al., 2024).

This response is likely multifactorial. Improved protein and energy nutrition may have increased the supply of metabolic precursors (e.g., cholesterol and amino acids) for steroidogenesis, while selenium may have supported luteal cell integrity through its antioxidant role in protecting steroidogenic tissues from oxidative stress (Kamada et al., 2014; Mehdi and Dufrasne, 2016). Although no direct biomarkers of oxidative stress (e.g., glutathione peroxidase or lipid peroxides) were measured in this study, the observed pattern is consistent with previous reports linking adequate selenium and energy balance to improved luteal function and progesterone synthesis in beef cows (Palomares et al., 2024).

Blood sampling in the present study was conducted at 90 days post-insemination, coinciding with pregnancy diagnosis. Therefore, the higher progesterone levels in the SUP group may partly reflect a greater proportion of pregnant cows at the time of sampling, given that functional corpora lutea persist and actively secrete progesterone during gestation. Nonetheless, improved nutrient intake and selenium supplementation likely enhanced luteal function in both pregnant and non-pregnant animals, contributing to the overall elevation in circulating progesterone. This interpretation aligns with the concurrent increase in pregnancy rate (46.67% vs. 10.00%) (Figure 1), suggesting that better metabolic and antioxidant support facilitated successful luteal development and maintenance of pregnancy. These findings underscore the multifactorial nature of reproductive responses to nutrition, where energy balance, protein supply, immune competence, and hormonal regulation interact to determine fertility outcomes (Meikle et al., 2018). Moreover, previous studies have shown that selenium supplementation reduces embryonic mortality, lowers the incidence of retained placenta, and enhances conception rates in cattle (Surai et al., 2019; Ullah et al., 2020).

The relatively low pregnancy rate observed in the control cows (10%) should be interpreted in the context of the feeding and breeding system used. Similar results have been reported in Brahman crossbred cattle under tropical conditions, particularly when maintained on rice-straw–based diets with low protein and energy density. Anam et al. (2021) observed pregnancy rates ranging from 3.3 to 20.7% in Brahman crossbreds, attributing these low values to inadequate nutrient intake. Using ultrasonography, Ervandi et al. (2020) observed several ovarian abnormalities in Brahman crossbred cows, indicating that reproductive inefficiency may involve multiple factors such as the frequent occurrence of ovarian hypofunction in crossbred females. Likewise, Sutiyono et al. (2018) documented reduced conception rates and high cases of ovarian hypofunction in Brahman crosses inseminated artificially, in contrast to higher fertility observed in Ongole or Simmental-Ongole cross cattle. These reports support the notion that low energy–protein supply, combined with breed-related reproductive constraints, is a major factor limiting conception in Brahman crossbreds. Accordingly, the marked improvement in pregnancy rate following selenium-enriched high-protein supplementation in the present study reflects nutritional restoration of reproductive function rather than a response to management deficiencies.

It is important to note that the current study did not include a “high-protein without selenium” group. Consequently, the positive responses observed cannot be unequivocally ascribed to selenium supplementation alone. The combined improvement in protein quality, amino acid profile, and selenium bioavailability likely acted synergistically to enhance metabolic and reproductive outcomes. This composite approach was chosen to mimic realistic smallholder feeding conditions, yet future factorial studies are warranted to disentangle these individual contributions.

Limitations and future directions

This study was conducted on a single farm and involved a limited number of Brahman crossbred cows; therefore, the results may not be applicable to other breeds or management systems. We only observed one breeding cycle, which means we could not assess long-term or generational effects. While selenium-enriched supplementation improved blood profiles and pregnancy rates, we did not investigate specific biological mechanisms, such as selenoprotein expression or oxidative stress markers.

Future research should investigate the effects of selenium-enriched supplementation over multiple breeding cycles and across various breeds and environments. Using molecular and genetic markers could help explain how supplementation works. Larger field trials are needed to confirm these results. It is also important to study the economic, productive, and environmental effects of selenium-enriched protein supplements in tropical cattle farming.

CONCLUSION

Adding selenium-enriched high-protein concentrate to the diets of Brahman crossbred cows fed rice straw improved their nutrient intake, blood biochemistry, hormone levels, and pregnancy rates. The cows that received the supplement had higher protein and energy intake, more red blood cells, more lymphocytes, and higher selenium in their blood. These changes led to higher progesterone levels and significantly better pregnancy rates than those of cows on the basic diet. This indicates that incorporating selenium into high-protein supplements can help alleviate the nutritional limitations of rice straw diets. Overall, selenium-enriched protein supplements could be a practical way to boost fertility and productivity in tropical beef cattle.

ACKNOWLEDGMENT

This research was supported and funded by the Program Academic Excellence (Skema C), Universitas Gadjah Mada, under Contract No. 684/UN1.P/KPT/HUKOR/2025.

NOVELTY STATEMENT

This study demonstrates, in a real farm setting, that adding selenium-enriched high-protein concentrate to rice straw-based diets improves blood health, increases selenium and progesterone levels, and results in higher pregnancy rates, without altering dry matter intake. Unlike earlier studies that examined protein or minerals separately, this research tested a combined supplement under normal farm conditions and directly linked changes in diet to hormone and fertility outcomes. These findings provide a straightforward and scalable approach to enhancing reproduction in herds fed low-quality forages.

AUTHOR’S CONTRIBUTION

MSA conceived and designed the study and methodology, led field implementation and sampling, curated the data and performed statistical analyses, prepared the original draft and visualizations, and administered the project; CH contributed to methodology development, conducted experimental and laboratory work, provided resources and facilities, validated the data, and reviewed/edited the manuscript; AA provided conceptualization and supervision, and reviewed/edited the manuscript, and all authors read and approved the final manuscript, with MSA holding primary responsibility for the final content.

Generative AI and AI-assisted technology statement

The authors declare that no Genrative AI was used in the creation of this manuscript.

Conflict of interest

The authors have declared no conflict of interest.

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