Goats can produce milk, meat, fiber, and skins, often the primary income source in low-input farming systems. In the last 50 years, the worldwide goat population has increased by over 240% (FAO, 2021). However, the goat population in Mexico has declined, primarily due to economic and climatic factors and poor reproductive efficiency (Mellado et al., 2020). Nevertheless, nowadays, Mexico ranks 24^th in milk and 19^th in meat goat production worldwide, with almost 174,314 tons of milk and 41,034 tons of meat annually (Panorama-Agropecuario, 2024). Given the growing demand for these products by an ever-increasing human population, there is a pressing need to improve the efficiency of goat farming to meet this demand and harness the associated benefits it generates.

In small ruminants, once females have reached sexual maturity, they can be successfully bred at 7-10 months of age (Panzuti et al., 2018; Thompson et al., 2021). In sheep, breeding females aged 7 to 10 months can significantly improve lifetime animal production (Kenyon et al., 2011) and farm profitability (Young et al., 2014). However, in many small ruminant production systems, a low percentage of females are bred to give birth at 12-14 months of age. For example, in Australia (5% of Merino and 20% of no-Merino ewe lambs) and in New Zealand (40% of Romney ewe lambs) are bred to lamb at 12-14 months (Kenyon et al., 2014; Kenyon & Corner-Thomas, 2022). In Africa, goats have their first parturition at 17 months (Wilson, 1989), yet the proportion of young females bred is not precise. Moreover, most indigenous goats in Ethiopia are reported to give birth between 12 and 14 months of age, depending on managing practices (Dereje et al., 2015). In Mexico, the information, to the best of our knowledge, is scarce; however, in extensive production systems in arid and semi-arid areas, doelings selected as replacements are joined together with male goats. Consequently, some native doelings are bred when their live weight is approximately 15 kg, increasing fetal losses and delaying body weight gains (Delgadillo & Martin, 2015). Consequently, goat farmers know that breeding young does is risky (Mellado et al., 2004; Kenyon et al., 2014) as young females' reproductive and maternal efficiency in their first breeding is reduced compared to adult females (Kenyon & Corner-Thomas, 2022). Live weight at breeding is essential for successfully breeding young females, as heavier females consistently are more fertile than lighter females (Rosales-Nieto et al., 2014; 2015; Thompson et al., 2021). Goat farmers also perceive that on-farm costs increase as the amount of concentrate offered increases to achieve a targeted live weight for breeding (Rosales Nieto et al., 2018). Successful breeding of young goats is challenging, especially under suboptimal feeding conditions.

Since dairy goats are usually seasonal breeders in latitudes > 35°, the precise timing of their first conception is affected by the season of birth; thus, environmental cues influence the age at first breeding (McManus et al., 2008; Espinoza-Flores et al., 2020). Therefore, attaining a certain weight and age at conception is essential to increase pregnancy rate in doelings. Indirectly, this can enhance the chance of successfully breeding young females as they age, as recent evidence indicates that age at first breeding is an essential factor for breeding success (Thompson et al., 2021). It is unknown if the reproductive response of doelings would be jeopardized or delayed when the grain is not offered to achieve maximum live weight at the start of the breeding period. Breeding females at a younger age can provide more animal protein, ensure food security, and alleviate poverty. The guidelines reported to breed young ewes successfully do not exist in goats (Kenyon et al., 2014; Rosales Nieto et al., 2018; Kenyon & Corner-Thomas, 2022). We hypothesized that the reproductive and maternal efficiency in doelings bred at nine months of age is not jeopardized when fed a diet that mimics the conditions currently used for raising goats in semiarid regions of northern Mexico and similar areas worldwide. The response will be the same for French Alpine and Saanen doelings.

The Institutional Animal Care and Use Committee of Universidad Autonoma de San Luis Potosi approved the animal procedures in this study. The animal procedures meet the ARRIVE guidelines for reporting animal research (Animals in Research: Reporting In Vivo Experiments; Kilkenny et al., 2010). The study was conducted at the Goat Unit of the Facultad de Agronomia y Veterinaria (22°13’ N, 100° 51’ W) from San Luis Potosi, Mexico. All doelings were constantly monitored by the veterinarian from the faculty.

The study results indicated that nine-month-old Alpine and Saanen doelings successfully ovulated by the buck stimulus, conceived, and kidded without negatively impacting maternal ability. Doelings' reproductive performance was satisfactory, and the progeny preweaning mortality rate was less than 10%, which is comparable to figures from other authors with pen-raised goats (Bélanger-Naud et al., 2021; Zamuner et al., 2024). Our results are relevant and have positive implications for the goat industry.

Information on doelings' reproductive success is scarce in Mexico. Espinoza-Flores et al. (2020) reported does mating at 13-month-old with or without nutritional supplements, which resulted in goats weighing 20 or 26 kg at breeding. The information is both relevant and consistent with our findings, indicating that in Northern Mexico, the growth rate of goats can be delayed if additional nutritional supplements are not provided (Mellado et al., 2020), with the growth delay being more pronounced in specific genotypes (Freitas et al., 2004). In other parts of the world and with similar genotypes, the age at breeding selected in the current experiment is not uncommon in well-fed dairy goats (Brazil: McManus et al., 2008; Turkey: Ince, 2010; Australia: Zamuner et al., 2023). What is noteworthy in the present study is the adequate reproductive response of goats with a reduced live weight at breeding, which was far below the recommended 65 to 70 % of their mature weight for doelings or at least 35 kg of live weight by the start of the breeding season (Mellado, 2011; Zamuner et al., 2023). The Saanen doelings were 45 % (23.5 kg), and the Alpine doelings were 51% (22.7 kg) of their mature live weight at the start of the breeding period. Doelings require an adequate energy and protein supplement to reach the desired weight (35 kg), but in the present study, grain was not provided. Lack of grain in the goat’s diet resulted in doelings weighing less than that recommended for breeding (Van Nevel et al., 2023). Recently, Van Nevel et al. (2023) reported that a low-protein diet in growing goats reduced growth hormone gene expression. Surprisingly, a high proportion (86%) of pregnant doelings conceived in their first induced ovulation. Doelings responded to the buck stimulus, highlighting the importance of exposure of does to sexually active bucks for inducing precocious activation of ovaries and puberty in these doelings and allowing them to conceive. Metabolic homeostasis is aligned with changes in live weight and composition through processes mediated by circulating metabolites and metabolic hormones with a strong relationship with reproductive efficiency (Rosales-Nieto et al., 2019), indicating that a certain minimum level of body fatness before the onset of puberty and successful ovulation must be attained (Rosales-Nieto et al., 2014; 2021). At this location, doelings born during January or February and with minimal physical activity can attain puberty the same year and approximately at seven months of age (Cuevas-Reyes et al., 2025). Other studies have documented the onset of puberty of some goat genotypes at 3.5 months of age (Moaeen-ud-Din et al., 2008), at 5 months of age in Saanen (Freitas et al., 2004), at 8.5 months of age in Anglo-Nubian (Freitas et al., 2004), or at 12 months of age in local breeds of Brazil (Simplicio et al., 1990). Therefore, the use of natural methods for increasing reproductive efficiency (Rosales-Nieto et al., 2024) could be used by producers to accelerate sexual maturation in underweight dairy goats using the “buck effect,” as documented in this and a previous study (Chasles et al., 2019).

The prolific rate of doelings in the present study was typical of well-fed, not artificially inseminated Saanen and Alpine goats (Alvarado-Espino et al., 2016; Sen & Onder, 2016), which indicated that breeding dairy goats at a reduced live weight did not impair the reproductive performance of these animals. Also, as expected from a previous study (Fonseca & Torres, 2005), no differences in reproductive performance were observed between Saanen and Alpine goats. Prolificacy in the present study was very low, which was expected because there is a tendency for dams fecundated young to gestate mainly singleton (Haldar et al., 2014), and this response is due to their not fully mature reproductive tract. Although few Alpine and Saanen goats had twins, ovulation rate is a complex trait that is primarily determined by genetic factors (Montgomery, 2024) but is strongly modulated by metabolic factors (Juengel et al., 2021), with heavier females at breeding (both young and adult) often being more fertile and more prolific than lighter females (Mellado et al., 2006; Panzuti et al., 2018; Rosales-Nieto et al., 2014; 2021). Therefore, based on our results and the literature, weight and adequate body score at certain age and parity seem essential to optimize metabolic provision to stimulate the hypophyseal-pituitary-gonadal axis for more ovulations (Rosales-Nieto et al., 2021; Thompson et al., 2021). Our findings allow us to determine in the following reproductive period if the prolificacy rate recorded in the current experiment was due to the doelings' immature reproductive tract or another unknown mechanism involved.

Our second hypothesis was that maternal efficiency is not reduced in doelings while giving birth at fourteen months of age, and the response will be independent of the genotype. Based on the results, Saanen and Alpine doelings could fully develop the fetuses and deliver them without problems. However, we recorded six stillborns, a figure very close to that reported by Mellado et al. (2006). Nevertheless, a higher proportion of stillbirths (14.7%) have been reported in Zaraibi goats mated at fourteen months of age (Hamed, 2010). The study also suggests that the number of stillborns observed may be positively affected by season, maternal age, and the low live weight of their mothers. The scarce information on goat kid stillborns highlights a need to quantify the causes of death and their relative importance. Further research is warranted to determine if the incidence of stillbirths has a genetic component beyond the dams' metabolic cues and body composition.

The birth weight of the Alpine and Saanen offspring in the current research was below that reported for well-fed Alpine and Saanen goats (McManus et al., 2008). We and others have reported a positive relationship between dam live weight at mating and offspring birth weight (Oldham et al., 2011; Paganoni et al., 2014; Flores-Najera et al., 2020). Furthermore, we recorded seven deaths during the progeny´s postnatal growth development. The average age to death of the goat kids was 25 ± 8 d, indicating that other factors may have contributed to the offspring's death, not the low milk yield produced by young dams or possibly reduced maternal ability. The mortality rate reported here is within the rate reported within the first two weeks of life for Saanen and Alpine genotypes (6 to 15%; Mellado et al., 2000; Sen & Onder, 2016). Indeed, the twinning rate is the major non-infectious cause of perinatal mortality in goats because being underweight at birth and having low maternal milk capacity impair goat kid survival (Robertson et al., 2020). However, the twinning rate in the current experiment was low, and lighter and perhaps weaker kids from multiple births did not appear the leading cause of death at birth (3 out of 22). The poor mothering ability of too-young dams probably provoked the mortality rate observed (9%). Robertson et al. (2020) indicated that perinatal mortality in goat kids was associated with poor mother ability. Furthermore, our results align with the findings of Snyman (2010), who observed that survival rates of preweaning kids from young does with breeding weights below 25 kg were reduced. This study suggests a similar adaptation ability of purebred Saanen and Alpine goat kids. The limitation of the current experiment is that we did not conduct an autopsy on those deceased kids, which could have allowed us to determine the exact cause of death. However, overall survival percentages were generally high for these genetic groups considered. Nevertheless, these observations warrant further research to determine the factors contributing to postnatal deaths.

As reported previously in Saanen (Sanahmmed et al., 2023) and Alpine (Sánchez et al., 1994), male kids were heavier than female kids. This response is basically due to physiological functions attributed to sex hormones; estrogen restricts the growth of long body bones, and androgens promote muscle growth rate (Assan, 2020). Also, in line with previous reports across various genotypes (Keski̇n et al., 2017; Flores-Najera et al., 2021; Castillo-Gutierrez et al., 2022), single kids were heavier than twin kids. This response is ascribed to the allocation of uterine space and utilization of uterine nutrients by the fetus of multiple births, resulting in lower birth weight (Assan, 2020).

Results of this study showed a positive response of doelings to the buck effect to synchronize and conceive at a young age. Importantly, we observed no adverse effect of low live weight (23.5 kg) and reduced mature weight (Saanen:45%, and Alpine: 51%) at breeding Saanen and Alpine goats on reproductive variables in nulliparous goats. Breeding underfed Saanen and Alpine goats at nine months of age can significantly enhance animal protein production, increasing food security and contributing to poverty alleviation. It is also feasible to meet market demands for goat products. Maternal efficiency was acceptable and within the rates for well-fed animals. Progeny´s weight variables were acceptable. Also, the results of this study suggest that fecundation of very light doelings may cause the occurrence of some stillbirths. Further research is warranted to determine if mammary development, colostrum composition traits, milk yield, and lifetime reproductive performance affect these females bred at a low weight and give birth at 14 months of age. Doeling age may need to be considered when selecting replacement female goats to increase productivity and long-term profitability.