Nutritional needs

Dual purpose cattle with 50 % Bos taurus genes represent the optimal genotypes for the humid tropical region (^{López et al., 2010}). Although ^{Arce et al. (2017}) found that genotype had no significant effect on calving interval. In contrast, the Holstein x Zebu cross is considered the most appropriate for milk production (^{Vite-Cristóbal et al., 2007}). In addition, this cross has lower nutritional requirements and different nutrient partitioning compared to cows with high milk merit. Thus, it would be expected that both the magnitude and duration of the negative energy balance (NEB) would be different in these cattle due to their lower milk production potential. Also, these genotypes show a greater response to additional energy supplementation in early lactation, improving or eliminating NEB and increasing their productive and reproductive performance (^{Aguilar-Pérez et al., 2009}). In this regard, ^{Garmendia (2005)} recommends supplementation when: 1) The forage supply is low, from 3 to 6 tons of dry matter/year, 2) There are energy limitations during the prepartum and postpartum periods, since feed restriction before calving affects the animal's body condition, and in the postpartum period it influences the secretion of essential hormones for the restart of the reproductive cyclic activity, 3) There is a low protein intake in the diet, 4) There are animals that have not finished their growth and in pregnant animals to guarantee the viability of the calf and the restart of the postpartum reproductive cycles, 5) There are lactating cows that rapidly lose body condition and are in a critical period of forage limitation (dry season), and 6) In the diet the phosphorus content is less than 0. 20 %, since it affects voluntary consumption and ruminal fermentation of dry matter, causing imbalances in the production of ruminal gases and microbial protein, affecting reproduction.

Restart of postpartum cyclic activity in cattle

The anestrus period in beef cattle can vary depending on the effects of lactation, mothercalf relationship, nutrition and calving problems (^{Pohler et al., 2020}), as well as uterine infection, protein deficiency, early mating, parasitism and environmental stress (^{Bolaños, 2020}); even calving season and parity (^{Hernández-Reyes et al., 2000}); the day of the week when calving occurs and herd size affect reproductive performance (^{Montiel-Olguín et al., 2020}). However, in dual-purpose cattle, suckling and nutrition are the main factors that can modify female ovarian cyclicity (^{Montiel & Ahuja, 2005}; ^{Rojo-Rubio et al., 2009}). However, it has also been found that body condition and energy balance (dry matter intake) are the factors that contribute most to the frequency of Luteinizing Hormone (LH) pulses to achieve the first ovulation. In addition, these factors have been observed to indirectly affect postpartum uterine health in beef and dairy cattle (^{Crowe et al., 2014}).

With less research, in dual-purpose cattle, mastitis has been reported to be a negative factor on cow reproductive efficiency (^{Bacha & Regassa, 2010}; ^{Nava-Trujillo et al., 2010}). Other factors to consider are semen management at the time of service, among many other factors, sperm deposition in the ideal reproductive tract site at the appropriate estrous cycle time and heat stress (^{López-Gatius, 2012}). Thus, postpartum anestrus or puerperium is defined as the time elapsed from the time of parturition until reproductive functions are restored so that effective service and consequently gestation can occur. Simultaneous to this regeneration, there is a progressive increase in the frequency of circulating LH pulses, which is essential for the establishment of normal estrous cycles after parturition (^{Pohler et al., 2020}). In this regard, ^{Diskin et al. (2003)}) in their literature review mentioned that the nutritional status of cattle is an important factor affecting follicular growth, maturation and ovulatory capacity and that the resumption of ovarian activity is related to a reduced frequency of LH pulses, which is controlled by the release of GnRH from the hypothalamus. Therefore, a delay from calving to first estrus with ovulation is caused by a lack of LH pulses, which is a result of calf suckling inhibition in beef breeds (30-130 days) and high production in dairy breeds (25-45 days; ^{Crowe, 2008}). When the above occurs, the cow cannot reach conception (^{Lucy et al., 1992}; ^{Robinson et al., 2006}).

In high milk producing cows, it has been shown that the main factors that determine the duration of postpartum anestrus are closely related to situations such as loss of body condition and NEB, metabolic disorders and uterine pathologies, udder health and lameness (^{Walsh et al., 2011}).

^{Walsh et al. (2011}) consider that, the points to be addressed to reduce the problem of low fertility in cattle are: firstly, minimize pre- and postpartum NEB; ensure estrous expression and detection in the cow followed by artificial insemination; achieve ovulation and fertilization of a high quality oocyte; followed by this, to provoke a rapid increase in corpus luteum progesterone synthesis, simultaneously, to ensure that the uterine endometrium rapidly produces an appropriate environment to stimulate embryo development and finally, to get the embryo to produce adequate amounts of interferon-τ, which is an antiluteolytic factor that prevents lysis of the corpus luteum in ruminants and in a coordinated and local way between embryo-uterus-ovary maintains pregnancy. Interferon-τ acts on uterine cells to inhibit the transcription of certain genes and to abrogate the development of the endometrial luteolytic mechanism. The molecular mechanisms involved in the silencing of the expression of such genes by interferon-τ are probably due to the fact that interferon-τ induces the expression of another factor, which is a potent repressor of uterine enzyme transcription (^{Thatcher, 2017}). However, all of the above may be limited due to nutritional deficiencies caused by poor forage quality or reduced postpartum dry matter intake. For example, blood progesterone concentration is a factor that becomes more relevant in high-producing dairy cows, because their corpora lutea produce less progesterone and this hormone is eliminated faster from the blood, due to elevated hepatic metabolism (^{Hernández, 2016}).

Nutritional factors affecting the resumption of postpartum cyclic activity

Dual-purpose cattle production systems are mainly found in humid tropical and dry tropical climate regions, grazing is the basis of feeding (^{González-Padilla et al., 2019}) on pasture and rangeland available in the region (^{Garay-Martínez et al., 2020}). In addition, dualpurpose cattle in the tropics present a lower nutritional requirement and a different nutrient partition compared to cattle with dairy merit, nutritional deficiency of forages causes low reproductive efficiency in both systems (^{Aguilar-Pérez et al., 2009}). Feed supplementation is most frequently used during climatic times with low pasture availability, mainly in drought (^{Granados-Rivera et al., 2018}), being the most common with commercial concentrates (^{Lassala et al., 2020}). However, offering feed supplementation in strategic physiological stages of dual-purpose cattle correlates with an increase in milk production (^{Bautista-Martínez et al., 2017}) and with better reproductive efficiency, so feed supplementation should be a permanent management practice and independent of the climatic season (^{Granados-Rivera et al., 2018}). The impact of nutrient deficiency on reproductive parameters in cattle, focusing mainly on dual-purpose systems, is presented below.

Energy deficiency

In ruminants, reproductive activity is associated with energy availability. Deficient glucose availability caused by lower dry matter intake and high demand for milk synthesis, commit the animal to enter a NEB (^{Chandra et al., 2011}). In addition, increased ketogenesis is one of the requirements for good hepatic adaptation to increase energy demand in the period immediately after calving, thus ketone bodies are a very important alternative energy resource for the tissue when glucose is being redirected to the mammary gland (^{Herdt, 2000}). However, it can be harmful to the animal causing ketosis.

Energy supplementation prevents or decreases NEB. Thus, cows that are not supplemented before and after calving show a deep postpartum NEB, whose lowest point is called "nadir", taking longer to reach a positive energy balance (^{Aguilar-Pérez et al., 2009}). In a study conducted in beef cows of varying age, supplementation during the last third of gestation consisting of whole soybean grain (1.36 kg/day) compared to a soybean meal/hull based supplement (1.56 kg/day), cows fed the first supplement at adulthood represented higher ovarian activity at the beginning and end of the reproductive season compared to those fed a soybean meal/hull-based supplement (^{Banta et al., 2008}).

Similarly, ^{Aguilar-Pérez et al. (2009)} found that supplementation with cereal-based concentrates improved reproductive parameters in cattle. In this study, supplementation consisted of sorghum (69 %), soybean (14 %), wheat bran (15 %) and minerals (2 %). These cereals together contained 878 g/kg dry matter, 168 g crude protein/kg dry matter and 11.8 MJ metabolizable energy/kg dry matter (2.82 Mcal ME/kg DM), providing 0.9 % of the live weight. For the control group only wheat bran and minerals were offered (15 % and 2 %, respectively). It is worth mentioning that the cattle used for this study were dual purpose (Holstein x Zebu) grazing in early lactation in the tropical regions of Mexico. The supplemented cows presented a higher ovulation percentage compared to the control group (58 and 30 %, respectively) and higher gestation rate at day 90 postpartum (47 and 22 %, respectively) and shorter interval from calving to first estrus (62.8±6.9 days and 68.2±3.8 days, respectively), so this supplementation tends to improve reproductive parameters.

In Cynodon nlemfuensis grazing cattle (Swiss x Zebu), the addition of an energy supplement in the diet (ground sorghum at 0.4 % of live weight) helped to improve postpartum cyclic activity, shortening the interval from calving to the appearance of the first corpus luteum (ultrasonography, twice a week during the study) in supplemented cows. Likewise, the higher gestation rate of supplemented dual-purpose cows (40±10 vs. 51±10 days) could not be explained by nutritional status, suggesting additional effects of supplementation energy mediated by metabolic hormones, which originate higher progesterone synthesis in luteal cells that are associated with an increase in conception rate in supplemented cows (42 % vs. 0 %; ^{Tinoco-Magaña et al., 2012}).

Recently, in a study by ^{De Souza et al. (2019)} in Holstein dairy cows they found that, when supplemented with palmitic acid during the first three weeks after calving, fresh period, they observed that palmitic acid increases plasma beta-hydroxybutyrate concentration during the first week after calving compared to the control group. The significance lies in the fact that ketogenesis, the process by which fatty acids are transformed into beta-hydroxybutyrate, takes place in the mitochondria of liver cells and may occur in response to a lack of blood glucose availability (^{Robinson y Williamson, 1980}). The production of beta-hydroxybutyrate is initiated to make energy available primarily from fatty acids. Therefore, elevated plasma beta-hydroxybutyrate concentrations as markers of excessive NEB reveal a decrease in reproductive performance (^{Chapinal et al., 2012}). Plasma beta-hydroxybutyrate concentrations of 10 to 14 mg/dL tend to increase the risk of a metabolic disorder (abomasal displacement, metritis, clinical ketosis) and 15 % less likely to become pregnant after a voluntary 70-day waiting period. Although in this study, palmitic acid increased plasma betahydroxybutyrate concentration in the first week of the fresh period, a pronounced decrease occurred immediately thereafter that does not represent a risk. Therefore, the researchers concluded that plasma beta-hydroxybutyrate concentrations from palmitic acid supplementation are probably associated with increased nutrient partitioning to the mammary gland rather than as an indicator of metabolic dysfunction. This feeding strategy reduces NEB and prevents loss of body condition of the postpartum cow.

Protein deficiency

^{Bolaños (2020)} suggests avoiding protein underfeeding of the cow in the diet during peripartum, since the immune system through the formation of antibodies (protein compounds) are responsible for capturing and digesting the bacteria that enter the reproductive system during parturition causing uterine infections. Inflammation in response to the presence of bacteria in the reproductive tract has negative effects on reproductive efficiency; in this sense, the researcher considers that the lack of an adequate protein level in the diet is a cause of metritis. On the other hand, ^{Garmendia (2005)} commented that once the protein reserves in the body are depleted, the lack of protein limits milk production in the first instance and the synthesis of immunoglobulins, so that immune competence is compromised. The result is a greater predisposition to the appearance of pathologies during and after parturition, such as the retention of fetal membranes.

The low protein quality of unimproved pastures, where ruminants are grazed, does not cover the nutritional requirements. Protein produces a greater response on dry matter intake. As crude protein content of forages increases, the magnitude of production responses due to additional protein supplementation may not be related to changes in forage intake, but to changes in forage digestibility or metabolic efficiency of nutrient utilization, including the effects of degradable and non-degradable protein. Due to the high degradable protein content of grasses, supplementation of non-degradable protein (overage) can improve grazing performance (^{Kawas, 2007}).

^{Lara et al. (2015)} conducted a study, the objective of which was to evaluate progesterone and LH concentrations during postpartum in response to different doses of surplus or protected methionine (0, 8, 16 and 24 g/day). The study was conducted on an experimental ranch in Mexico with Holstein dairy cows for 96 days in the early postpartum period. Although the results showed that there was a higher concentration of progesterone in the first three cycles in those cows that were fed the protected or overage amino acid, the concentrations of LH, which produce the preovulatory peak, were not statistically different. Therefore, they concluded that supplementation with protected methionine to cows during postpartum appears to improve corpus luteum function, without achieving a higher LH peak compared to cows that are not supplemented. In the literature review by A^{lvarez-Cardona et al. (2019)} mention that the action of neurostimulatory amino acids stimulate the secretion of adenohypophysial gonadotropins and, therefore, regulates the control of gonadal physiological events.

Feeding management strategies to improve reproduction in dual-purpose cattle

The negative effects exerted by suckling and grazing, without being supplemented, inhibit the reestablishment of postpartum ovarian activity, lengthening the interval between calving, which can exceed 500 days, an extensive period of postpartum anestrus in dual purpose cattle (^{Rojo-Rubio et al., 2009}), which is no longer economically profitable for a production unit to keep a cow empty for a year or more (^{Martínez et al., 2021}).

Use of improved pastures

^{Samadi et al. (2013)}demonstrated that pasture-based nutritional control before and after calving can be used to induce marked differences in weight, body condition, and metabolic hormonal status in postpartum beef cows. These researchers concluded that cows receiving improved pasture had higher plasma concentrations of glucose, insulin, insulinlike growth factor-I and leptin compared to cows receiving less digestible pasture. These differences in glucose and metabolic hormone profiles were associated with earlier resumption of ovulation for cows on improved pasture. The reduction in the postpartum anestrus period in beef cows grazing improved subtropical pastures may result in improved metabolic homeostasis. The earlier resumption of cyclic ovarian function of cows on improved pasture probably resulted from the general positive actions of glucose and metabolic hormones on somatic tissues and more specifically on the hypothalamus and ovaries. The potential actions on different tissues within the reproductive axis probably reflect the nature of the interrelationships between nutrition, metabolic regulators and reproductive function, with information available that allows for an integration of the effects of leptin and insulin-like growth factor-I, on GnRH secretion, which appears to involve an indirect action through kisspeptin and leptin neurons; and a more direct action on GnRH (insulin-like growth factor-I) neurons. Since in the experiment the restart of postpartum ovarian cyclicity (ovulation) at 12 and 15 weeks was 100 % for cows kept on improved pasture compared to those kept on less digestible pasture, where only 14 % showed ovulation at 15 weeks.

Technification of production units and manipulation of nutritional components in the diet

^{Velázquez et al. (2020)} evaluated an induction protocol to restart ovarian activity in Bos tauros x Bos indicus crossbred cows in the tropical region of Mexico. The hormonal treatment consisted of the placement of CIDR with and without estradiol in dual-purpose production units with three technification statuses described as: High technification level, where grazing is done on pastures with improved pastures, they perform productive and reproductive records and nutritional supplementation during milking of the animals; medium technification level, the same conditions as the previous one; however, they do not perform productive and reproductive performance recording; low technification level, where grazing is on available natural forage and without productive and reproductive records or nutritional supplementation of the animals. In this research, this group of researchers concluded that cows respond to the induction of ovarian activity of the combination of progesterone and estradiol in a high proportion in all levels of technification. These results confirm the benefits of production units with medium and low technification status, since fixed-time artificial insemination programs, for example, can be carried out. However, economic issues should be evaluated. ^{Bautista-Martínez et al. (2019)} in their economic-productive characterization of the dual-purpose cattle system in three tropical regions of Mexico found that practices related to reproductive management are the least frequently performed in production units, the promotion in the application and adoption of technologies related to this area, could be an alternative to improve productive and reproductive variables in the long term.

On the other hand, in the same Mexican region described above, ^{Zárate-Martínez et al. (2022)} studied the reproductive behavior of dual-purpose systems with two levels of neutral detergent fiber, inducing ovulation with a hormonal protocol of fixed-time artificial insemination. The experiment consisted of: treatment 1 fed 4.5 kg of concentrate cow/day and treatment 2 fed 3 kg of concentrate cow/day. Forage was offered ad libitum in both treatments. The results obtained were that cows in treatment 2 consumed more neutral detergent fiber than cows in treatment 1, 38 and 44 % of the total diet, respectively. This group of researchers commented that the increase in forage intake to where the neutral detergent fiber allowed compensating the energy and metabolizable protein intake, being similar for both treatments during the first 15 days before calving. Cows in treatment 2 (6.25 dry matter/day) tried to compensate for nutrient deficits by consuming more forage than those in treatment 1 (7.37 kg dry matter/day). In addition, they comment that metabolizable protein was always deficient during lactation for both treatments. In treatment 2 cows, the poor metabolizable protein resulted in lower milk nitrogen content. Finally, they found that cows in treatment 1 lost less body condition by 75.2 %, this was reflected in a higher gestation rate at first service in the treatment compared to treatment 2 with 42.8 %, although they did not produce more milk. They concluded that the decrease of neutral detergent fiber in the diet from 44 to 38 % and the application of the hormonal protocol of fixed-time artificial insemination decreased the open period to 105 days in lactating cows in the tropics of Mexico.