Animal performance
The BW, DMI, ADG, and FCR of treatments according to the energy and protein levels in the feed of Hanwoo heifers are presented in
Table 3. Initial (0 day) BW of T-0.2, −0.4, and −0.6 were 316.7, 318.3, and 320.6 kg, respectively. There was no significant difference in BW among treatments during the whole experimental period. However, T-0.2, −0.4, and −0.6 in final BW (63 day) were 348.3, 353.6, and 361.2 kg, respectively, and the weight gain difference during the whole period between T-0.2 and −0.6 was 9.0 kg.
The DMI of T-0.2, −0.4, and −0.6 was 5.75, 5.88, and 6.07 kg/d, respectively, and a constant amount was ingested with almost no residual throughout the entire period of the experiment. Thus, there was no significant difference among treatment groups. This is because the level of energy and protein required for weight gain by treatment groups was adjusted by setting different ratios of rice straw and formula feed, not DMI. In other words, the feeding ratios of rice straw for T-0.2, −0.4, and −0.6 were 55% (3.07 vs 2.67 kg for rice straw and formula feed), 45% (2.85 vs 3.04 kg) and 40% (2.63 vs 3.44 kg), respectively. Accordingly, the TDN intakes of T-0.2, −0.4, and −0.6 were 2.58, 2.75, and 2.95 kg/d, and the CP intakes were 0.54, 0.59, and 0.64 kg/d, respectively.
In the 1st period, the ADG of T-0.2, −0.4, and −0.6 was 0.59, 0.57, and 0.62 kg/d, respectively, and there was no significant difference. This is probably because the nutritional status of heifers before the experiment period was relatively good (ADG about 0.62), so even if the nutritional level was lower than the required amount for T-0.2 and −0.4, the difference between the treatment groups did not appear immediately after one month. The ADG in the 2nd period was 0.36, 0.56, and 0.72 kg/d, respectively, and showed a significant difference (p<0.05) among treatments. However, the ADG values of the T-0.2 and −0.6 treatment groups differed only twice, not three times. Although there was no significant difference in the first period, the result from the 1st period was not excluded from the results of the entire period because the weight appears as an accumulated result. When combined with those of the 2nd period, the ADG of the whole period was 0.48, 0.56, and 0.65 kg/d (p<0.05), respectively. The target ADG (T-0.2, 0.4, and 0.6 kg/d) was set based on NIAS (2017) [
2], but the experimental results showed higher values than the target ADG in all treatment groups. Based on the results of this experiment, the ADG presented by NIAS (2017) [
2] is probably underestimated, and more experiments on the energy and CP requirements of Hanwoo heifers are deemed necessary in the future.
In addition, the lower the target daily weight gain, the greater the difference from the predicted weight gain with a relatively higher weight gain, i.e. T-0.2 (target ADG) vs 0.48 (result). This is probably because the regression equation of nutrient requirement is linear. In general, net energy (NE) for maintenance and weight gain is measured through the ratio of the changes in energy used for accumulation in the animal body (retained energy, RE) and energy consumed through feed (intake energy, IE); NE = ΔRE/ΔIE. The measurement of NE using this method assumes that the relationship between ingested feed and stored energy is linear. However, in the actual animal body, it has a curved reaction rather than a straight line, and the rate of energy stored decreases as feed intake increases (
Figure 1) [
3]. Thus, the higher the weight gain, the lower the efficiency of use of the ingested feed. Conversely, the lower the weight gain, the higher the feed utilization efficiency. The curvilinear relationship between ingested feed and stored energy consists of two aspects: i) ingested feed and reduced body tissue (− RE) and ii) ingested feed and increased body tissue (+ RE). Comparing the slopes between − and + RE, it can be seen that the feed consumed for net energy for maintenance (NEm) is more efficiently used than for net energy retained (NEr). Therefore, while the animal’s response is curved, the energy requirement is determined through a single linear regression equation. Accordingly, it is interpreted that the treatment group with a relatively low ADG showed a higher weight gain.
As a result of the experiment in this study, regression equa tions for calculation of TDN (
Figure 2) and CP requirements (
Figure 3) were derived according to the observed BW by the level of ADG. In addition, based on the ADG results, regression equations for TDN [
Eq. 1] and the CP requirements [
Eq. 2] according to the target ADG level and BW were derived as follows:
Comparing results from regression equations [
Eq. 1,
2] and NIAS (2012, 2017) [
1,
2] for TDN and CP requirements according to the target daily gain based on the 300 kg BW of heifer are presented in
Table 4. When compared with NIAS (2012) [
1], TDN requirements were 26.66%, 28.19%, and 29.37% lower, respectively, and CP requirements were 5.81%, 10.05%, and 13.63% lower, respectively. In addition, compared with NIAS (2017) [
2], 20.27%, 23.46%, and 26.01% lower values for TDN and 4.04%, 6.89%, and 9.38% lower values for CP were derived, respectively. Compared with the Korean Feeding Standard for Hanwoo, the TDN and CP requirements derived from this experiment show that the reduction rate increases as the target daily gain increases from 0.2 to 0.6 kg/d. This is because the coefficient of both TDN and CP for ADG in the regression equation derived from this experiment is lower than that of the regression equation [TDN = 0.169+(0.009×BW)+(1.176×ADG); CP = 0.009+ (0.002×BW)+(0.119×ADG)] of NIAS [2]. It is also because the nutrient requirement value at the daily gain of 0 kg/d was lowered.
There was no significant difference in FCR between treat ment groups in the 1st period, but during the 2nd period, they were16.20, 10.57 and 8.29, respectively, with T-0.2 being higher than T-0.4 and −0.6 (p<0.05). The whole period was 11.79, 10.66, and 9.54, respectively, indicating that the lower the target daily gain, the higher the feed requirement. This is because, although there was no difference in DMI among treatment groups, T-0.2 fed a relatively high proportion of rice straw resulting in low daily gain.
Animal behavior
The results from animal behavior according to the energy and protein levels in the feed of are shown in
Table 5. The experiment was observed by dividing day time (0700 to 1800, 11 h) and night time (1800 to 0700, 13 h) throughout the day. It was confirmed that the majority of standing, eating, walking and drinking occurred during the day time, while the majority of lying and rumination occurred at the night time. In other words, standing, eating, walking, and drinking in the day time were 73%, 100%, 65%, and 94%, respectively, if the average value of the three treatments was calculated as a percentage based on the time data. Lying and rumination in night time were 86% and 76%, respectively.
Lying, standing and walking of T-0.2, −0.4, and −0.6 showed no significant difference in day time, night time and a whole day. Lying, standing, and walking are behaviors that can confirm the degree of anxiety, discomfort and comfort, and no significant difference was found in this experiment [
10]. In addition, in the case of anxiety, crying is accompanied while walking [
10], but such behavior was not observed during this experiment. The purpose of this study was not to focus on changes in animal behavior due to hunger stress but to examine changes in the BW of heifers while minimizing these stresses. Therefore, it was confirmed that there was no change in normal behavior, such as increased standing and walking time due to hunger stress, even if the energy and protein levels in the feed were lower than the required amount when the DMI was constant by adjusting the roughage and concentration ratio.
Eating time was longer (p <0.05) at T-0.2 than T-0.6 during the day time. The eating time becomes shorter when the amount of feed intake is low or the ratio of forage intake is low [
12]. The NDF intake also affects chewing activity (eating and rumination), and is known to be from 111 to 209 minute (min) per kg of ingested NDF [
13,
14]. In this experiment, since the DMI was constant, it is thought that this difference appeared due to the effect of the difference in the ratio of the roughage intake and the resulting NDF intake.
The rumination affected by the ratio of roughage intake [
12] also showed a significant difference (p<0.05) by the treatment groups at whole day (281.91, 331.57, and 247.28 min/24 h) and day time (65.31, 99.68, and 44.75 min/11 h). However, the longest time was observed at T-0.4, not T-0.2 where the ratio of roughage intake was highest. Since there was no significant difference in rumination at night time, the significant difference found in this behaviour is due to day time. Day time is the time of highest activity during a 24 hour period, and rumination is not the main activity. Therefore, it is considered that the significant difference may be due to the individual difference, not the feed.
Drinking is affected by the type of feed [
15] and moisture content [
16]. In this experiment, a significant difference between treatment groups was shown as 0.17, 0.19, and 0.61 times (p<0.05) in the number of drinking at night time, but the difference between among groups was less than 1 time, so it is unreasonable to judge the effect of feed according to energy and protein levels.