4.1. Pasture Characteristics, Forage Removal, and Animal Performance
The results of our study show that an increase in HM causes changes in the structure of the pasture, resulting in an increase in the pasture height and size of the plants, which is expressed in a longer length [
2] and lamina width. These structural characteristics promote an increase in DMI [
17] in the first 60 min of the GS, that was reflected in the greater amount of forage removed in the HHM treatment (
Table 1), which is consistent with the results obtained by McEvoy et al. [
2] and Wales et al. [
14].
In the present study, it was observed that MHM contained lower amounts of pre-grazing ryegrass pseudostems, but higher concentration of pre-grazing ryegrass leaves than HHM. Similar findings have been reported in other studies that analyzed the effect of HM on pasture characteristics and DMI [
2,
18,
19]. The content of pre-grazing ryegrass leaves reported in our study was higher than observed by Muñoz et al. [
18], and similar to those reported by Wims et al. [
19] and Curran et al. [
1], particularly in MHM. This is due to the fact that, in our work and in Wims et al. [
19] and Curran et al. [
1], the pasture samples were taken above 4 cm, while in Muñoz et al. [
18], the samples were taken at ground level. The content of pre-grazing ryegrass pseudostems was higher in HHM in relation to MHM, which according to Tharmaraj et al. [
20] would cause a negative effect on the DMI due to the increase in pasture resistance to prehension. According to Curran et al. [
1], the negative effect of a high HM on DMI due to the increase in the proportion of stems and the resistance offered to the grazing process by them, would be reduced with a high post-grazing HM which coincides with the results of our study, where the post-grazing HM was greater than 1500 kg DM/ha in both treatments.
These results contrast with that reported by Muñoz et al. [
18], who obtained a higher DMI with lower pre-grazing HM evaluated above 3 cm, however the difference in pre-grazing HM among treatments was ca. 3000 kg DM/ha and therefore was affected by differences in the proportion of pseudostems, and therefore different nutritional value of the forage. Thus, the comparison of DMI between experiments should be carried out with caution since the results will depend on the height of the allocation of herbage allowance [
2,
8,
21]. According to Pérez-Prieto et al. [
8], when comparing the effect of the HM at the same herbage allowance above ground level, it is observed a positive effect of the HM on DMI, which is consistent with our experiment. On the other hand, when comparing HM at the same herbage allowance above 4 or 5 cm, it was observed a negative effect of HM on DMI [
1].
In our experiment, the nutritive value of pre-grazing pasture was relatively similar between treatments, so the structural characteristics of the pasture could become more relevant to the forage removed than the nutritional value of the pasture [
21].
Despite the greater amount of pasture removed in the HHM treatment, milk production was higher in the MHM treatment, which is consistent with the results obtained by Muñoz et al. [
18] and Curran et al. [
1]. The higher milk production in pastures managed with low HM compared to high HM is related to a higher nutritive value of the forage offered [
8,
18], which was not observed in our study. According to Amaral et al. [
22], treatments that promote an increase in defoliation depth, result in a higher proportion of stems and sheaths consumed by dairy cows which makes the forage selection process more difficult. In our study, a tendency towards a greater defoliation depth was observed in the HHM treatment, which could have influenced diet selection by the cows in that treatment. Moreover, we observed that MHM cows had a lower nutrient selection for ADF and tended to a greater selection of DOMD compared with HHM cows, which could explain the greater milk production in the MHM treatment. On the other hand, the increase in milk fat concentration under HHM is in accordance with other studies normally linked to relatively higher fiber concentrations [
19].
4.2. Grazing Behavior, Herbage Intake, and Herbage Disappearance during the First Grazing Session
During the first GS, grazing behavior was not affected by the HM treatments, with similar grazing, rumination and idling times. The increase in bite rate was the behavioral adaptation of MHM cows trying to maintain a high DMI, whereas in HHM the structure of the pasture (longer lamina length and greater lamina width) allowed a higher bite mass [
22,
23]. However, it should be noted that the process of DMI during the GS generated changes in the pasture structure in both treatments, which indicates that only during the first 60 min after the start of the GS the bite mass was greater in HHM.
Several studies have indicated that bite mass is the most influential component of the DMI in grazing dairy cows [
4,
23,
24], thus cows offered a HHM presented a higher DMI during the first 60 min of the GS, associated with the greater bite mass registered in this treatment during that time interval [
22]. Thereafter, the bite mass was similar between treatments, due to the decrease in pasture height and therefore the available HM, which is consistent with the results of Stakelum and Dillon [
25] and Gregorini et al. [
4]. Bite rate was higher in MHM throughout the whole GS, which is a behavioral adaptation of cows to the pasture height and available HM to maintain daily DMI [
24].
During the first 60 min of the GS, DM disappearance was greater in the HHM treatment than in the MHM treatment, which is consistent with the greater bite mass and amount of pasture removed in this treatment. After 60 min, the process was similar between treatments according to the ingestive behavior of the cows [
22]. The ease of the grazing-down process is reduced to the extent that the cows graze lower strata of the pasture, an effect that increases with high HM, which would explain the similarity in DM disappearance between the treatments after the first 60 min of the GS [
19,
22].
It is important to highlight the meaning of the first GS on ingestive behavior and DMI [
26]. The pasture removed after 24 h of grazing (
Table 1) is related with DMI during the first GS (
Table 3) and that a substantial amount of daily DMI is realized during first 60 min after the grazing event started, which is consistent with the results of studies by Enriquez-Hidalgo et al. [
9] and Gregorini et al. [
4].
4.3. Nutritive Value of the Pasture during the Grazing Down Process in the First GS
The HM influenced the DMI and the nutritive value of the residual HM during the GS. Despite the difference in HM between treatments, the nutritional value of the pasture offered was relatively similar, with only significant differences in the pre-grazing content of ADF, which is consistent with that obtained by Wims et al. [
19] in pastures managed in the range of 1150 to 2000 kg DM/ha and by Pulido and Leaver [
27], with ranges of HM between 1680 kg DM/ha to 2790 kg DM/ha, however the difference between MHM and HHM (+10.8 g/kg for HHM) is of relatively low biological importance, and would not influence the productive performance of animals. In several studies where significant effects of HM on the productive performance of cows were observed [
8,
18,
27], it is feasible to observe that, when comparing high and low HM, the differences between treatments in the pre-grazing ADF content are in the range of 40 to 60 g/kg, values higher than those observed in our study. The HHM treatment was generated by delaying the regrowth period before the start of the study in relation to the regrowth period of MHM (+15 days of regrowth), which may influence fiber concentration in the pasture and, consequently, its digestibility [
2,
28]. However, the higher content of pre-grazing ADF and during the first 120 min of the first GS in HHM did not affect the concentration of DOMD, as fiber digestibility in a vegetative leafy spring pasture is high (>800 g/kg) and therefore an increase in ADF does not necessarily result in lower DOMD [
29]. According to Chapman et al. [
30], the decline in the digestibility of the pasture is more related to the progress of the growing season and, therefore, by the location of the leaves insertion points on the tiller axis during the pasture growth cycle. Given the above, it is likely that the difference in the regrowth period between treatments was not sufficient to generate changes in the DOMD.
The variation in ADF content during the first GS is directly related to changes in the morphological composition of the pasture (
Table 1), due to the process of grazing and selection of cows. The highest ADF content in HHM in the first 120 m of the GS coincides with the highest proportion of pre-grazing ryegrass pseudostem that were observed in this treatment in relation to MHM, which is consistent with the results reported by Muñoz et al. [
18]. Then, the ADF content was similar between treatments, which are consistent with the morphological composition at the end of the GS.
Several studies have observed that the CP content is higher in low HM pastures [
31,
32,
33], which contrasts with that obtained in our study and Pulido and Leaver [
27]. Even though there were no significant differences in the pre-grazing CP content in this experiment, it was numerically higher in MHM (+11 g/kg). Throughout the GS, selection processes were conducted by the cows and may have resulted in a higher CP concentration after the first GS for MHM compared with HHM.
However, the nutrient selection differential at the end of the first GS was similar for most of the variables, except for ADF content and a trend for DOMD. This result differs from that reported by Tharmaraj et al. [
20], who observed that the pre-grazing pasture height (which caused different HMs) did not affect the selection differential. In the aforementioned study, post-grazing HM was higher in the treatment with higher pasture height (high HM), while in the present experiment post-grazing HM was similar between the treatments, as a consequence of a restricted herbage allowance (25 kg/cow/d at ground level). As a result of the above mentioned and, due to the higher pre-grazing ADF content in HHM, those cows were forced to consume a greater proportion of ADF than cows offered a MHM. On the other hand, MHM cows tended to select for DOMD.
In general, a higher intake of ADF correlates with a decrease in the availability of nutrients digested by cows, due to the greater rate of ruminal filling and decrease in the rate of digestion and passage of forage, which negatively affects milk production [
34].
In the present study, the difference in ADF content was of low biological importance. However, if the highest DMI obtained in HHM is considered, it is likely to produce changes in nutrient digestion and, therefore, in the productive performance of the cows. Similarly, it should be considered that, to the extent that the following GS occur after the initial 240 min of grazing, the nutritional value of the forage in HHM should tend to be lower than in MHM, due to the progressive decrease of the amount of leaves in the pasture.
This may explain the greater productive performance of cows offered MHM compared with those offered HHM despite the greater herbage removal for cows offered HHM and that at the beginning of the GS the nutritional value was similar among HMs.
These results are relevant for strip-grazing systems, particularly when determining to allocate one or more strips per day, or to define the best moment (based on HM) at which a new grazing sector should be offered to dairy cows with the objective to combine a high pasture DMI with high nutritional value.