INTRODUCTION

Animal welfare refers to the internal state of an animal when it faces the environment that surrounds it, so it includes its state of health, its perception of the environment and its mental states (^{Greiveldinger, 2007}). It has been defined as the state in which the animal has its basic physiological, health and behavioral needs satisfied, in the face of changes in its environment (^{Broom, 1986}). For this reason, it is increasingly perceived as an integral element of the global quality of food, with important implications for animal health and food safety, among others.

However, the foregoing, intensive production systems have been designed to obtain increasingly higher productions at lower and lower costs, adopting in many occasions inappropriate management that causes acute or chronic stress, leaving aside the behavioral needs of the animals, with what compromises your well-being; calf raising does not escape this situation. Currently, in most dairy production units, the newborn calf is raised under a scheme that includes various situations that generate stress, one of the most common is to house them in pens or individual cages as a sanitary control measure, to avoid excessive sucking and feeding behaviors; although most of the time the cages are so small that they restrict and even suppress individual and social behavior patterns, not allowing the animal to turn around or preen the back of the body, among others (^{Flower and Weary, 2003}; ^{Galindo and Orihuela , 2004}). The isolation of calves has several adverse effects on their well-being: it increases the frequency of vocalization and the presentation of abnormal behaviors, also increasing the physiological response to stress (heart rate and plasma cortisol levels), thereby affecting the use of nutrients, and consequently, the productivity of the animals. It can also impede the learning of essential social skills necessary to cope with the group environment that all dairy cattle are subjected to later in their adult life (^{Duve and Jansen, 2011}; ^{Horvat and Miller-Cushon, 2019}).

In a previous study, housing in pairs in lactation stage, resulted in more relaxed animals, but the evidence of improvement in productive indicators was not conclusive (^{Chua et al., 2002}). One of the main objectives of the science of animal welfare is to offer management strategies that reduce animal suffering, thus increasing their welfare state.

Therefore, the objective of this research was to evaluate the productive and behavioral response of lactating Holstein calves housed in pairs, compared to calves housed individually.

MATERIAL AND METHODS

Location

This research was carried out in the facilities of the University Ranch of the Institute of Agricultural Sciences (ICAP) of the Autonomous University of Hidalgo State, located in the municipality of Tulancingo de Bravo, Hidalgo, Mexico. The climate of Tulancingo de Bravo is temperate-cold, with an average annual temperature of 14 °C and a rainfall that ranges between 500 and 553 mm per year and is located at 2,580 m.a.s.l. (^{García, 2017}). Tulancingo is a fertile valley whose dairy basin produces approximately 40,000 L of milk per day, contributing a significant amount to the total state production, thus contributing to Hidalgo state occupying 10th place in the national production of cow's milk (^{SIAP, 2018}).

Experimental animals and management

30 one-week-old Holstein calves were used. Prior to the experiment from day 0 to 7 post-birth, all the calves received the same management, which consisted mainly of separating them from their mother and housing them in an individual pen where they were offered two 2 L colostrum intakes, each with a 6 h interval during the first day of birth; From day 2 after birth, they were fed 4 L of milk daily, two in the morning and two in the afternoon (without rejection), and from the first week of age they were offered commercial starter concentrate and free access water.

Treatments

The animals were housed for 8 weeks of the test in two treatments:

a) Treatment 1: calves housed in pairs. At one week of age, 20 calves were housed in pairs, using 10 pens of 2.40 x 1.25 meters, designed with an outdoor area and another covered area, having ten experimental units.

b) Treatment 2: calves housed individually. At one week of age, 10 calves were housed individually in pens of 1.20 meters x 1.25 meters, designed with an outdoor area and another covered area, having ten experimental units.

Los animales fueron alojados durante 8 semanas de duración de la prueba en dos tratamientos:

Variables of response

a) Productives:

b) Behavioral:

Two days a week the behavior catalog of each calf was recorded, observing them for three hours each day for eight weeks; A sweep sampling was used in combination with a recording at five-minute intervals for the states, and a behavioral sampling for the events (^{Martin, 2007}); obtaining a total of 576 sweeps for each calf during the experiment.

The behavioral variables considered in the behavioral catalog were:

The final percentage of sweeps recorded in each behavior was calculated for the states, and the frequency for the events.

Statistical analysis

A completely randomized experimental design was used; behavioral variables were analyzed using a completely randomized model in repeated measures; while the productive variables were analyzed using a completely random model; in both using the PROC GLM procedure of SAS (^{SAS, 2009}).

RESULTS AND DISCUSSION

Table 1 shows the means of the productive variables by treatment, finding no significant difference (p> 0.05) in the IW (T1 = 38.18 vs T2 = 39.58, kg), thereby confirming the homogeneity of the experimental units between treatments with respect to this variable. The type of accommodation did not significantly modify (p> 0.05) the total dry matter consumption; this variable being statistically the same in both treatments (T1 = 56.84 vs T2 = 56.85, kg). However, the type of housing significantly modified (p <0.05) the final weight, finding an increase of 14.52 % in calves that were housed in pairs, with respect to those housed individually (FW = T1: 75.288 vs T2: 65.747 kg). The variables total weight gain and daily weight gain were also modified by the effect of the type of housing, with increases of 41.81% and 43.04%, respectively, in calves housed in pairs (TWG = T1: 37.108 vs T2: 26.167, kg ; DWG = T1: 0.6683 vs T2: 0.4672, kg). Regarding feed conversion, significantly lower values (p <0.05) were obtained in 41.83% in the calves housed in pairs (T1 = 1.53 vs T2 = 2.17), which means that this group of animals used significantly less feed to achieve a kilogram of body weight gain, compared to calves that were housed individually.

The results of this research agree with what was found in other studies, where it is reported that calves housed in the company of another or other individuals of the same age, establish strong social ties with their companions, which allows them to better face different situations of stress; which results in a lower biological cost when facing these stressful situations, showing a greater productive response and a wide range of behaviors (^{Christensen et al., 2002}; ^{Chua et al., 2002}; ^{Tapki, 2007}; ^{Duve and Jansen, 2011}; ^{Jansen and Larsen, 2014}). It has also been found that calves housed in groups of 10 or less have better growth rates and lower morbidity rates associated with bovine respiratory disease, and with better levels of welfare (^{Nabais, 2011}; ^{Jansen and Larsen, 2014}).

Table 2 shows the means of the behavioral variables by treatment; It was found that the type of accommodation significantly modified (p <0.05) the percentage of time that the calves devoted to expressing the behaviors "standing", "walking", "drinking" and "vocalizing", finding that the calves that stayed in couples spent 16.26% less time standing (T1 = 26.57 vs T2 = 31.73), 24.77% less time walking (T1 = 2.52 vs T2 = 3.35), 23.52% less time drinking (T1 = 5.17 vs T2 = 6.76) and they vocalized 41.16% minus (T1 = 2.83 vs T2 = 4.81); compared to calves that were housed individually. During the experiment, the presence of diseases or abnormal behaviors was not observed in the animals of both treatments. The behavioral results of this research suggest that partner accommodation decreases the expression of stress-related behaviors, such as vocalizations. In this regard, ^{De-Paula et al., (2012)} indicate that under natural conditions, calves interact with other animals of the same age, so that individual housing limits or disappears this necessary interaction.

Calf housing methods have an important impact on the health and potential of the animal in its future production (^{Quigley, 2001}). Individual housing is believed to help control disease, abnormal sucking behavior, and food consumption. While it has been proposed that individual housing simplifies disease detection and general management of calves, the practice has been highly criticized for the fact that it limits the degree to which the calf can behave naturally (^{Nabais, 2011}; ^{Jansen and Larsen, 2014}).

Social isolation is stressful for calves and can impede the learning of essential social skills necessary to cope with the group environment to which all dairy cows are subjected in their adult life (^{Morisse et al., 2000}; ^{Quigley, 2001}; ^{Horvat and Miller-Cushon, 2019}). Individually housed calves show behavioral alterations, such as standing for long periods, walking more insecurely, are reluctant to change posture, and as they grow up they develop stereotypies motivated by lack of social activity.

Isolation at pre-weaning could also affect adaptation to group housing after weaning, especially with animals that had been in groups during the lactation period (^{Morisse et al., 2000}; ^{Quigley, 2001}; ^{Mandel et al., 2016}). In contrast, group-housed calves are safer (^{Nabais, 2011}), and show a greater repertoire of activities typical of the species, including the expression of social behaviors, such as play and learning from their peers (^{Duve and Jansen, 2012}; ^{Costa et al., 2016}). Social interactions can help them cope with stressful situations. Calves housed in groups or pairs are less stressed during handling and transfer and they are quicker to initiate solid feed consumption after weaning, compared to those housed individually; indicating that they are better able to cope with the elimination of liquid feeding (^{Cobb et al., 2014}).

The small group or couples accommodation system can be considered viable and economical in terms of health, development and behavior (^{Lakshman et al., 2009}; ^{Krachun and De-Pasillé, 2010}; ^{Faerevik et al., 2010}; ^{Duve and Jansen, 2011}). Even though the quality of housing may not improve nutrition and management when they are poor, it can ruin the effectiveness of an excellent management and feeding program. Herein lies the importance of studying the behavior of calves in relation to housing systems (^{Morisse et al., 2000}; ^{Costa et al., 2016}). This was verified in the present study, where under the same feeding scheme and with similar consumptions of milk and concentrate, the animals housed in pairs showed a calmer behavior and a better repertoire of behaviors typical of the species. This better behavioral performance was reflected in a better feed conversion, probably due to a better use of nutrients. The difference in the best feed conversion represents a greater economic benefit with the use of this system.

A final point of importance for the future of animal production units is the “ethical cost” in production (^{Capó, 2005}). If the ethical cost is high, it means that the animals are poorly treated and their welfare is low; those production units in which traditional management practices persist in causing unnecessary suffering to animals, will be less competitive in the market. The change has to do, not only with the fact that the animals must present absence of suffering, but also that they must experience pleasure, enjoyment (play behaviors, food of good nutritional/sensory quality, sex and social contact), which is essential to provide complete animal welfare (^{Balcombe, 2009}).

CONCLUSIONS

Under the conditions in which this research was carried out, in order to compare the housing systems in Holstein calves, the results found suggest that housing in pairs is more appropriate, compared to individual housing, since it increases the productive response and decreases the expression of behavioral patterns, indicators of stress such as vocalizations. Therefore, it is possible to conclude that housing with a partner improves the well-being of the calves.