INTRODUCTION

Milk production in the country is carried out under three production systems: intensive, family and dual purpose ^{(Vera et al., 2009)}. The family system contributes with a third of the national milk production (^{FAO, 2014}), represents little more than 70% of dairy farms and has 20% of the dairy cattle inventory ^{(Hemme, 2007; Plata-Reyes et al., 2018}). This production system also contributes with social benefits, because it improves food security and the nutritional status of families in rural areas (^{Hemme and Otte, 2010}). Furthermore, it represents a source of income for families in the countryside, which contributes to the roots of the population in their places of origin (^{Espinoza-Ortega et al., 2005}). Despite the importance of these stables, it has been reported that the production units operate in a suboptimal way, which compromises their survival (^{Posadas-Domínguez et al., 2014}; ^{Montiel-Olguín et al., 2019a}). In order to make strategic recommendations and thus improve their productivity, it is necessary to make an analysis of the factors that negatively impact the performance of these stables' operations.

In the family system, the producers and their family are the main operational support of the barn's activities ^{(García et al., 2012; Jiménez Jiménez et al., 2014}). These activities are time-consuming and have a high degree of impact on productive performance; some examples are the work of the field, milking, feeding, and attending to the rearing and the calving area and the detection of estrus ^{(Vera et al., 2009)}. Therefore, it is reasonable to think that due to the large number of tasks carried out by family members, perhaps on some days of the week they do not pay the same attention to some tasks, for example, attending to the tasks correctly deliveries on weekends. In addition to this, in stables of the intensive system it has been observed that as the size of the stables increases, reproductive performance is affected ^{(Oleggini et al., 2018; Washburn et al., 2018)}. Also, in the family system it has been reported that cows in larger stables have a higher risk of requiring assistance at calving (^{Montiel-Olguín et al., 2019b}). Therefore, in the present study we challenge the hypothesis that the day of the week in which calving occurs and herd size have an impact on days to first service and days open as indicators of reproductive performance. The objective of the present study was to determine if the days of the week in which calving occurs and the size of the herd impact on the reproductive performance indicators in stables under the family production system in Mexico.

MATERIAL AND METHODS

RESULTS AND DISCUSSION

In other countries, it has been reported that the day of the week impacts some reproductive events, such as the conception rate ^{(Berry et al., 2011)}, birth attendances rate ^{(Mee et al., 2011)} and the rate of artificial insemination ^{(Tesfaye et al., 2015)}. However, as far as we know, this is the first report on the effect of the week day on some indicators of reproductive performance, such as days at first service and days open in dairy farms in Mexico. The results indicated that, for days to first service and for days open, the interaction Calving day X Herd size was significant (P≤0.05). Table 1 shows the adjusted means of the interaction Calving day X Herd size for days at first service and days open. Cows that calved in small stables on Wednesdays and Thursdays had more days to first service (106.1 ± 10.1 and 120.8 ± 11.5, respectively) than cows that calved the same days in large stables (86.6 ± 4.3 and 87.3 ± 4.3, respectively). Furthermore, it is striking that in large stables there are no differences (P> 0.05), due to the effect of the day of calving on the days to first service (range between 83.9 ± 4.1-93.2 ± 4.4). However, in small stables, the cows that calved on Wednesday and Thursday had a greater number of days to first service (106.1 ± 10.1 and 120.8 ± 11.5), compared to cows that calved on the rest of the days of the week (range 70.8 ± 11.9-91.0 ± 11.5) (P≤0.05).

Regarding the days open, the cows that calved on Saturday and Sunday in large stables had a greater number of days open (136.4 ± 7.0 and 128.8 ± 6.5, respectively), compared to the cows that calved the same days in small stables (89.8 ± 16.1 and 98.3 ± 12.9, respectively) (P≤0.05). In contrast, cows that calved on Thursday in large stables had fewer days open (113.4 ± 6.8), compared to cows that calved the same day in small stables (146.4 ± 18.2) (P≤0.05). Furthermore, the cows that calved in large stables on Saturday had a greater number of days open (136.4 ± 7.0), than those that calved during the rest of the week (range 112.5 ± 7.0-128.8 ± 6.5) (P≤0.05). While, in small stables, the cows that calved on Wednesday and Thursday had a greater number of days open (136.4 ± 16.1 and 146.4 ± 18.2, respectively), than those that calved during the rest of the week (range 89.8 ± 16.1-133.8 ± 18.2) (P≤0.05).

These results suggest that in large stables of the family system (≥33 cows), the effect of the day of the week on which calving occurred is less important, and due to this the days to first service are more homogeneous. However, it is worth noting that the cows that calved on Sunday in large stables had a greater number of days open. In other countries it has been reported that cows calved on Sunday are at greater risk of requiring assistance at calving ^{(Mee et al., 2011)}; however, newborn mortality is not affected Mee et al., 2008;^{Bleul, 2011)}.

In relation to the above, another report indicates that the pregnancy rate was decreased when artificial insemination was provided on weekends, specifically on Saturday (^{Berry et al., 2011}). This information as a whole suggests that, in our study, the higher rate of attendance at calving is the result to a greater extent by a human factor and not by biological reasons. In addition to this, attendance at delivery is a risk factor for long open days in the family system (^{Montiel-Olguín et al., 2019a}), which, based on our results, could be partially associated with greater uterine contamination due to unnecessary care, which prolongs the postpartum period and reduces the conception rate (^{Sheldon and Owens, 2018}). On the other hand, in other production systems it has been reported that as the size of the stables increases, reproductive performance is affected ^{(Oleggini et al., 2018; Washburn et al., 2018)}.

Stables in the family system are generally smaller than stables with fewer cows in the intensive system (^{Val-Arreola et al., 2004}). Our results suggest that despite being large stables within the family system, they can still be managed reproductively without suffering a negative effect due to herd size, as occurs in intensive stables ^{(Oleggini et al., 2018; Washburn et al., 2018)}. Another factor that would be contributing to these results is the number of people who work in family herds. Unpaid family labor plays an important role in reducing production costs in this production system (^{Jiménez Jiménez et al., 2014}). In addition, it has been reported that in family stables, as the number of cows’ increases, a greater number of people outside the family are hired (^{Arriaga Jordán et al., 1999}; ^{García et al., 2012}). The above explains why the care provided in medium or large stables is more homogeneous throughout the week compared to what occurs in small stables.

In the family system in Mexico, attendance at delivery and retention of the placenta are risk factors for late days to first service and long open days (^{Montiel-Olguín et al., 2019a}). Therefore, we challenge the hypothesis that the day of the week on which parturition occurs and the size of the herd impact the proportion of deliveries and placental retentions. The reasoning behind this hypothesis was that this condition could be occurring due to management failures associated with rest days (weekends), and not due to biological issues. Regarding delivery assistance and placental retention, the results of the logistic regression analysis indicated that the main effect Herd size was significant (P≤0.05); Whereas Day of calving and the interaction Day of calving X Herd size did not (P> 0.1). The cows that calved in large stables had a odds ratio of 1.7 for calving assistance (95% CI, 1.01-2.96) and 1.7 for placental retention (95% CI, 1.04-2.87); that is, cows calving in large stables have a higher risk of requiring assistance at calving and presenting with retention of the placenta than cows calving in small stables, which has been previously reported ^{(Mee et al., 2011; Montiel-Olguín et al., 2019a}; ^{Montiel-Olguín et al., 2019b}).

In a complementary analysis, we found that herd size impacts the proportion of calving attendances that occur on weekends (Friday, Saturday, and Sunday). Small herds had a higher proportion of calving attendances (21.0%) than large herds (10.4%) during those days (P≤0.05). Taken together, these results suggest that, although large stables generally report higher calving attendances and placental retentions, the effect of weekends has a greater impact on small stables (twice calving attendances). This could be explained again from the number of workers available to carry out the tasks during these days (^{Arriaga Jordán et al., 1999}; ^{García et al., 2012)}.

In other countries with small-scale production systems, it is reported that some activities of reproductive importance are not carried out during weekends and holidays, such as artificial insemination ^{(Tesfaye et al., 2015)}. In addition, it is likely that in small stables there are not personnel dedicated exclusively to the task of attending deliveries on weekends, and those who participate in this work have less experience and rush care unnecessarily ^{(Mee et al., 2011)}. Previously, it was reported that the sex of the male calf is the main risk factor, associated with attendance at birth in family stables (^{Montiel-Olguín et al., 2019a}). In this study it is stated that this risk factor triggers a series of events that negatively affect the reproductive performance of family stables, placing it as a key factor to control (sexed semen as a possibility). However, the findings that we report here place care at delivery as another key factor, which could be independent and additive to the biological component (sex of the male calf). A practical recommendation that emerges from these last results is that in extension programs, the importance of attending births in small herds of the family system (specifically during weekends) should be emphasized.

CONCLUSIONS

The day of the week on which calving occurs and the size of the herd impact first service days, open days and attendance rates in cows under the family production system. In large stables (≥33 cows), the averages of days to first service and days open are more homogeneous throughout the week, compared to the values in small stables (<33 cows). Likewise, cows in small stables have a higher proportion of calving attendances on weekends, this probably associated with failures in calving care during rest days.