Relationship of Longevity with Productive and Reproductive Variables in Nelore Cows of the Bolivian Tropic

Facultad de Ciencias Veterinarias, Universidad Nacional de Rosario, Argentina Centro Latinoamericano de Estudios de Problemáticas Lecheras (CLEPL), Argentina Facultad de Ciencias Agrarias, Universidad Nacional de Rosario, Argentina Carrera del Investigador Científico (CIC-UNR) Cooperativa Agropecuaria Integral San Juan de Yapacaní (CAISY) – Bolivia Universidad Autónoma Gabriel Rene Moreno – Bolivia


Introduction
Age at first calving, survival and longevity are economically important characteristics in beef cattle (Dakay et al., 2006;Carvalho et al., 2015;Damiran et al., 2018). Longevity, includes from the first to the last calving, being an economically important trait. An extension of longevity, allows increasing the number of adult cows that have a higher accumulated production (Ikeda et al., 2020) and also reduces annual production costs, which are associated with replacement heifers and with the number of involuntary culls of cows (Rogers et al., 2004). This coincides with what was reported by (Ikeda et al., 2020) where adult Nelore cows older than five years were more productive in total kg of weaned calves and in reproduction than younger animals. Not achieving this distribution, the nutritional requirements are higher, because there would be more cows in a growing state (first and second calving cows), leaving the pregnancy relegated to the background; therefore they will be less efficient. Keeping the largest number of adult cows in the herd close to 80%, would make it possible from a zoo technical concept that the traits associated with biological efficiency such as longevity and reproduction of animals produce greater sustainability of the productive system (Ikeda et al., 2020). To obtain efficient productivity in beef cows, in addition to taking into account important factors of the system, such as health aspects, age at first calving, birth and weaning weight, nutritional, reproductive and genetic factors, it is necessary to control body weight and body condition of the animals (Carrizales Montealegre, 2005;Orozco et al., 2013;Diskin and Kenny, 2014). In addition, the production of beef cattle is influenced, among other factors, by the genotype of the animal associated with the environmental conditions to which it is subjected (Holgado and Rabasa, 1999). A previous work by (Ikeda et al., 2019), showed results on the relationship between the longevity of the productive variables, in this case the parturitionparturition interval is introduced as a reproductive variable and a greater number of years of information to be able to continue deepening the understanding of why some cows live longer than others.
For this reason, the objective of this study was to evaluate the relationship between longevity with productive and reproductive efficiency variables of Nelore cows in grazing systems in the Bolivian tropics.

Materials and Methods
Retrospective data corresponding to the period between 1992 and 2019 were used for the research work belonging to the Cooperativa Agropecuaria Integral San Juan de Yapacaní (CAISY) located in Japanese Community San Juan 16°59′0″ south latitude, 63°58′0″ west longitude and Technology Center on Agriculture and Livestock in Bolivia (FUNDACION CETABOL) in Japanese Community Okinawa (17°13′12″ south latitudes, 62°53′39″ west longitude) Santa Cruz, Bolivia. The communities are located at 286 m above sea level and they present a tropical climate, with significant rains in most months of the year and a short dry season with little effect on the general climate.
The annual average temperature is 24.3°C with average rainfall of 1805 mm in Japanese Community San Juan and 986 mm in Japanese Community Okinawa, Santa Cruz. Rainfall is minimal in the month of July, with average values of 50 mm. The wettest month in January with an average rainfall of 330 mm. The highest average temperatures correspond to the month of January, with values of 26.5°C and the lowest to the month of July with records of 20.7°C.

Animals
The data corresponding to 774 Nelore breed cows, primiparous and multiparous discarded with a total of 4050 calvings were used. The primiparous cows calve between the months of May and July of each year, while the rest do so between the months of July and September. Weaning occurs between seven and eight months in two or three stages depending on the body condition and general state. Gynecological control is performed routinely at weaning, by a technical advisor, as well as health. Natural conceptions were used in the years analyzed.

Body Weight Record
The cows had at least two individual weight controls every year. The weighing of the animals was always carried out at the same time (8:00 am). The calves were weighed with a brand manual scale (POCKET BALANCE; Made in Germany) on the same day of birth. From two months of age, calves and dams were weighed with a brand electronic scale (ICONIX New Zealand Ltd.).

Variables Used
Variables

Statistic Analysis
The cows were grouped by date of birth, where they were ordered in three decades [(1985-1995; 1995-2005; 2005-2015)]. A multiple regression model was applied that describes longevity as a function of the other variables. To select the best predictors, the stepwise method is used based on the bidirectional approach, which in each step tests which variables are excluded or included in the model. The quality of the model will be evaluated using the Akaike criteria. The interpretation of the model was carried out based on the estimated regression coefficients. The statistical analyzes were carried out with the R 4.0.0 Table 1 shows the descriptive results of the variables studied, showing that cows born in the year (1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995) are those that arrive later to have their first calving, have greater calving intervals, have greater longevity, similar weaning weight of calves, lower birth weight of calves, lower live weight and higher total calf weight. While cows born in the year (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) are those that arrive younger to have their first calving, they have shorter calving intervals, have shorter longevity, similar wearing weight of calves, greater birth weight of calves, intermediate live weight and lower total calf weight.   Table 2 shows that there is a significant interaction between the year of birth and the age at the first calving and the year of birth and the variable calving interval. That is, the relationship between age at first birth and life length is not the same for all birth year intervals. The same happens with the relationship length of life and the calving interval. Regarding the variable total weight (kg), it is significant by itself but its interaction with year of birth is not significant.

Model Results
In Table 4 the results of the equations can be interpreted as follows.
For the Year (1985Year ( -1995  In Fig. 2 it is observed that the predicted values of the length of life as a function of the calving interval behave in a similar way as for the previous case (Fig.  1). The relationship between the calving interval and the length of life is positive in the three intervals of year of birth. However, the slope or increase in the average life length for each unit increase in the calving interval is greater for cows born between 1985-1995 than for those born between 1995-2005 and 2005-2015 (p-value = 0.006076).  Year (1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995) Year (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) Year ( 1985, 1995) [1995; 2005) [2005; 2015) Life length predicted by the model 1000 1500 2000 2500 Age at first calving Year of birth 5000 4500 4000 3500 3000 [1985, 1995) [1995; 2005) [2005; 2015) Life length predicted by the model 400 600 800 1000 Labor-delivery Interval behave Year of birth  Fig. 3 it can be seen that as the total calf weight in kg increases, the average life length also increases. However, since there is no significant interaction between total weight and year of birth (p-value = 0.224477), the increase in average life length for each unit increase in total weight is similar in the three intervals of year of birth. That is, there is a linear relationship between total weight and life length (p-value < 2e-16).

Discussion
The period in which the cows remain productive is an important factor in the profitability of the systems, because their permanence and productivity affect production costs. Therefore, the profitability of beef cattle production is directly related to productive efficiency and costs (Bernardes et al., 2018). It is logical to expect that there are differences in the longevity of cows between different production systems, which may be associated with environmental factors, herd size and management (Segura-Correa et al., 2013). The results found in this study with respect to the ages at the first calving were different and significant with respect to the year of birth.
Where cows born in the decade 1985-1995 reached their first calving at 45 months, while in the other two decades (1995-2005; 2005-2015) it was 34.9 and 33.7 months respectively. Observing a decrease in the age at first calving in the last two decades studied. The results for the decades 1995-2005 and 2005-2015 are below the 38.7 months found by (García et al., 2003) and are similar to the 35.6 months presented by (Flores and Ortiz, 2010) both in Nelore cattle. The age at first calving can influence the number of calvings of a cow during its useful life in the herd, because the greater sexual precocity of the cows would lead to a longer productive life (Nunez-Dominguez et al., 1991, Vukasinovic et al., 2001Rogers et al., 2004;Aranda-Avila et al., 2010). In this study, the results do not coincide with the aforementioned authors, since in the model estimates for the three periods studied, an increase in the age at first calving by one unit increases the longevity of the cows. The greater probability of a greater longevity of Nelore cows occurs when they arrive at an older age at the first calving. This could be explained by a set of factors, including the environment in which they develop (nutritional and climatic). This means that those cows that manage to reach their first calving going through the difficulties that they had to overcome in the breeding and rearing stage, acquire a level of adaptation that allows them to face the different future calvings, in addition to being attributed to the greater rusticity and adaptability of the race analyzed to the tropical climate. Marulanda (1996Marulanda ( ), 6000 4000 2000Marulanda ( [1985Marulanda ( , 1995Marulanda ( ) [19952005) [20052015) Life length predicted by the model 1000 2000 3000 Total calf weight in kg Year of birth reported that the ideal is that the age at first calving coincides when the heifer is mature enough to withstand the stress of lactation, making it clear that there would not be a target time to meet since it could be subject to the factors previously cited.
To be efficient in the production of cows for meat, it is necessary to identify profitable females and for that, reproductive traits should be included when the objective of selection is the highest economic profitability for the breeder (Oyama et al., 2004;Laske et al., 2012;de Lima Silva et al., 2019). Brumatti et al. (2011) showed the need to readjust the current selection indices, thus giving greater emphasis to reproductive traits, since they are economically more relevant. The results found in this study regarding the calving interval were different and significant regarding the year of birth.
Where cows born in the decade 1985-1995 had a greater calving interval than in the other two decades (1995-2005; 2005-2015). The calving interval values found are below to those reported by (Perotto et al., 2006), where they reported that the calving interval in Nelore herds is 492 days and 483 days found by (McManus et al., 2002), also equivalent to the 465 days reported by (Silveira et al., 2004). In this study, the model estimates for the three periods studied (Table 3) showed that an increase of one unit in the calving interval increases the longevity of the cows.
The interpretation of this result may be that the cows go through traumatic nutritional and environmental situations (few or excessive rains, high temperatures) that do not allow them to carry out a gestation that allows a shorter calving interval, although this does not affect their longevity of the themselves, showing that in this environment the cows the longest intervals are part of their adaptation.
The variable total calf weight in kg is the third variable that explains the model, being the most important from the productive point of view and that in general this type of aggregate indicator is not used for the analysis of the systems. It is necessary for the cows to achieve the highest amount of kg of total calf weight at the end of their productive life, being within the model the one with the greatest economic impact and also, having the need for the cows to meet the highest life expectancy possible.
The results found in this study regarding the total calf weight were different and significant with respect to the year of birth. Where cows born in the decade of 1985-1995 had a higher total calf weight, than in the other two decades (1995-2005; 2005-2015), the latter being the one with the lowest value. The model estimates for the three periods studied showed that an increase in one unit in the calving interval increases the longevity of the cows. These results reinforce what was expressed by (Parish, 2010) where the herds made up of a higher proportion of mature cows tend to have a higher percentage of weaned calves (kg of total calf), reduce production costs and increase the sale of kilograms of calf total.
In addition, a greater number of adult cows in the herd close to 80%, would allow from a zootechnical concept that the traits associated with biological efficiency such as longevity and reproduction of the animals contribute to greater sustainability to the productive system (Ikeda et al., 2020). Nunez-Dominguez et al. (1990) suggested that the cumulative production of weaned calves per replacement female is an appropriate statistic that takes into account the fertility, maternal instinct and longevity of the cow to measure the reproductive behavior of the evaluated herd.
The results found in this study allow us to reflect that in livestock production and specifically in breeding in systems with very little environmental control, care would have to be taken just by thinking of maximizing weaning weight as the only objective, instead, it should be more comprehensive vision and think about the inclusion of the entire life of the cow (longevity), which would imply including reproductive success and rearing efficiency in the analysis.

Conclusion
It is concluded that it was possible to find a model that explained that the greater longevity of Nelore cows in a grazing system in the Bolivian tropics and it was related to the older age at first calving, the greater calving interval and the greater kg of weights of calves.
This will imply in the future replicating the model found to be able to explain the longevity of the cows in the systems that are required.