A tethering system makes it easier to manage cows than grazing and free-stalls, and can prevent agonistic behavior and feed competition between cows. In dairy farming, cows fed in tethering systems produce more milk than those fed in free-stall and grazing systems [1,2]. Therefore, the tethering system is widely used worldwide [3–6]. However, the tethering system results in decreased expression of normal behavior [7], increased expression of abnormal behavior [8], and deterioration of the health status of dairy cows [9], thus reducing animal welfare (AW).

Tethered cows introduced for outdoor access have been known to have higher AW levels than those tethered throughout the day [10]. Outdoor access for one-hour per day (ODA) can promote normal behavior expression [11], reduce nipple infection [12], and enhance the immune function of dairy cows [13]. It has been suggested that outdoor access for ODA is one of the measures used to increase AW levels in tethered cows. However, their effects on milk yield and quality have not been clarified. A previous study suggested that tethered cows introduced to outdoor access produce less milk than tethered cows that spend the whole day tethered [14]. In their study, the time and frequency of outdoor access were unclear. Another study showed that extreme walking in cows reduced milk yield and altered milk composition [15]. The methods used in their study involved forcing the cows to walk. This forced walking, may cause stress to the cow, and may also create a labor burden on the farmer.

In this study, we adopted an outdoor access method in which the farmer dose not drive cows but treats them gently and evaluated the effects of one-hour daily outdoor access on milk yield and composition, and behaviors of tethered dairy cows. The one-hour daily outdoor access treatment period in this study was set at two weeks. The reason for setting this period is that when cattle changed from grazing to indoor tethering, urinary cortisol levels were higher in the first week and remained at the same level as during grazing from the second week [16]. This finding suggests that the response of the cattle to environmental changes may have weakened during the second week. In addition, the milk yield and composition analysis in this study used data collected from cows over 40 days of lactation. The reason for this is that the milk yield and composition vary with the number of lactation days. Holstein cows in Japan have an obvious peak in milk yield at approximately 40 days of lactation, which then decreased gradually, but milk fat, non-fat solids and milk protein were characterized by a marked decrease at the 40th day of lactation, followed by a moderate increase [17].

This study was performed in accordance with the Animal Experimental Regulations of Shinshu University (Approval No. 020031). Additionally, this study was also conducted in accordance with the rules and regulations of the Basic Guidelines for the Conduct of Animal Experiments at Research Institutes under the Jurisdiction of the Ministry of Agriculture, Forestry and Fisheries, Japan.

Tethered cows introduced for outdoor access produce less milk than cows that spend all day tethered [14]. However, in their study, the duration and frequency of outdoor access were unclear. In present study, a short duration of ODA treatment also reduced the milk yield of dairy cows. In addition, the cows in the middle and late lactation responded similarly to ODA. ODA promotes walking in tethered cows, thereby improving their AW [11]. However, although walking is one of the main behaviors of dairy cows, it also increases their energy consumption and decreases milk yield [15]. These changes in milk yield are due to the lack of additional supplement intake by milking cows to compensate for increased energy requirements during walking [15]. Thus, many studies have focused on the effects of walking distance (1 to 12.8 km) on milk yield in dairy cows [15,21–23]. However, in these studies, walking was promoted in cows; because it was a mandatory exercise, the cows may have been stressed. In this study, the cows were allowed to act freely for the one-hour of ODA. However, the spontaneous walking distance in this study was much shorter than those reported in previous studies. This suggests that the effects of ODA on milk yield may be related to factors other than walking. However, the reason for this requires further investigation.

Loberg et al [11] reported that there was no difference in milk yield between cows fed ODA and those tethered all day. They divided the cows into four groups (13 cows each): exercise every day, two days per week, one day per week, and no exercise. The effects of the feeding treatments on milk yield were analyzed considering the effects of cow age and lactation stage. Lactation was divided into four stages: 1 to 3 months after parturition, 4 to 8 months after parturition, dry cows, and heifers. However, milk yield varies with the number of days of lactation, with a clear peak at approximately 40 days, followed by a decrease [1,17]. Therefore, in the study by Loberg et al [11], owing the long duration of each lactation stage, the analysis of the effect of feeding treatment on milk yield may inevitably be influenced by the difference in individual lactation days. In present study, we analyzed the difference in milk yield after 40 days of lactation to exclude the effect of lactation days on milk yield.

In the present study, milk fat rate was increased by ODA. This result is consistent with previous studies that investigated the effects of walking on milk quality [15,24]. The increase in milk fat rate by ODA may be due to a decrease in milk yield [15]. In addition, the milk protein rate in ODA tended to increase in this study. This may also be attributed to decreased milk yield [15]. On the other hand, the lactose rate was decreased by ODA treatment in present study. Daytime outdoor access has been reported to reduce lactose rate in milking eyes [25]. Moreover, the lactose content of dairy cows under pasture-raising conditions is lower than that of indoor-raised cows [26]. However, the cause has not yet been elucidated, and further investigation is required.

ODA increased MUN concentration in this study. MUN concentration can be used to monitor the nutritional status of dairy cows during lactation [27]. An increase in MUN may indicate that cows consume excessive protein [28]. It suggests that ODA might promote protein digestion in dairy cows. In addition, ketone bodies and free fatty acids were increased by ODA. During the early stages of lactation, feed intake decreases with an increase in milk yield, resulting in a negative energy balance in cows. The animal draws on body fat reserves to provide the energy needed for milk production, thereby increasing the concentrations of free fatty acids and ketone bodies [29]. In this study, the cows subjected to ODA may have consumed more energy than those left tethering, which may have led to an increase in the concentration of free fatty acids and ketone bodies.

The AW grade evaluation criteria recommend measuring the lying time of cows [30]. Our results showed that the mean total duration per day of lying during ODA was significantly lower than that before ODA, but the normal behaviors such as walking and social behaviors increased. Similar results have been reported in dry cows [13]. This suggests that ODA may have promoted normal behavioral expression, except for lying, owing to the increased available space for tethered cows gained through outdoor access for one-hour [13]. The expression of social behavior is used as an evaluation criterion for AW levels [10]. In addition, it has been reported that head butts in tethered cows with outdoor exercise is more frequent than those in cows that are left tethering all day [10]. However, increased agonistic behavior may indicate uncomfortable or stressful situations [31]. In this study, observed social behaviors were gently pushed through head-to-head interactions. This is probably because the cows were together during daily milking or during the two weeks outdoor access period, so they did not see each other as enemies and had lower levels of aggressive behavior.

Because milk yield varies depending on various environmental factors, this study used all milking cows housed in the same environment. Therefore, this study determined the extent of milk yield decrease during, before and after ODA's introduction. However, because the study was conducted in the fall when the weather was relatively mild, further research should be conducted in different seasons and outdoor paddock sizes.

ODA resulted in a decrease in milk yield and a change in the composition of dairy cows in the tethering system. These results suggest that ODA can lead to a significant reduction in benefits to farmers. However, ODA may have promoted normal behavior expression, owing to the increased available space for tethered cows gained through outdoor access for one-hour, towards that typically found in grazing or free barn feeding systems. This is highly effective in improving the AW of tethered cows. Our results provide useful scientific knowledge on the dissemination of AW in tethering dairy cattle production sites. However, the balance between AW improvement and production efficiency remains problematic. This may require government support or incur a consumer burden. Further studies are necessary to confirm this hypothesis.