The performance of single-rearing ewes and their lambs offered ryegrass pasture or herb–clover mix during lactation

ABSTRACT

Lamb growth to weaning has important implications for the profitability of farms producing lambs for meat. Alternative forage crops such as chicory, plantain, red clover and white clover have the potential to increase lamb growth rates to weaning percentage sold to slaughter at weaning. A study was conducted over two years to examine the growth of single-born lambs offered either pasture or herb-mix (chicory, plantain, white clover and red clover) from birth to weaning. Each year, 80 single-bearing ewes were allocated to either a pasture or herb-clover mix. In 2014, ewes and lambs that were offered the herb-clover mix had greater live weights at weaning than those grazed on ryegrass pasture. In 2015, however, no differences in lamb weaning weight was observed. In both years of the study, lambs reared on the herb clover mix had growth rates in excess of 400 g/day, from birth to weaning.

KEYWORDS:

• Lolium perenne
• Trifolium repens
• Trifolium pratense
• Cichorium intybus
• Plantago lanceolata
• ewe
• lamb
• growth
• carcasses

Introduction

The New Zealand sheep production system is based primarily on year-round grazing of pastures containing perennial ryegrass (Lolium perenne L.) and with a small percentage, usually less than 5%, of white clover (Trifolium repens, Waghorn and Clark 2004). There has been recent interest both nationally and internationally in the use of alternative forages such as chicory (Cichorium intybus L.), plantain (Plantago lanceolata) and red clover (Trifolium pratense), either as monocultures or in a herb–clover mix, for both sheep and cattle production systems (Marley et al. 2003; Sanderson et al. 2005; Reed et al. 2008; Andersen et al. 2009; Raeside et al. 2014; Lombardi et al. 2015; Somasiri et al. 2015a; Pembleton et al. 2016).

Recently it has been shown that herb–clover mixes can improve the pre-weaning growth rates of twin and triplet lambs born to mixed-age ewes, and of singleton lambs born to ewe lambs (Hutton et al. 2011b; Corner-Thomas et al. 2014). This effect was likely due to a combination of increased milk production of the ewe and greater nutritional value of the feed when consumed by the lamb (Hutton et al. 2011b; Cranston et al. 2015). In addition, the herb–clover mixes have been reported to support the early weaning of twin lambs born to mature ewes (Cranston et al. 2016) and improve the post-weaning growth rates of lambs (Somasiri et al. 2014; Somasiri et al. 2015a, 2015b). Prior to weaning, lamb growth rates of singletons born to ewe lambs and offered the herb–clover mix ranged between 358 and 333 g/day have been reported (Corner-Thomas et al. 2014). In addition, twin and triplet lambs born to mature ewes achieved growth rates of 205 g/day (Hutton et al. 2011b). To these authors’ knowledge the growth rates of singleton lambs born to mature ewes have not been reported.

Under New Zealand's pasture-based lamb production system, in which lambs are born in early spring (August/September), the most efficient lamb is one that is sold directly for slaughter at weaning at 10–14 weeks of age. Slaughter statistics, however, show that approximately 20% of lambs are slaughtered before the end of December (Ministry for Primary Industries 2017). Selling lambs at a younger age reduces production costs in terms of feed consumed, much of which is for maintenance only (Nicol and Brookes 2007), animal health remedies and labour. In addition, an earlier sale date can result in increased income as the schedule prices peak in early November and decrease steadily on a per kg basis until at least mid-March (JDJL Limited 2016). The impact of offering single-rearing mature ewes and their lambs a herb–clover mix in lactation has not been examined. We hypothesise that ewe and lamb live weight at weaning will be increased by grazing herb–clover mixes in lactation such that more lambs will reach target slaughter live weight in comparison to a ryegrass-based pasture.

Materials and methods

Study design

In each of two years, singleton-bearing mixed-age ewes, bred during a 17-day period, were selected and managed as one mob in late-pregnancy under commercial grazing conditions on ryegrass–white clover pastures. The intention of the feeding treatments was to offer ewes and lambs ad libitum intakes; therefore, the herb–clover mix was not grazed below 7 cm surface sward height and pasture had a minimum mass of 1200 kg DM/ha (Morris and Kenyon 2004). Ewes (n = 80) were allocated to one of two feeding treatments: (i) Ryegrass-based sward (Pasture) or (ii) a chicory, plantain, red- and white-clover mix (Herb). Total areas of 7 and 5 ha were utilised for the Pasture and 5 ha for the Herb treatments in 2014 and 2015, respectively.

Prior to the predicted start of lambing, ewes were placed in their lambing paddocks at a rate of 10 and 14 ewes per hectare on the Pasture and Herb treatments, respectively. Ewes remained at these stocking rates until weaning. The intake of the ewes allocated to the Herb treatment was adjusted over three days at the start of the experimental period by increasing the duration spent on the mix (4 h, 8 h and then continuously on days 1, 2 and 3, respectively). Both studies were approved by the Massey University Animal Ethics committee.

The 2014 Study

Singleton-bearing Romney ewes (n = 80) were allocated to the two feeding treatments using a stratified random sampling approach to ensure that both treatment groups had similar live weights at the start of the study (Pasture = 63.8 ± 0.8 kg, Herb = 63.6 ± 0.8 kg). From the start of the lambing period (139 days after the mid-point of the breeding period; P139) until 30 days after the mid-point of the lambing period (L30), ewes were continuously stocked; thereafter they were rotationally grazed within their feeding treatment until weaning (L85; a total duration of 95 days). The threshold for the movement of ewes between paddocks was 1200 kgDM/ha in the pasture and a sward height of 7 cm in herb-mix to ensure that ewe intakes were unrestricted. Ewes were weighed and body condition scored (BCS, scale 1–5, including half units, Jefferies 1961; Kenyon et al. 2014) within one hour of removal from herbage at P139, L42, L67 and L85.

During the lambing period, ewes were observed twice daily to identify new lambs. Within 24 h of birth, lambs were identified to their dam, ear tagged, weighed and had their sex recorded. Lambs were weighed again at L42, L67 and L85.

The 2015 Study

Singleton-bearing Romney ewes (n = 80) were allocated to the two feeding treatments as per the 2014 study to ensure similar live weights among treatments (Pasture = 71.5 ± 1.1 kg, Herb = 70.4 ± 0.9 kg). From the start of the lambing period (P139) until L20, ewes were continuously stocked; thereafter they were rotationally grazed within feeding treatment until weaning (L87; a total duration of 103 days). Ewes moved to a new paddock if the pasture mass decreased to 1200 kgDM/ha or the herb-mix sward surface height decreased to 7 cm. Ewe live weights and BCS were recorded at P139, L33, L55 and L87. Lamb measurements were recorded as per study one with lamb live weights being recorded at birth, L33, L55 and L87. In addition, at L87 any lamb that had achieved a live weight of 35 kg was slaughtered at a commercial abattoir (Silver Fern Farms, Takapau, New Zealand) and the hot carcass weight, dressing out percentage (hot carcass weight divided by live weight 24 h prior to slaughter) and GR (soft tissue depth, 110 mm from the mid-line of the 12th rib) were recorded.

Herbage measurements

The 2014 Study

During the lambing period (P139 to L30) the herbage mass (HM) of the Pasture and Herb treatments was measured weekly using a rising plate metre (50 readings per paddock, Ashgrove Pastoral Products, Ashhurst, New Zealand). Post L30, when the ewes and lambs were rotationally grazed the HM was recorded both pre- and post-grazing. HM of the Herb treatment was determined based on the following equation: HM = 84.2 × average metre reading + 1677.6 (Somasiri et al. 2014). HM of the Pasture treatment was determined using the equation: HM = 158 × average metre reading + 200 (Hodgson et al. 1999).

The botanical composition of each feeding treatment was determined from four samples made up of 10 random herbage grab samples per treatment collected across the paddock being grazed at L1, L49 and L82. Approximately 20 and 40 g of the Pasture and Herb treatment, respectively, were sorted to determine the composition. Pasture samples were sorted into ryegrass, other grasses, red clover, white clover, weeds and dead material. Herb samples were sorted into chicory, plantain, white clover, red clover, grasses, weeds and dead material. These individual components were dried in a draught oven at 70°C for at least 24 h before botanical composition was established. To determine the nutritional composition of each treatment an additional subsample was frozen at −20°C before being freeze-dried and ground. Freeze-dried samples were analysed for neutral detergent fibre (NDF; using Tecator Fibertec system, AOAC 2002.04), digestibility (digestible organic matter (DOMD); Roughan and Holland 1977) and metabolisable energy (ME; Roughan and Holland 1977).

The 2015 Study

The herbage measurements collected in 2015 were similar to those in 2014. HM was estimated using a raising plate metre weekly during the lambing period (P140 to L20) and thereafter pre- and post-grazing masses were recorded for each paddock change as per the 2014 study. Herbage botanical composition, digestibility and ME content were determined from samples collected at P145, L48 and L84.

Statistical analyses

Statistical analyses were conducted on data collected from ewes that gave birth to a single lamb. Ewes euthanised due to lambing difficulties were excluded from the analyses (n = 2; 1 from herb treatment in 2014 and 1 from herb treatment in 2015). Ewes were also removed from the analyses if there was no record that they had lambed (n = 2 in the Herb treatment in 2015). Ewes were also excluded from the analyses of lactation live weight if their lamb was born dead, or died within 24 h of birth (n = 5; all 5 were in the pasture treatment in 2014). Ewes that had no weight recorded at weaning were excluded from the analyses of lactation live weight (n = 3; all 3 were in the herb treatment in 2014). Statistical analyses were therefore conducted on data collected from 71 ewes in 2014 and 77 in 2015).

The effect of feeding treatment was analysed within the year due to the differences in the age of the lambs at each weighing event. The effect of feeding treatment on ewe live weight was analysed using a generalised linear model which included the fixed effect of feeding treatment and the covariate of lambing date. The effect of feeding treatment on ewe BCS was analysed using a general model using a Poisson distribution and logit transformation, which included the fixed effect of feeding treatment and the covariate of lambing date.

HM during the lambing period and the pre- and post-grazing masses during the remainder of the study were analysed using a generalised linear model that contained the fixed effect of feeding treatment and period during which the sample was collected (lambing period or lactation). The ME content and DOMD of the forages were analysed using a generalised linear model that contained the fixed effect of feeding treatment and collection day. The botanical composition of the herbages was analysed separately for each feeding treatment using a generalised linear model that contained the fixed effect of collection day.

The effect of feeding treatment on lamb live weight, live weight gain, dressing out percentage and GR depth was analysed using a generalised linear model which included the fixed effects of feeding treatment and sex of the lamb with the covariate of birth date. Lamb survival up to weaning was analysed using a generalised linear model using a binomial distribution and a logit transformation. The model contained the fixed effects of feeding treatment and sex of the lamb. In addition, the total live weight gained per hectare was calculated as follows:

Results

HM and quality

The 2014 Study

During the lambing period HMs were greater (P < .05) in the Herb than Pasture treatment (2603 ± 106 vs. 1419 ± 106 kg DM/ha, respectively). During the remainder of the lactation period the pre-and post-grazing masses were also greater (P < .05) in the Herb than the Pasture treatment (3124 ± 82 and 2824 ± 82 kg DM/ha and 1612 ± 92 and 1347 ± 87 for the Herb and Pasture treatments, respectively). The ME and digestibility (DOMD) of the Herb treatment were greater (P < .05), and the NDF lower (P < .05) than the Pasture treatment on all three sampling dates (Table 1).

Table 1. The nutritional composition (ME, NDF and DOMD) of the feeding treatments (Herb or Pasture) at L1, L49 and L82 in 2014 and P145, L47 and L84 in 2015.

		Nutritional composition
Treatment	Study day	n	ME (MJ/kg DM)	n	DOMD (% DM)
2014
Herb	L1	4	11.4 ± 0.14^b	4	71.3 ± 0.29^c
	L49	4	11.0 ± 0.20^bc	4	68.5 ± 1.30^bc
	L82	4	10.5 ± 0.20^c	4	65.7 ± 1.30^bc
Pasture	L1	4	9.7 ± 0.20^a	4	60.2 ± 1.30^a
	L49	4	9.6 ± 0.20^a	4	60.2 ± 1.30^a
	L82	4	9.6 ± 0.20^a	4	60.2 ± 1.30^a
2015
Herb	P145	4	10.7 ± 0.08^c	4	66.9 ± 0.52^c
	L47	4	10.5 ± 0.08^b	4	65.4 ± 0.52^b
	L84	4	11.3 ± 0.08^d	4	70.9 ± 0.52^d
Pasture	P145	4	9.8 ± 0.08^a	4	61.0 ± 0.52^a
	L47	4	9.6 ± 0.08^a	4	60.3 ± 0.52^a
	L84	4	9.9 ± 0.08^a	4	61.6 ± 0.52^a

Note: ^abcwithin columns for each year, means with different superscripts differ significantly.

The 2015 Study

During the lambing period HMs were greater (P < .05) in the Herb than in the Pasture treatment (3687 ± 79 vs. 1951 ± 79 kg DM/ha, respectively). During the remainder of the lactation period the pre-and post-grazing masses were also greater (P < .05) in the Herb than in the Pasture treatment (3701 ± 100 and 3100 ± 100 kg DM/ha for the Herb and 2453 ± 84 and 1872 ± 84 for the Pasture treatments, respectively). The ME and DOMD of the Herb treatment was greater (P < .05), and the NDF lower (P < .05) than the Pasture treatment on all three sampling dates (Table 1).

Botanical composition

The 2014 Study

The Pasture treatment on L1, L49 and L82 was composed primarily of ryegrass and other grasses and contained small amounts of white clover, weeds and dead material (Figure 1). The Herb treatment was composed primarily of chicory and plantain and variable amounts of red clover, white clover, grasses, weeds and dead material (Figure 2).

Figure 1. Botanical composition of ryegrass, other grasses, white clover, weeds and dead material in the pasture sward on days 1, 49 and 82 of lactation (L1, L49 and L82) in 2014 and on day 145 of pregnancy and days 48 and 85 of lactation (P145, L48 and L85) in 2015.

Figure 2. Botanical composition of chicory, plantain, white clover, red clover, weeds, grass and dead material in the herb-mix sward on days 1, 49 and 82 of lactation (L1, L49 and L82) in 2014 and on day 145 of pregnancy and days 48 and 85 of lactation (P145, L48 and L85) in 2015.

The 2015 Study

As in 2014 the pasture was largely composed of ryegrass and other grasses (Figure 1). The herb–clover also was predominantly made up of chicory and plantain with clovers (red and white contributing 20–31% of the herbage dry matter (Figure 2).

Ewe live weight and body condition

The 2014 Study

At P139, ewe live weights and BCS did not differ (P > .05) between feeding treatments (Tables 2 and 3). Throughout lactation, ewes in the Herb treatment were heavier (P < .05) than those in the Pasture treatment. At L42, Herb ewes had greater (P < .05) BCS than Pasture ewes (P < .05); however, at L67 and L85 ewe BCS did not differ (P > .05, Table 3).

Table 2. Effect of nutritional treatment (Herb or Pasture) on ewe live weight (kg; mean ± S.E.) of ewe prior to the start of the lambing period (P139 in both 2014 and 2015) and during lactation (L42, L67 and L85 in 2014 and L33, L55 and L87 in 2015).

Treatment	Live weight (kg)
2014
	n	P139	n	L42	n	L67	n	L85
Herb	36	72.8 ± 0.83	36	87.2 ± 1.07^b	36	89.4 ± 1.15^b	36	91.8 ± 1.15^b
Pasture	35	73.8 ± 0.84	35	78.0 ± 1.08^a	35	83.6 ± 1.17^a	35	85.5 ± 1.17^a
2015
	n	P139	n	L33	n	L55	n	L87
Herb	37	71.1 ± 1.12	37	85.0 ± 1.20	37	87.0 ± 1.25	37	89.7 ± 1.36^b
Pasture	40	73.6 ± 1.02	40	82.8 ± 1.15	40	85.4 ± 1.20	39	84.8 ± 1.33^a

Note: ^abcwithin columns, main effects and years, means with different superscripts differ significantly.

Table 3. Effect of nutritional treatment (Herb or Pasture) on ewe body condition score (BCS, back-transformed mean with the 95% confidence interval in parentheses) prior to the start of the lambing period (P139 in both 2014 and 2015) and in lactation (L42, L67 and L85 in 2014 and L33, L55 and L87 in 2015).

Treatment	Body condition score
2014
	n	P139	n	L42	n	L67	n	L85
Herb	36	2.7 (2.2–3.2)	36	4.2 (3.5–4.9)^b	36	4.8 (4.1–5.6)	36	4.8 (4.1–5.0)
Pasture	35	2.7 (2.2–3.3)	35	3.1 (2.5–3.7)^a	35	4.2 (3.5–4.9)	35	4.2 (3.6–5.0)
2015
	n	P139	n	L33	n	L55	n	L87
Herb	37	2.5 (2.1–3.1)	37	3.8 (3.3–4.5)	37	3.8 (3.2–4.5)	37	4.1 (3.5–4.9)
Pasture	40	2.6 (2.2–3.2)	40	3.3 (2.8–3.9)	40	3.6 (3.1–4.3)	40	3.8 (3.2–4.4)

Note: ^abcwithin columns, main effects and years, means with different superscripts differ significantly.

The 2015 Study

Ewe live weight and BCS did not differ (P > .05) between treatments at P139, L33 or L55 (Tables 2 and 3). At L87, however, Herb ewes were heavier than Pasture treatment ewes (P < .05) but their BCS did not differ (P > .05).

Lamb live weight, live weight gain and survival

The 2014 Study

At birth and L42 there were no differences (P > .05) in the live weight of lambs born to ewes in either the Herb or Pasture treatment (P > .05; Table 4). At both L67 and L85, however, lambs born to Herb ewes were heavier (P < .05) than those born to Pasture ewes (P < .05). Lambs born to ewes in the Herb treatment had a greater (P < .05) live weight gain from birth to L85 (412 ± 8 g/day) than lambs born to ewes offered Pasture (371 ± 8 g/day). The pasture treatment produced a total lamb live weight gain per hectare of 313 kg compared with 484 kg from the Herb treatment. Lambs born to Herb ewes had greater survival rates up to weaning than those born to Pasture ewes (P < .05; Table 5). The percentage of lambs that achieved the target slaughter weight of 35 kg was 10% greater in the herb treatment compared to pasture; however, this difference was not significant (P > .05; Table 5).

Table 4. Effect of nutritional treatment (Herb or Pasture) on lamb live weight (kg; mean ± s.e.) at birth and in lactation (L42 and L87 (weaning) in 2014 and L33 and L85 (weaning) in 2015.

Treatment	Live weight (kg)								Live weight gain (g/day)
2014
	n	Birth	n	L42	n	L67	n	L85	Birth to L85
Herb	36	5.9 ± 0.13	36	21.8 ± 0.51	36	33.2 ± 0.67^b	36	40.7 ± 0.76^b	412 ± 8
Pasture	35	5.7 ± 0.13	35	20.7 ± 0.50	35	30.3 ± 0.66^a	35	37.1 ± 0.76^a	371 ± 8
2015
	n	Birth	n	L33	n	L55	n	L87	Birth to L87
Herb	35	6.4 ± 0.15	34	19.8 ± 0.37	35	29.2 ± 0.51	35	41.2 ± 0.71	402 ± 8
Pasture	36	6.3 ± 0.15	36	20.5 ± 0.35	36	28.9 ± 0.50	36	40.5 ± 0.70	395 ± 8

Note: ^abcwithin columns, main effects and years, means with different superscripts differ significantly.

Table 5. Effect of nutritional treatment (Herb or Pasture) on lamb survival to weaning (back-transformed mean with 95% confidence interval in parentheses) (L85 in 2014 and L87 in 2015), the percentage that reached 35 kg at weaning (L85 in 2014 and L87 in 2015) and carcass characteristics.

Treatment		Survival		35 kg at weaning	Slaughter data
	n	(%)	n	(%)	n	Carcass weight (kg)	n	Dressing (%)	n	GR (mm)
2014
Herb	36	100.0 (0–100.0)^b	36	89.1 (73.9–95.9)	–	–		–		–
Pasture	40	87.7 (73.3–94.9)^a	35	79.1 (61.8–89.8)	–	–		–		–
2015
Herb	38	92.1 (78.2–97.4)	35	34.3 (20.6–51.2)	33	20.2 ± 0.34	33	47.7 ± 0.3	33	13.0 ± 0.92
Pasture	40	90.0 (76.2–96.2)	36	33.3 (20.0–50.0)	30	19.6 ± 0.35	30	47.3 ± 0.3	30	11.5 ± 0.95

Note: ^abcwithin columns, main effects and years, means with different superscripts differ significantly.

The 2015 Study

Neither lamb live weight, percentage to achieve target slaughter weight, slaughter characteristics or survival up to weaning (L87) differed between treatments (P > .05: Tables 4 and 5). The live weight gain from birth to L87 of lambs born to ewes in the Herb and Pasture treatments did not differ (Table 3). The pasture treatment produced a total lamb live weight gain per hectare of 338 kg compared with 490 kg from the Herb treatment.

Discussion

Offering single-rearing ewes and their lambs a herb–clover mix during lactation improved the live weight of ewes at weaning in both years but only increased lamb live weight in 2014. In both years of the study, the aim was to offer ewes and lambs pasture allowances to allow unrestricted intake of their respective forages. Previous studies have indicated that ryegrass–white clover HMs above 1200 kg DM/ha result in ewe intake being unrestricted (Morris et al. 1993; Morris and Kenyon 2004). The post-grazing heights in the pasture group in 2014, although greater than the 1200 kgDM/ha target, were lower than in 2015 and suggest that intakes may have been limited which in turn may have influenced lamb growth. Further, there was some variability in the ryegrass pasture composition between the two years. In 2014, there was little white clover present (maximum of 2%), whereas in 2015 the minimum clover content was 3% with a maximum of 10%. This greater percentage of clover, however, had little effect on the ME content of the pasture offered. This lack of a difference in the ME of the pasture treatment reflects only the average ME for the pasture. Edwards et al. (2008) reported that in a grass–legume mix the proportion of legume in a sheep's diet can be greater than the proportion that is present in the sward due to selective grazing. In support of this Pain et al. (2014) have shown that sheep select clover in preference to ryegrass. It is also known that as the percentage of clover in the sward increases so does the growth rate of weaned lambs (McEwan et al. 1988). In 2015, ewes and lambs offered the Pasture treatment were provided with an opportunity to have a greater proportion of clover in their diet, compared to 2014, potentially explaining the lack of a difference in lamb growth between the pasture and herb treatments. The lamb production per hectare on the Herb treatment, however, exceeded that of pasture by 170 kg in 2014 and 152 kg/ha in 2015 due to its ability to support a higher stocking rate. This increase in productivity makes the herb mix an attractive option for farmers to maximise lamb growth rates prior to weaning.

Lamb live weight gains from birth to weaning on the herb–clover mix exceeded 400 g/day in both years of the study, although differences between treatments were only observed in 2014, not in 2015. The live weight gains observed in the current study were higher than those previously reported for twin and triplet lambs born to mixed-age ewes of 238 and 253 g/day by Kenyon et al. (2010) and Hutton et al. (2011b), respectively. Single lambs born to ewe lambs (bred at 8 months of age) showed greater live weight gains of 301 and 443 g/day in each of two years of a study by Corner-Thomas et al. (2014). To these authors’ knowledge, no other studies have reported the live weight gains of single lambs born to mixed-age ewes when offered the herb–clover mix. When compared to ryegrass pastures, herb–clover mixes have been reported to result in greater live weight gains between birth and weaning for twin- and triplet-born lambs born to mixed-age ewes and twin lambs born to ewe lambs (Hutton et al. 2011b; Corner-Thomas et al. 2014). Kenyon et al. (2010), however, found no difference in the live weight gains to weaning of twin and triplet lambs born to mixed-age ewes offered the herb–clover mix from late pregnancy. In that study, at the end of the feeding period, the ME content of the herb mix was lower than in the current study at 9.5 MJ ME/kgDM and was similar to that of the pasture in that study at 8.9 MJ ME/kgDM. It is likely, therefore, that the lack of difference in live weight gain of the lambs in that study was a result of the similar nutritional value of the treatments. In the present study it is not clear why live weight gains’ differences were observed in 2014, but not 2015. Given that the live weight gains of the lambs offered the herb–clover mix were similar between years it is likely that the lack of difference was due to the greater performance of the lambs offered the ryegrass pasture in 2015.

In the current study, lamb survival was greater for lambs in the herb treatment compared to pasture in 2014, but not 2015. Offering herb–clover mixes during lactation has been reported to have a positive effect on the survival of twin and triplet lambs born to mixed-age ewes (Kenyon et al. 2010; Hutton et al. 2011b). In the study of twins born to ewe-lambs, however, survival was greater for lambs in the herb treatment than pasture in only one of two years although live weight differences were observed in both years (Corner-Thomas et al. 2014). Hutton et al. (2011a) reported that the improvement of lamb survival was not related to ewe behaviour or lamb colostrum intake but hypothesised that greater ewe milk yields one week post-lambing may have resulted in greater lamb survival rates.

In both years of the present study, ewes offered the herb–clover mix had greater live weights at weaning than ewes offered pasture although BCS did not differ. Ewe live weight at the start of breeding can impact on ewe reproductive performance (Scaramuzzi et al. 2006). Ewes with greater live weights prior to breeding have been reported to have an earlier onset of oestrus (Doney and Gunn 1981), greater ovulation rates (Allison 1968; Knight 1980; Rattray et al. 1980; Rattray et al. 1983; Smith 1991) and lower levels of barrenness (Smith 1982) compared with lighter ewes. Similarly, greater body condition is associated with improved ovulation rates (Gunn et al. 1969; Bastiman 1972; Rhind et al. 1984; Forcada et al. 1992), conception rates (Bastiman 1972), embryo survival (Gunn and Doney 1975) and lambing percentage (Gunn et al. 1969; Gunn et al. 1972; Newton et al. 1980). In New Zealand weaning generally occurs in early summer, which can make it difficult for ewes to regain live weight and condition prior to breeding, particularly in dry areas (Moot et al. 2007). Ewes that are heavier at weaning, therefore, have a greater chance of achieving optimal pre-breeding weights.

In New Zealand, the price a farmer receives for a lamb at slaughter is dependent on the weight of the carcass, fat content and the requirement for trimming (New Zealand Meat Producers Board 2004). Carcasses that weigh between 14.5 and 23.0 kg with a GR up to 9 mm achieve premium prices (New Zealand Meat Producers Board 2004). In addition, lamb prices show annual variation with maximum prices typically observed in early November and dropping to a minimum in mid-March (JDJL Limited 2016). Assuming an average dressing out percentage for Romney lambs is 41.0% (Muir et al. 2008), a minimum carcass of 14.5 kg equates to a live weight of 35 kg. The 2014 data suggest, therefore, that a greater percentage of lambs would have reached the minimum target slaughter weight at weaning although this was not apparent in 2015. The most profitable lamb is one that is sold directly for slaughter at weaning (Morris and Kenyon 2014).

The dressing out percentage in the current study did not differ between the pasture and herb treatment groups. This finding is in contrast with a previous study that reported that in Spring weaned cryptorchid male lambs offered a herb–clover mix had greater dressing out percentages than their counterparts offered pasture (Somasiri et al. 2015b). Dressing out percentage is known to have a positive relationship with live weight (Muir et al. 2008) and is influenced by weaning status (not weaned vs. weaned; Kirton et al. 1984). It is perhaps unsurprising that no difference was seen given that in the 2015 study the live weights of lambs in each treatment did not differ and all lambs had the same weaning status prior to slaughter. In the years that the herb–clover mix results in greater live weights at weaning a concurrent increase in dressing out percentage would provide additional financial returns.

Conclusion

Grazing on the herb–clover mix during lambing and lactation increased ewe live weight above that of ewes offered ryegrass–white clover pastures in both years of the study. Although the results of only one year showed the herb–clover mix increased lamb live weights at weaning compared with ryegrass pasture, in both years of the study, lamb live weight gains exceeded 400 g/day.

Acknowledgements

The authors wish to acknowledge Mr Dean Burnham, Mr Geoff Purchas and Dr Catriona Jenkinson for their technical assistance and Beef & Lamb New Zealand and Massey University for funding this study.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Peter D. Kemp http://orcid.org/0000-0002-2239-2012