4.1. Load Bearing Capacity
Load bearing capacity was generally higher for kurzrasen than for strip-grazing, but this was only statistically significant during July 2016, November 2016 and March 2017. Load bearing capacity is an important parameter that has a large impact on grassland utilization, as it affects the length of the grazing season (turn-out and turn-in dates) but also the potential for grazing during periods with high rainfall during the grazing season (such as in June 2016). In general, a load bearing capacity in excess of 0.7 MPa is assumed to be adequate to prevent sward damage during grazing [16
]. The weeks around the measurements in November 2016 and particularly March 2017 were wet periods, during which, the difference in load bearing capacity affected whether this threshold value was achieved or not. Load bearing capacity may depend on a combination of factors, including soil moisture content, sward density, and root density [17
], as outlined below.
Soil moisture content (SMC) showed a strong negative correlation with load bearing capacity (Figure 7
a), both within and between measurement periods, which is in line with literature [18
]. The high SMC in March and April (>50%) resulted in a low load bearing capacity (<0.5 MPa) during those periods. Whereas during the dry periods (<50% SMC) the load bearing capacity ranged from 0.8 to over 2 MPa. The negative correlation between load bearing capacity and soil moisture content is in line with other studies showing that a higher soil moisture content decreases the load bearing capacity of the soil due to decreased soil strength [18
]. Under these conditions, grazing can result in soil compaction, followed by pugging at high moisture contents and poaching on saturated soils [17
Soil moisture content tended to be lower for kurzrasen compared to strip-grazing (Figure 2
). This could be related to higher levels of evaporation from the soil, due to increased wind speed at soil surface at low sward heights [19
]. However, this is somewhat counterbalanced by the higher sward density for kurzrasen, limiting the exposure of bare soil to the air. At the same time, the lower grass growth rate for kurzrasen in comparison to strip-grazing may result in lower water usage and hence evapotranspiration. Indeed, evapotranspiration from grass has been shown to be positively correlated to sward height [20
]. Also, Donkor et al. [21
] found a lower soil moisture level for rotational grazing compared to continuous grazing as a result of higher evaporation and transpiration rates of the standing crop.
Load bearing capacity was positively correlated to the sward density (Figure 7
b), which is in line with literature [17
]. Dense swards allow less direct hoof/soil contact and offer a higher degree of protection. Sward density was higher for kurzrasen compared to strip-grazing. This increase in tiller density is the result of lower shading by large leaves in the kurzrasen sward [22
]. Additionally, more frequent defoliation may affect the hormonal distribution, resulting in faster cell division and tiller formation [23
]. Therefore, kurzrasen may be employed as a strategy to increase the load bearing capacity and therefore increase the number of days available for grazing and decrease sward damage as a result of treading.
Load bearing capacity showed no statistically significant correlation with root density (Table S2
) and there were no statistically significant differences in root density between the two grazing systems (see also below).
The number of grazing hours and number of steps during grazing may affect the impact of grazing on sward damage during grazing at times with low load bearing capacity. In the kurzrasen system, both the time spent in pasture and the number of steps per hour were higher compared to strip-grazing. This is probably related to the low available herbage mass at kurzrasen, resulting in a lower intake rate per bite. It is unclear to what extent this higher level of treading for kurzrasen will counteract the positive effects of increased sward density on damage during grazing at higher soil moisture levels. Several studies have reported a negative impact of stocking density on soil damage during grazing [24
]. However, in most studies (as in the current study) the effect of increased treading cannot be disentangled from the grazing system, i.e., higher stocking densities are often associated with lower sward height [17
]. Also, for kurzrasen, the daily grazing area was 2 ha, whereas for strip-grazing, the daily available area was on average 0.18 ha. As a result, the actual stocking rate during grazing was more than 10 times higher for strip-grazing compared to kurzrasen, but for strip-grazing, the sward had rest-periods in between grazing sessions. Therefore, when soil load bearing capacity is low, strip-grazing potentially results in a lot of damage on a relatively small area, whereas with kurzrasen the sward damage is limited. In practice, this means that, regardless of differences in load bearing capacity associated with sward density, with strip-grazing, farmers will more readily choose to keep their livestock indoors to avoid excessive sward damage. Indeed, during 2016 and 2017, the strip-grazing groups were kept indoors during the grazing season due to low load bearing capacity on a number of occasions, whereas with kurzrasen this was never the case. However, kurzrasen systems often only have one access point into the paddock, which is used throughout the grazing season, and trampling around this area may therefore be more apparent. There was no statistically significant effect of system on the soil penetration resistance at 0–10 cm soil depth, indicating that there was no evidence of differences in soil compaction between the two systems.
4.3. Water Infiltration Rate
A high water infiltration rate is important for flood prevention, because soils can take up water during rain showers more rapidly, which reduces the peak loads of drainage water in ditches and rivers. This is particularly relevant in view of future climate change scenarios, in which the occurrence of extreme rainfall events is predicted to increase (KNMI, 2014). The range in water infiltration rate measured in the current experiment is in line with Deru et al. [28
], who reported an average water infiltration rate of 32 mm min−1
on relatively dry production grassland soil and 6.2 mm min−1
on dryer semi-natural grassland soils.
At high soil moisture levels (i.e., April 2016, March and October 2017), the water infiltration rate was less than 4 mm min−1 (too low for measurement during the first and the latter month). At these times, we found no statistically significant differences between the two grazing systems. In March 2017, there was a negative correlation between water infiltration rate and soil dry matter content, indicating that during wet periods, the water infiltration was faster on dryer soils. During November 2016, when the soil was relatively dry, the water infiltration rate was over 40 mm min−1. Even though at this point the water infiltration rate was statistically significantly higher for strip-grazing compared to kurzrasen, it is questionable whether this difference is practically relevant. During this dry period, there was no statistically significant correlation between SMC and water infiltration rate, and a negative correlation between penetration resistance and water infiltration rate.
4.4. Herbage Growth
Herbage growth rates in 2016 were relatively low, which was related to the extremely wet conditions during the middle of June, followed by very dry conditions during September and October. Growing conditions were generally better in 2017, and the mean herbage growth rate in the pasture was on average 39 and 52 kg DM ha−1 d−1 during 2016 and 2017, respectively.
The herbage dry matter production rate on grazed pasture, excluding area temporarily used for silage production, was on average 13% and 24% lower for kurzrasen compared to strip-grazing in 2016 and 2017, respectively. This is in line with [9
], who reported a 36% increase in herbage DM production for simulated rotational grazing (6 cuts per season) compared to kurzrasen (9 cuts per season). The yield reduction for kurzrasen can be related to the young leaf stage at time of grazing (leaf stage 1–1.5 and 1.5–3 leaves tiller−1
for kurzrasen and strip-grazing, respectively, Figure 5
b), and the short post-grazing height (on average 4.5 and 6.1 cm for kurzrasen and strip-grazing, respectively) associated with the kurzrasen system, as outlined below.
A number of publications have shown that the highest herbage yields are achieved by grazing at the two to three leaf stadium (corresponding to approximately three-week regrowth period) to a residual grazing height of approximately 5 cm [29
]. Just after grazing, at the start of the regrowth period there is relatively little photosynthetically active plant material (i.e., lamina), and growth is mainly dependent on sugar reserves in the stubble and roots. As soon as sufficient lamina material has developed, further growth is based on photosynthesis, and at this stage, sugar reserves can be replenished [31
]. If swards are grazed too often (before the sugar reserves have been restored), this can result in decreased growth capacity and reduced root growth.
Similarly, a too low stubble height may result in low sugar reserves and reduced or at least delayed growing capacity. In contrast, when the regrowth length is too long, or the stubble too high, this will result in senescence of older plant material, and lower grass utilization [31
]. However, the yield penalty for kurzrasen is smaller than could be expected, as a result of the morphological changes in the perennial ryegrass plants in response to these different systems, including (1) the proportion of free-leaf lamina, (2) the sward density and (3) the root density.
(1) In the kurzrasen system, the amount of lamina material per tiller (expressed as free-leaf lamina length per tiller (Figure 5
c) was much smaller than the pre-grazing free-leaf lamina length of strip-grazing. However, the free-leaf lamina length for kurzrasen tended to be larger than the post-grazing free-leaf lamina length for strip-grazing. Furthermore, when the free-leaf lamina length is expressed as a proportion of the total tiller length, there is only a small difference between kurzrasen and pre-grazing strip-grazing, and the proportion of free-leaf lamina is significantly higher for kurzrasen compared to post-grazing strip-grazing. This might imply that the amount of photosynthetically active material just after defoliation is not limiting for the regrowth of the kurzrasen sward (but is even higher for kurzrasen compared to strip-grazing) and therefore the depletion of sugar reserves just after defoliation is smaller. At the same time, the kurzrasen tillers may not reach the stage in which the sugar reserves are actively replenished [31
(2) Additionally, the kurzrasen system resulted in a much higher sward density, and the lower tiller productivity was partly compensated by an increase in the number of tillers, which is in line with [33
], as discussed above.
(3) The root density at 10 and 20 cm soil depth tended to be lower for kurzrasen compared to strip-grazing. However, the differences tended to be small and only significant at 10 cm during April 2016. In general, the root density was high in comparison to measurements in the same region by [28
]. Also, there was no evidence of a shift to more shallow soil layers for kurzrasen. This is in line with [34
], who did not show a clear difference in root biomass at 0-10 cm soil depth when comparing kurzrasen to a rotational grazing system. Starz et al. [34
] did report a slight shift in the proportion of roots in the top 5 cm from 0.99 for rotational grazing to 0.95 for kurzrasen. Generally, root mass shows a decline under high defoliation frequency and intensity [10
], which could be related to the reduced availability of stored carbohydrates in the roots and stubble. For the kurzrasen system (in contrast to strip-grazing), the grass morphological measurements have shown that the herbage is never entirely dependent on stubble and root reserves, because there is always a relatively constant proportion of green lamina material present in the sward (Figure 5
d), which, in combination with the higher sward density, may explain the lack of effect on root density. Indeed, [33
] reported that the root biomass of grazed swards was larger under continuous grazing compared to rotational grazing, which was associated with a higher tiller density under continuous stocking. Moreover, the infrequent severe grass cuttings under rotational stocking lead to a standstill in root production of 25 to 45 days during the growing season as carbohydrate reserves and assimilates will in first instance predominantly be used to re-establish the shoot/root ratio [35
In the current research we found no evidence of lower drought resistance under kurzrasen, i.e., the relative yield difference between the two systems did not increase during dry periods in September and October 2016 and April and May 2017 (Figure 4
b). As discussed above, this may be related to the lower water requirement for growth at lower herbage production rates.
4.5. Herbage Quality
For kurzrasen, the herbage intake consists of young lamina material, whereas for strip-grazing, the average age of the plant material is higher and contains more sheath and stem material. The CP content is generally higher for lamina compared to sheath and stem material [36
], whereas the water soluble carbohydrate (WSC), NDF and ADF content is generally lower [37
]. Also, increased sward age results in a decrease in CP, whereas WSC, NDF and ADF show an increase with increasing sward age [31
]. Chapman et al. [29
] reported that the OM digestibility, CP and WSC content was higher for the newly emerged leaves, compared to older leaves, whereas NDF and ADF contents were lower. Therefore, the CP content, and digestibility is expected to be higher for kurzrasen compared to strip-grazing. Indeed, Starz et al. [9
] reported higher net energy and CP content for simulated kurzrasen compared to rotational systems, whereas the reverse was found for NDF. In 2016, we found only a small difference in the nutritional value of the herbage for grazing between kurzrasen and strip-grazing, and differences were often in contrast with expectations: the CP content tended to be lower for kurzrasen and the ADF content was significantly higher for kurzrasen. This unexpected result was probably related to the sample collection method during 2016 (see material and methods), which was not representative of the actual plant parts (leaves) selected during grazing. Therefore, the sampling protocol was adjusted in 2017 (using hand scissors), and in 2017 we found a 13% increase in CP content for kurzrasen compared to strip-grazing, whereas the NDF and ADF content was significantly lower for kurzrasen compared to strip-grazing.
Additionally, the difference in botanical composition between the kurzrasen and strip-grazing fields may have affected the difference in nutritive value: the proportion of highly palatable grasses (perennial ryegrass and timothy) was lower for kurzrasen compared to strip-grazing. However, there is some evidence that the difference in nutritional value between highly palatable and less palatable grasses is less apparent at early plant growth stages [42
]. As a result, the less favorable botanical composition of the kurzrasen fields, in which the grass is grazed at a very young stage of growth, is unlikely to have a large impact on nutritional value.
The current experiment was limited to two experimental years and the sample size was relatively small. While this is to a large extent the results of limitations associated with systems approach, it may have limited the power for statistical testing and the robustness of the responses.