Compared to lactating dairy cow systems, there is a paucity of information on the effect of PGSH on herbage production and nutritive value, and DMI, grazing behaviour and growth performance of steers within weanling-to-beef suckler systems. The aim of this study was to address this knowledge deficit in the context of evaluating suckler steer genotypes contrasting in maturity within a temperate, rotationally stocked, grass-based production system. In terms of production systems and beef markets, consumer demand for beef produced from pasture-based diets, and ultimately “grass-fed” beef (i.e., no concentrates), is rising as it is perceived to be healthier, animal friendly and good for the environment.
4.1. Post-Grazing Sward Height
This study indicates that the currently recommended PGSH (4 cm) restricts steer ADG at pasture compared to PGSH-6. The poorer individual animal growth performance on the lower PGSH agrees with the findings obtained in previous research comparing 3.5 vs. 5.0 cm PGSH with dairy-bred steers [
10,
11] and 4.1 vs. 5.3 cm PGSH with suckled calves [
36] in Ireland. Similarly, dairy cow studies in Ireland [
15,
16] and New Zealand [
17] comparing low (2.7–4.9 cm) and high (4.2–8.7 cm) PGSH found lower milk solids production in cows grazing the lower sward residual. In contrast, Minchin and McGee [
37] detected no statistically significant difference in ADG in replacement beef heifers grazing to either 4.4 or 5.6 cm, although numerically growth rate was in favour of the higher sward residual. Reasons for this inconsistency could be due to the smaller differential between the two PGSHs investigated, a ‘less severe’ low-PGSH and the lower growth potential of the older breeding heifers employed compared to other studies [
10,
11].
In the current study, the superior 0.10 kg ADG of steers grazing PGSH-6 equated to an additional 21 kg live-weight gain at the end of the grazing season compared to PGSH-4. The proportionate difference in ADG between PGSH treatments in the current study (0.11) was lower than the 0.21 and 0.17 proportionate differences reported by O’Riordan et al. [
11] and O’Riordan et al. [
10], respectively, which were equivalent to an additional 27–33 kg live-weight gain at the end of the grazing season. The relatively lower differential in the current study can be at least partially attributed to period 2 of the grazing season when, due to atypical dry weather, both treatments grazed silage area to a common PGSH of 6 cm for 43 days. This is likely to have favoured the PGSH-4 steers. Using the animal growth trajectories observed in periods 1 and 3 and excluding period 2, the estimated difference in ADG over the entire grazing season would be 0.15 kg (proportionately 0.16) rather than the 0.10 kg (proportionately 0.11) observed. Additionally, the higher proportionate differences in ADG found by O’Riordan et al. [
10] and O’Riordan et al. [
11] may also be due to the fact that these studies grazed tighter (3.5 cm) than the current study (4.2 cm), which could magnify the negative effects associated with grazing lower sward residuals. Collectively, these studies indicate that grazing temperate pasture excessively tightly restricts the growth rate of yearling beef cattle by 0.13–0.15 kg/day.
The superior animal growth of PGSH-6 at pasture was associated with a greater DMI of higher (statistical tendency) OMD herbage compared to PGSH-4. Herbage DMI or grazing behaviour was not measured in previous studies evaluating PGSH for beef cattle in temperature pastures [
10,
11,
36]; however, grazing behaviour parameter values obtained for PGSH-6 steers were similar to those reported in the literature for beef cattle grazing pasture to a similar height [
38,
39]. In the current study, the greater herbage DMI for PGSH-6 than PGSH-4 was mainly due to grazing behaviour differences that emerged during the last 24 h rather than the first 24 h of a 48-h allocation of fresh herbage. The differences in DMI can be largely attributed to a lower bite mass and intake rate for PGSH-4 than PGSH-6, as both parameters decrease linearly with sward depletion height [
40,
41]. Furthermore, the shorter eating time, lower grazing bites and bite rate for PGSH-4 during the last 24 h of the allocation may be due to sward structure increasingly becoming a limiting factor, whereby animals may not have a desire to graze for longer periods of time to select out small quantities of herbage [
40]. As sward structure becomes a limiting factor under such circumstances in a rotational grazing system, animals do not increase grazing time but instead stand and wait to enter a new ‘paddock’ in the rotation, leading to reduced herbage intake [
42,
43]. It also appears that upon entering a new paddock, PGSH-4 steers did not change their grazing behaviour or increase their DMI to compensate for the lower herbage consumption in the previous day. Consequently, offering steers ‘new’ pasture before sward structure becomes a limiting factor can increase steer DMI and subsequent ADG at pasture.
Despite the differences in DMI, with the exception of ruminating mastication rate, PGSH did not affect ruminating behaviour over the 48-h allocation. This could be due to the similarity in herbage NDF and ADF concentrations between the PGSH treatments. The greater ruminating mastication rate for PGSH-6 could be due to the higher intake rate [
44], as a significant positive, albeit low, correlation (0.25) between herbage DMI and ruminating mastication rate in grazing steers has been reported [
38].
In the current study, the absence of a statistically significant effect of PGSH on herbage OMD concurs with previous research [
13,
15], although a decrease in OMD in high compared to low PGSH has been reported elsewhere [
16,
17,
45]. However, the decrease observed in some of the later studies [
45] may be attributed to the very wide range of PGSH employed (6.6 vs. 10.5 and 14.6 cm), which are extreme in relation to modern temperate grazing systems. Similar to the current experiment, Mayne et al. [
16] did not mechanically top the pasture, but they found no difference in herbage OMD between PGSH of 4.9 and 6.0 cm determined using a sward stick; however, herbage OMD decreased as PGSH increased from 6.0 to 8.7 cm (accompanied by a higher pre-grazing herbage mass).
The absence of large effects of PGSH on herbage OMD broadly supports the hypothesis of Ganche et al. [
13], that animals harvested herbage from pastures to a consistent PGSH at each rotation, i.e., the animals were consuming only the herbage that had regrown since the last rotation and thus were not grazing lower into the sward profile where older herbage had accumulated and which was likely to be of lower nutritive value. This is reinforced by the similar leaf and stem proportions in the respective grazing horizons of both PGSH treatments in the current study.
Due to the lower ADG at pasture, PGSH-4 steers tended to exhibit compensatory growth during the indoor finishing period across both genotypes and finishing diets. It is surprising there was no additional compensatory growth (i.e., treatment interaction) for PGSH-4 steers offered the higher (SC) compared to the lower (SO) plane of nutrition during the finishing period. However, the compensatory index was only 0.42, which was insufficient to negate the extra weight gain achieved by PGSH-6 during the grazing season and consequently, PGSH-6 tended to have an 11 kg heavier carcass weight irrespective of genotype and finishing diet. Similarly, O’Riordan et al. [
11] and Minchin et al. [
36] reported that for growing beef cattle, differences in live-weight at the end of the grazing season in favour of the higher PGSH were still evident immediately post-housing, implying that ‘gut-fill’ was not a primary contributory factor.
Although there was no difference in carcass fat score between the PGSH treatments for EM genotype, LM PGSH-6 were almost 1-unit fatter than LM PGSH-4 (reflected in the statistical interaction), implying that, in addition to growth advantages, more lax grazing could be particularly beneficial for finishing LM genotypes in grass-based systems. However, there is no published literature with which to compare these findings, and this requires further research. Alternatively, if steers were not slaughtered at a similar time point and were drafted for slaughter based on carcass weight, PGSH-4 steers would require an additional 19.7 and 19.5 days on SO on SC, respectively, to obtain the same carcass weight as PGSH-6. Reducing the slaughter age of cattle can have substantial effects on lowering greenhouse gas emissions from beef production systems [
46].
4.2. Genotype
In the current study, during the grazing season and indoor finishing period, EM had a higher DMI and ADG than LM. Although there was no statistically significant difference in grazing and ruminating behaviour parameters between the genotypes, key parameters that determine grazed herbage DMI (bite rate, intake rate and bite mass) were all numerically higher for EM than LM steers. There is relatively little published information evaluating the DMI of EM and LM beef cattle genotypes grazing pasture, and those are equivocal with higher [
47] and lower [
48] pasture DMI reported for EM compared to LM. Likewise, when offered a pre-dominantly forage diet supplemented with concentrates indoors, higher [
49] and similar [
4] DMI was reported for EM compared to LM. In terms of growth performance, most previous research has found EM to have a lower [
48,
50,
51] ADG than LM at pasture, but not always [
4], whilst the ADG differences reported during the indoor finishing period are more inconsistent, with a lower [
23,
50], similar [
52,
53] or higher [
4,
54] ADG reported for EM than LM, respectively. In the current study, it is important to note that the progeny of the two sire genotypes were all from LM dams. Additionally, this discrepancy across studies may be attributed to breed genetic selection [
23]. Nonetheless, in this study, the higher DMI for EM at pasture and during the indoor finishing period (on SO and SC) underpins the higher ADG for EM than LM. The feed conversion ratio (based on live-weight) did not differ between genotypes at any time point in this study; however, due to the lower kill-out proportion for EM than LM [
4,
49,
55], EM had inferior feed efficiency when expressed as carcass gain. As the higher forage DMI of EM did not yield additional carcass gain compared to LM and this resulted in a 4% lower stocking rate for EM than LM at pasture.
An objective of this study was to identify the optimum genotype for a grass-forage-only and conventional grass-forage plus concentrate steer production system. In this study, there was no genotype × finishing diet interactions. Although EM had an inferior feed efficiency, the findings of this study suggest that EM may be more suitable to grass-forage-only systems than LM as carcass weight was relatively similar (351 vs. 355 kg, respectively), but carcasses were fatter (7.4 vs. 6.2 units). These differences in carcass attributes between the genotypes on a grass-forage-only diet are similar to the values reported in Regan et al. [
4] (314 vs. 319 kg and 8.5 vs. 6.1 for EM and LM, respectively). The lower carcass fat score for LM steers within both PGSH shows that a higher proportion of those carcasses failed to achieve a commercially-acceptable fat score (>6.0). A consequence of this can mean a lengthening in the production cycle or, in practical terms, a longer indoor finishing period or returning to pasture at the end of the ‘second’ winter for finishing. However, Herron et al. [
47] reported that unfinished animals returned to grass for a ‘third’ grazing season with subsequent slaughtering at 28 months of age exhibited compensatory growth and superior weight gain and have a lower environmental footprint per kg carcass produced than those finished indoors on grass-forage-only at 24 months-of-age. However, the impact of a third grazing season on stocking rate in a weanling-to-beef system on a fixed land resource was not considered.
In the current study, when concentrates were supplemented during the finishing period (SC), all EM and LM steers exceeded the commercially-acceptable fat score (9.6 vs. 7.4, respectively). Therefore, LM may be more suitable than EM in a conventional forage plus concentrate input system, as in accord with previous studies [
4,
23], LM steers generally achieve a superior carcass weight (383 vs. 396 kg), conformation score (7.7 vs. 9.4 units) and feed efficiency than EM under that feeding regime.
4.4. Additional Considerations
In grass-based systems, herbage production has a positive impact on key profit drivers, such as stocking rate and LWG/ha [
1,
60]. In this study, although PGSH-6 increased steer LWG, it decreased herbage production compared to PGSH-4, but the differences in herbage production were relatively small (±506 kg DM/ha). The greater herbage accumulation for PGSH-4 agrees with the herbage regrowth principles summarised in Chapman et al. [
14]. Furthermore, the longer regrowth interval [
61] and a tendency for the higher sward density for PGSH-4 than PGSH-6 also contributes to a higher herbage accumulation for PGSH-4. The lower sward density for PGSH-6 could be attributed to the decreased tiller numbers as a result of lower light penetration to the base of the sward [
62].
As a result of lower accumulation and greater daily demand for herbage, PGSH-6 used a 15% greater grazing area per rotation than PGSH-4, which is consistent with the literature [
9,
13]. In practice, this can either reduce the quantity of excess herbage removed as silage or reduce the animal stocking rate. Silage preserved per animal unit in this study was 135 kg DM (proportionately 0.07) lower for PGSH-6 than PGSH-4, resulting in 7.5 days less feeding per animal unit (assuming a combined daily demand of 17.9 kg DM for both a weanling and finishing steer when assuming an edible silage recovery of 0.78 (field to fed losses) as outlined in Keating and O’Kiely [
35]). The extra 7.5 days of indoor feed supply for PGSH-4 could lead to an additional 3.2 kg carcass gain in a SO system. Minchin and McGee [
37] also reported a lower silage supply (147 kg DM/heifer) for 5.6 than 4.4 cm PGSH from a one cut-silage system.
Although the stocking rate is identified as a key profit driver in temperate pasture-based beef systems [
1], relatively speaking, most Irish commercial beef farms [
63] are under-stocked (1.6 LU/ha) compared to research beef farms (2.6 LU/ha) [
63] and commercial dairy [
64] (2.1 LU/ha) farms [
1]. Consequently, the 13% lower stocking rate on the grazing area (not the whole farmlet) for PGSH-6 compared to PGSH-4 is less of an issue for commercial beef systems.
The results indicate that LWG/ha was similar (2% lower for PGSH-6) for both PGSH treatments. The effect of PGSH on animal output/ha remains unclear in the literature; for example, dairy cow studies on temperate pastures reported an increase [
65] (3.6 vs. 4.0 vs. 4.5 cm) and decrease [
16] (5 vs. 6 cm) in milk production/ha as PGSH decreased.
Regarding genotype, the environmental footprint must be considered as outlined by Herron et al. [
47], who reported similar animal production-related results to the current study. They observed that EM steers had a lower environmental footprint than LM when expressed on an LWG basis; however, the opposite occurred when expressed per kg of meat weight gain, due the superior kill-out and carcass lean proportion for LM.
Regarding the finishing diet, from an environmental perspective, it must be recognised that although concentrate supplementation increases emissions per animal, it reduces emissions per kg meat produced [
47]. Therefore, strategic concentrate supplementation is often advised during the indoor finishing period where steers are slaughtered at 24 months of age [
47].
Of the total steer LWG achieved from the start of the first winter to slaughter, proportionate LWGs achieved during the first winter, grazing season and indoor finishing period were 0.15, 0.57 and 0.28, respectively, in the grass-forage-only system. Corresponding values for the grass-forage + concentrate system were 0.15, 0.50 and 0.35, which is similar to Drennan and McGee [
2]. This illustrates the importance of pasture management to maximise steer growth at pasture whilst producing sufficient quantities of high digestibility silage (>750 g/kg DMD) to reach carcass specification during the finishing winter.