Introduction

Calf morbidity and mortality are major problems for dairy cattle farms worldwide (^{Romha, 2014}). Raising healthy calves is one of the main requirements to ensure farm profitability, and it depends highly on successful calf feeding and management programs. The rate of calf mortality is estimated to be over 10% in Turkey and 5-10% in Europe (^{Küçükoflaz et al., 2022}).

Calves are born as monogastric animals since their rumen is not fully developed and functional. Thus, milk is the main source of nutrients for the newborn calves. Initiation of rumen activity is only possible by consumption of solid feed, such as starter diets. Transition from milk to solid feed is a major cause of stress for calves especially when the rumen is not fully prepared for digesting solid feeds. However, in dairy cattle farms, the pre-weaning period is intended to be as short as possible since milk is the primary source of income. If calves are raised together with their dams, the milk feeding period takes approximately 10 months (^{Eckert et al., 2015}). While weaning at a greater age causes economic losses for the farm, early weaning may result in deficiencies related to growth, development, health, and performance of calves due to poor nutrient supply during pre-weaning (^{Khattak et al., 2018}). Hence, several studies have been conducted to determine the optimum weaning time of dairy calves (^{Kehoe et al., 2007}; ^{Laswai et al., 2007}; Passillé et al., 2011; ^{Özkaya and Toker, 2012}; ^{Koçyiğit et al., 2015}; Khattak et al., 2018; ^{McCoard et al., 2019}; ^{Schwarzkopf et al., 2019}; Schwarzkopf et al., 2022).

Early-weaning methods are based on encouraging calves to consume solid feed by keeping milk constant at 10% of calves’ birth weight (^{Özhan et al., 2001}). Several weaning methods are widely used in dairy farms, most of which focus on age or certain amount of feed consumed by calves to stop milk feeding. In intensive dairy cattle farms weaning is performed mostly when calves are 7-8 weeks of age (^{Schwarzkopf et al., 2019}; ^{Palczynski et al., 2020}). It is also common to wean calves when their daily starter feed consumption reaches over 750-1000 g in two consecutive days (^{Koçak and Güneş, 2005}). Even though feed consumption may provide an idea about rumen development, the age of a calf alone is not enough to decide whether it is ready to be weaned. Weaning calves according to body weight (BW) instead of age was first suggested by ^{Bell (1958}). This method could be more appropriate since the physiological age of calves is equalized. However, it is not widely practiced and there is little information about it. Therefore, the aim of this study was to determine the effect of weaning weight on growth performance, feed efficiency, and behavioral traits, as well as the feed cost of dairy calves.

Materials and Methods

The study was carried out at the research farm of the Food and Livestock Application and Research Center of Atatürk University, at Erzurum, Turkey. Sixty-seven newborn purebred dairy calves [Holstein-Friesian (n=37) and Brown Swiss (n=19)] were used in the study. For the first three days after birth, the calves were housed together with their dams to receive colostrum. Subsequently, they were randomly allocated into four weaning weight groups [weaning at 60 kg (n=18), 65 kg (n=19), 70 kg (n=18) and 75 kg (n=12)]. The calves were housed in individual pens equipped with feeders and water buckets. Each pen was 150 cm long and 120 cm wide. Straw bedding was provided to the calves. Straw was replaced daily to keep the bedding clean and dry.

Calves were fed milk form plastic bottles until weaning. The amount of milk offered was kept constant at 10% of birth weight until weaning, as suggested by ^{Eckert et al. (2015}). Calves were fed milk twice a day and had ad libitum access to water. From birth to the end of the study starter feed and dry hay were available in the pens. The starter feed offered contained barley grain (30%), wheat bran (20%), wheat grain (10%), molasses (5.7%), cotton meal (30%), salt (1%), limestone (3%), vitamin premix (0.2%), and trace mineral mixture (0.1%). The chemical composition of the feeds used in the study is presented in Table 1. Calves were offered starter feed and dry hay every morning and the leftovers from the previous day were weighed and recorded to determine consumption.

Body weights and measurements including chest depth, body length, height at withers, fore shank circumference, and hearth girth were measured and recorded at birth, weaning and four months of age. A measuring stick and a tape were used to determine body measurements. A 10-g precision scale was used to weight the calves.

The instantaneous sampling method was used to collect behavioral data (^{Kartal and Yanar, 2011}). Behavior was recorded for each of the following activities: eating (head of the calf is in the feeder), drinking water (head of the calf is in the water bucket), lying (calf is lying on the ground), standing (calf is standing still). Throughout the study, calf behavior was determined and recorded weekly by walking from 9 to 12 a.m. through the calf barn at least 2 m from the pens, at 15-min intervals. The scores obtained for each activity were divided into the total behaviors and the results were multiplied by 100 to calculate percentage of time spent in each behavior.

Results

Least square means and standard errors for weight and daily weight gain of calves are presented in Table 2. There was no significant difference between breeds in terms of weight at birth and at four months of age. However, daily weight gain of Brown Swiss calves between weaning and four months of age was significantly higher than Holstein calves (p<0.05). Sex had no effect on weight or weight gain. No significant difference was observed among the four weaning weight groups in terms of weight at birth and four months of age as well as daily weight gain from birth and weaning to four months of age. Nevertheless, daily weight gain of calves weaned at 60 kg of BW was significantly (p<0.05) lower than the other three groups during the pre-weaning period.

Feed efficiency at different time periods is shown in Table 3. While breed had no significant effect on feed efficiency during pre-weaning, feed conversion ratio of Holstein-Friesian was significantly higher than Brown Swiss calves between weaning and four months of age, as well as between birth and four months of age (p<0.05). Sex and weaning weight had no significant effect on feed conversion at various time periods.

Least square means and standard errors for body measurements at different time periods are presented in Table 4. Holstein-Friesian was significantly superior to Brown Swiss calves in terms of body length and hearth girth growth in the pre-weaning period (p<0.05). Similar results were also observed in the period from birth to four months of age (p<0.05). Breed had no significant effect on other measurements such as height at withers, chest depth and cannon bone girth. Growth of different body measurements was not influenced by sex. No statisticalifference was found among weaning weight groups in terms of growth of body measurements in the periods from weaning and birth to four months of age. However, growth of chest depth in the group weaned at 60 kg was lower than the other three weaning weight groups (p<0.05).

Least square means and standard errors for behavioral activities of calves at different periods are presented in Table 5. Breed had an effect (p<0.05) on standing and lying behavior of calves. Brown Swiss spent more time standing and less time lying compared to Holstein calves in the pre-weaning stage. Similar results were also observed in the period from birth to four months of age. However, in the post-weaning period only the time spent lying was significantly higher in Holstein calves.

Sex effect on behavior was also analyzed. There was no significant difference between sex groups in the pre-weaning period. Male calves spent more time lying between weaning and four months of age (p<0.05). Standing behavior was significantly higher in female calves in the period from birth to four months of age (p<0.05).

Weaning weight had no effect on behavior in the period from weaning to four months of age. Comparable results were also observed in the 4-month period starting from birth. While there was no difference among groups in terms of standing, lying or feeding, the water drinking behavior was notably (p<0.05) higher during the pre-weaning period in calves weaned at 75 kg of BW.

Discussion

Greater weight gain in the early life of calves might influence later performance of dairy heifers (^{Nikkhah and Alimirzaei, 2022}). There was no significant difference between both breeds in terms of daily weight gain in the pre-weaning period and first four months. However, Brown Swiss calves had notably higher weight gain in the period between weaning and four months of age (p<0.05). These findings agree with ^{Güler et al. (2003}), who reported higher daily weight gain for Brown Swiss calves compared to Holstein-Friesian. Sex had no effect on daily weight gain of calves in any stage of the first four months of life. Comparable results were also reported by ^{Lambertz et al. (2015}).

Early weaning may result in deficiencies in weight gain and growth performance. Daily weight gain of calves weaned at 60 kg was significantly inferior compared to the other three groups in the pre-weaning period (p<0.05). This result is supported by ^{Miller-Cushon et al. (2013}), ^{Yavuz et al. (2015}), ^{Abbas et al. (2017}) and ^{Khattak et al. (2018}) who reported that late weaning increases BW. In contrast, there was no difference among weaning weight groups in terms of four-month weight and daily weight gains in the periods between weaning to four months of age as well as birth to four months of age. Our findings show that the gap between light and heavy weaned calves in terms of daily weight gain and BW is close in the post-weaning period. Our results agree with the findings reported by ^{Hopkins (1997}) and ^{Koçak and Güneş (2005}).

Genotype is a major factor affecting calf efficiency. Brown Swiss were superior to Holstein calves in terms of feed efficiency in the post-weaning and 4-month periods (p<0.05). Our results agree with ^{Güler et al. (2003}). Sex had no significant effect on feed efficiency. These results are comparable to the findings by ^{Koçyiğit et al. (2015}). Although feed conversion efficiency of calves increased gradually as weaned weight increased, this difference among groups was not significant. Similarly, ^{Winter et al. (1985}) and ^{Kehoe et al. (2007}) reported that weaning at different ages had no effect on feed efficiency of calves.

Adequate body development in early life is essential for calves to achieve acceptable body structure later in life. Breed is a source of variation for changes in different body measurements. Increases in body length and hearth girth of Holstein-Friesian were higher than Brown Swiss calves in the pre-weaning period. However, in the post-weaning period until four months of age, Brown Swiss calves were superior to Holsteins in terms of body length and hearth girth (p<0.05). The differences for these measurements were not significant in the period between birth and four months of age. ^{Ertugrul et al. (2000}) reported that Holstein calves had higher average values for body measurements than Brown Swiss from birth to 6 months of age. Their findings differ with our results. Weaning weight did not have significant effect on body measurements, except for chest depth, which was higher in the pre-weaning period for the group weaned at 75 kg. Similar results have been reported in several studies (^{Kehoe et al., 2007}; ^{Rashid et al., 2013}).

Behavioral activities are used as indicators of animal comfort (^{Anderson, 2001}). Lying bout of Holstein-Friesian was significantly longer than Brown Swiss calves in the pre-weaning and 4-month period (p<0.05). In contrast, Brown Swiss calves spent more time standing in both post and pre-weaning periods (p<0.05). Lying bout of male calves was longer in the post-weaning period (p<0.05). On the other hand, female calves spent more time standing compared to male calves between birth and 4-months of age (p<0.05). There was no difference between sex groups in other activities and periods. Comparable results were also reported by ^{Koçyiğit et al. (2015}). Weaning may cause severe stress for calves, especially when the rumen is not developed enough. Changes in the behavior may be used to determine stress and discomfort. In the present study there was no difference among weaning weight groups in terms of behavioral activities, except for water drinking. Our results showed that weaning at lighter weights did not significantly affect the behavioral patterns of dairy calves. The findings by Koçyiğit et al. (2015) agree with the results of the present study.

Different weaning weights had a remarkable effect on the cost of calf feeding. Average feeding cost for calves weaned at 60, 65, 70 and 75 kg were USD$2.72, USD$2.77, USD$2.84 and USD$2.96 per kg live weight gain, respectively. Increasing weaning weight resulted in higher feeding costs due to higher amount of milk consumed. Calves weaned at 75 kg consumed approximately 73 L more milk compared to calves weaned at 60 kg. Our results agree with the findings by ^{Bjonklund et al. (2013}) who reported that feeding cost of calves weaned at 30 days of age (62 kg; USD $3.02) was lower than that of calves weaned at 90 days (108 kg) in an organic production system. Similarly, ^{Koçyiğit et al. (2015}) reported that feeding cost of calves weaned at lighter (53.7 kg) and heavier (74.3 kg) weights were USD$2.18 and USD$2.41, respectively. Comparable results were also found by ^{Khattak et al. (2018}) who reported lower feeding costs for calves weaned at 70 days compared to those weaned at 90 and 110 days of age.

In conclusion, our findings provide clear evidence that weaning dairy calves at 60, 65, 70, and 75 kg of BW does not have a significant effect on growth performance, weight gain or feed efficiency during a 4-month period. Moreover, no sign of stress was observed in the early weaned calves as compared to calves weaned at heavier weights. However, higher weaning weights resulted in higher feeding cost. This study suggest that dairy calves can be weaned at 60 kg of BW without any detrimental effect on performance and the feeding cost per kg weight gain is significantly lower when compared to higher weaning weights.