4.1. Grazing Management
In this study, all the cows grazed in similar pre-grazing pasture conditions. The average values recorded for pre-grazing HM and pre-grazing CSH (2292 kg DM/ha and 8.1 cm, respectively) agree with those reported by Ruiz-Albarrán et al. [
22] and King and Stockdale [
42] for cows grazing in an autumn pasture and fell within the range of recommended pre-grazing HM in Southern Chile during the autumn [
43]. Although it is widely accepted that an increase in pasture utilization can be achieved by decreasing DHA [
44] or increasing the stocking rate [
45], the total amount of herbage utilized is seldom reported. Average pasture utilized per grazing event was 825 kg DM/ha and 784 kg DM/ha for low and high DHA, respectively. Given that cows offered a low DHA had approximately a 29.4% less offered area (−32.4 m
2/cow.day) than those on the high DHA regime, the post-grazing HM and post-grazing CSH values decreased by 7.5% and 10.8%, respectively. In addition, the treatment under the low DHA conditions exhibited an increased instantaneous stocking rate (+1.1 cows/ha/day) compared to those of the high DHA condition, whereas the efficiency of harvesting remained unaffected. Despite these differences in the post-grazing residual, no differences were observed in the apparent forage DMI per hectare between DHA treatments. Therefore, the apparent forage DMI per cow increased on average by 0.28 kg DM per kg of increment in DHA. This agrees with the range reported previously of 0.10 to 0.30 kg DM when the DHA is estimated above the ground level [
6,
10,
46,
47,
48]. The increased forage DMI per cow in the high DHA treatments might be explained by the greater amount of forage allocated per cow [
49] and the greater grazing area.
4.2. Sward Characteristics
Given that the experiment was carried out in autumn and early winter with pastures dominated by
L. perenne L. in the vegetative stage, a high proportion of leaves and high nutritive quality of the forage offered (>4 cm of height) were found. The forage CP and ME concentrations in available pasture found in this study fall within the range of high-quality pastures from Southern Chile in the autumn [
50]. Regarding the distribution of HM in the vertical plane, the greater proportion of HM was positioned below 4 cm in height, which averaged 53.1% DM between both DHA levels, which is in agreement with the study carried out in Valdivia (Southern Chile) during a four-year period [
15].
The absence of differences between the DHA levels in botanical and chemical compositions, pasture density, and plant weight could be explained by the short experimental period used and by the fact that all the pastures used in the present study had similar characteristics, according to the measurements carried out during the pre-experimental period. Moreover, the pre-grazing HM was controlled for each of the four herds and the post-grazing sward height corresponded to the range of 4 to 6 cm, as recommended by Donaghy and Fulkerson [
51] for an effective grazing management system for ryegrass-based pastures. The lack of short-term DHA effects on all sward characteristics evaluated in the present study are consistent with the long-term effect reported by Merino et al. [
15] when comparing DHA of 20 kg DM/cow.day and 30 kg DM/cow.day in the spring. Coincidently, in a recent study performed in Uruguay studying the effect of DHA (high, 38.4 kg DM/cow.day, medium, 30.3 kg DM/cow.day and low, 26.8 kg DM/cow.day, measured above the ground level) on botanical composition of the sward for one year, they found no effect of DHA on a pasture comprising
Festuca arundinacea,
Trifolium repens, and
Lotus corniculatus in any season [
52].
4.3. Herbage Depletion and Changes in Morphological Components during Grazing
Herbage depletion showed an exponential reduction as the grazing session progressed. Cows consumed close to 31% of the HM and reduced approximately 55% of the USSH with respect to the corresponding initial value during the first 120 min of the grazing session. The amount of pre-grazing HM available to be eaten (above 4 cm) at the start of the grazing session was 1198 kg DM/ha and 1286 kg DM/ha for low and high DHA, respectively, from which the amount of lamina was on average 1112 kg DM/ha for the low DHA and 1160 kg DM/ha for high DHA. Cows grazing under the low DHA conditions consumed the pasture upon a faster offer than those under the high DHA conditions as a consequence of the lower herbage availability and the greater grazing pressure. However, the USSH reduction rate was unaffected by the DHA conditions, likely as a consequence of the absence of differences when the cows were fed with 4.5 kg DM/cow.day of maize silage and because of the similar pre-grazing pasture conditions in terms of the vertical distribution of HM.
As the sward was grazed down, the proportion of lamina was reduced while the proportion of stem and dead material increased, which coincided with the findings of McGilloway et al. [
53]. Thus, after 120 min, the reduction in the amount of available leaf-lamina (>4 cm) was, on average, of 459 kg DM/ha whereas dead material was increased by 61 kg DM/ha, respectively. Despite the fact that the dynamics of change throughout the grazing session observed in all proportions of sward morphological components in the pasture on offer (>4 cm) did not vary between DHA treatments, the amount of leaf-lamina present after 120 min from the star of the grazing session was 106 kg DM/cow.day lower at the low DHA when compared with high DHA treatments.
Cows with a low DHA and a low level of MSS achieved greater bite rates at the beginning of the grazing session than those under a high DHA condition, likely because the motivation to graze was greater compared with the other feeding regimes and because the substitution of pasture by supplements decreases as the DHA decreases [
11]. The lower bite rate at the end of the grazing session in cows grazing on low DHA is likely attributable to a decreased motivation to continue grazing, which is caused by a higher sward bulk density [
54] and a lower proportion of green lamina [
55] in the lower strata compared with the upper strata. Moreover, the lower bite rate and grazing activity observed near the end of the grazing session can also be explained by a higher sward bulk density [
54] and a lower amount of green lamina [
55] available to be eaten. While the total HM was reduced by 1187 and 924 kg DM/ha under low and high DHA condition, respectively, the reduction in the USSH reach 12 cm on average without differences between DHA. Regarding the total available green-lamina reduction, this reached 571 and 579 kg DM/ha under low and high DHA conditions, respectively. The sward height [
56] and the proportion of green lamina in the deeper strata [
57] are the most important structural sward variables that affect the short-term herbage intake rate. In addition, it is largely accepted that dairy cows are able to adapt their grazing behaviour, according to the grazing conditions, and have the ability to anticipate access to a new strip after milking, as described by Peyraud et al. [
6]. Accordingly, the restrictive herbage availability and the relatively high supplementation level (from 4.5 to 9 kg DM/cow.day) would also have decreased the intake rate.
4.4. Pasture Regrowth
Net production of HM represents the balance between pasture growth, senescence, and tiller population density [
35]. Pasture growth is a function of the leaf growth and leaf production as well as the tiller (or stolon) production rates [
58]. Moreover, the lamina growth rate is linearly related to the mean daily temperature [
16]. In turn, leaf and tiller production are continuous processes regulated by environmental conditions (temperature, luminosity) and pasture management (especially nitrogen fertilization). The results of our study showed that DHA did not affect the leaf production rate. That likely was because the pre-grazing HM did not differ between DHA treatments and because this process depends mostly on the temperature [
59]. On the other hand, the greater tiller production rate at a high DHA could be the result of the grazing management that resulted in a greater amount of water-soluble carbohydrates available in the stubble for regrowth [
60].
The pasture growth rates decreased progressively as the autumn season progressed and was 28% greater in the high DHA than in the low DHA conditions. It can be attributed to the greater photosynthetically active area remaining in the post-grazing residue [
52]. The mean pasture growth rates agreed with the values that were expected in the permanent pastures of Southern Chile during the autumn, which were in the range of 30 to 50 kg DM/day [
16].
4.5. Animal Performance and Grazing Behaviour
As part of two parallel studies conducted on the same herd and paddock with a focus on nutritional aspects [
61] and energy metabolic response [
62,
63], the intake of MSS and concentrate was measured. Cows, in total, consumed 12.1 kg DM forage per day (pasture plus MSS) and 3 kg of concentrate, with no difference among groups.
The greater pasture intake obtained at the high DHA (representing 54.3% of the total DMI vs. 39.7% at the low DHA) was sufficient to increase the individual milk production, which increased by 0.15 kg per kg of increased DHA. This result was associated with a decreased grazing severity [
64] and, consequently, the ability of the cow to select greater quality herbage within the sward was greater compared to cows grazing at a low DHA [
65]. Moreover, it can be assumed that the increase in herbage DM intake when cows grazed on the high DHA condition could increase the energy intake and decrease the extent of the negative energy balance commonly found in the first months of lactation, and, thus, improve milk production [
66]. This assumption could be supported by the results reported by Morales et al. [
63] who indicated that increasing DHA from 17 to 25 kg DM per cow decreases
B-hydroxybutyrate in plasma from 1.12 to 0.91 mmol L
−1.
The increased milk production at the high DHA shown in our study was similar to the findings of Pulido et al. [
48] and Pérez-Prieto et al. [
44], who reported that individual milk production per cow increased by 0.12 and 0.09 kg per kg of DHA offered, respectively. Greater responses were observed by Wales et al. [
67], who reported that the milk yield increased by 0.99 kg per kg of DHA increase. In contrast, Ruiz-Albarrán et al. [
22] and Kennedy et al. [
68] did not find evidence of the effect of DHA when 17 and 25 kg DM/cow.day (measured above ground level) and when 13, 16, and 19 kg DM/cow.day (>4 cm height) were used, respectively.
Experiments using the stocking rate (SR) as the main factor governing milk output per cow and per unit pasture area have shown that, as the SR increases, the individual milk production is reduced because DHA, and, consequently, the offered area and forage DMI are reduced [
49]. Despite the individual milk production decreased by 1.2 kg/day (92.4 kg over a 77-day period) at a low DHA, the results of this study showed that, with a 1 cow/ha increase in SR, the milk output per hectare increased by 24.3% (+1510 kg). These results are consistent with the long-term effect of DHA on milk production, as reported by Merino et al. [
15], who found that, with a 1 cow/ha increase in SR, the milk output per hectare increases by 27.3%.
The milk fat concentration decreased by 0.33 g/kg per kg of increase in DHA. The lower milk fat concentration observed in this study was a consequence of the greater individual milk production when cows grazed at a high DHA. This effect was relatively greater than those of Pérez-Prieto et al. [
44] in an experiment carried out in the winter using DHA between 19 to 46 kg OM/cow.day. The absence of an interaction effect between DHA and the level of MSS on milk production and milk fat concentration observed in this study has been previously described by Delaby et al. [
69], who applied a DHA ranging from 12 to 22 kg DM/cow.day.
The CP supply from high-quality pastures in the autumn usually exceeds requirements for milk production of dairy cows, whereas the energy intake is the main limiting nutrient [
70]. Therefore, the use of supplementary feed is necessary to ensure a stable supply in quantity and quality of energy to avoid the energetic costs associated with excreting N through urea synthesis [
71] and to optimise rumen microbial synthesis, which improves dietary N utilization (milk N in relation to N intake) [
72].
The higher milk protein observed in our study at the high DHA was not related to the quality of the feed provided (because forage and MSS quality did not differ in CP nor ME concentrations) or to the total DM intake. The increase in milk protein content when more herbage was offered could be explained by the decrease in the plasma concentration of
B-hydroxybutyrate reported by Morales et al. [
63], which represents a positive effect on the energy metabolism. The milk protein concentration increased by 0.01 g/kg per kg of increase in DHA, which is similar to the results reported by Delaby et al. [
69].
4.6. Final Remarks
In a high-quality pasture, allocating herbage at 17 kg DM/cow.day with 4.5 kg/cow.day MSS was appropriate for improving both herbage utilization and milk production per hectare while maintaining the short-term conditions of a pasture grazed by dairy cows in the autumn. This would represent an economical benefit, as long as the potential marginal increase in costs does not counteract the marginal increase in the revenues from the additional milk produced, compared with offering more grazing area or a greater amount of supplement to animals, as well as a strategy to enhance the sustainability, both economic and biological, of pasture-based dairy production systems. However, the beneficial effects found in this research are not guaranteed when the overall management does not satisfy the cow’s nutrient requirements. Therefore, the health status and welfare of lactating dairy cows must be monitored. Moreover, the level of DHA must be adjusted in the following spring and summer periods.