Dual-purpose wheat (Triticum aestivum) refers to wheat crops that are sown earlier than usual with the intention of grazing livestock on crops during the vegetative phase prior to stem elongation (Growth Stage 31). Livestock are then removed from the crop so that grain can be harvested at the end of the season. Good management of dual-purpose crops can result in grain yields similar to ungrazed crops sown at optimum time, resulting in increased producer returns due to the additional livestock production from grazing the crop. Other advantages of dual-purpose crops include high livestock production and reducing the annual winter feed gap and the opportunity to increase farm stocking rates. Most research with dual-purpose wheat has used young livestock grazing wheat with a focus on grain yields and growth rates of the young animals. This thesis explored the potential to utilise these crops for grazing by reproducing ewes. It was hypothesised that grazing of dual-purpose wheat with reproducing ewes in southern New South Wales (NSW) provides a strategy to manage the winter feed gap, allowing farm stocking rates to be increased. Reported concentrations of calcium (Ca), magnesium (Mg) and sodium (Na) in wheat forage are below requirements of ewes during late pregnancy and lactation. This could be expected to increase the risk of metabolic disease in ewes grazing wheat at these reproductive stages, thus restricting the ability of producers to graze ewes on these crops. A comparative survey of producers in southern NSW identified that when high ewe mortality rates associated with grazing dual-purpose wheat occurred, a significantly (P<0.05) lower proportion of respondents had supplied mineral supplements that included magnesium or sodium to ewes, a higher proportion had fed grain, and ewes were reported as being in lower average body condition score (BCS). Producer reported diseases associated with ewe mortalities when grazing wheat included dystocia, metabolic diseases (pregnancy toxaemia, hypocalcaemia and hypomagnesaemia) and foot abscess. The hypothesis that grazing reproducing ewes on dual-purpose wheat may increase ewe ill-health and mortality rates was supported, and metabolic diseases implicated. It was identified that good management, in particular supply of mineral supplements and having ewes in adequate body condition score, could assist to reduce or prevent these incidence occurring. It was hypothesised that providing a loose-lick containing Mg, Ca and Na to lambing ewes would reduce the incidence of clinical or sub-clinical hypomagnesaemia and hypocalcaemia in ewes grazing a dual-purpose wheat crop. Replicated field experiments were conducted with Merino (2010) and Coopworth x Merino (2011) ewes grazing wheat with treatments ± ad libitum access to a loose-lick supplement consisting of magnesium oxide, calcium carbonate and sodium chloride in a 2:2:1 ratio. Ewe BCS was monitored, and lamb birth weights and growth rates recorded to lamb-marking. A high incidence of ewe health problems or death was not observed in either experiment, and there was no difference in ewe BCS between treatments. Twin-born Merino lambs had higher growth rates to lamb marking when ewes had access to the mineral supplement (259 vs 243 g/head.day; P = 0.002), however growth rates of single-born Merino lambs or twin-born cross bred lambs did not differ significantly when ewes were supplied a mineral supplement. There were no significant effects on lamb survival from providing ewes access to a supplement. Average apparent consumption of supplement was 24 g/ewe.day for Merino and 16 g/ewe.day for crossbred ewes. There was no association of sub-clinical hypomagnesaemia and hypocalcaemia (based on serum samples) with supplementation regimen. It was concluded that the experiments could not identify whether supplying a mineral supplement to correct expected deficiencies would assist to reduce the incidence of disease, but this practice may increase the growth rate of twin-born sucking lambs. Recommendations for pasture availability for reproducing ewes have been developed on permanent pastures (e.g. ryegrass and clovers). It was hypothesised that the difference in the height:biomass relationship in wheat crops compared to pasture would allow ewes to graze crops at lower feed on offer (FOO, kg DM/ha) compared to the recommendations for pasture. Ewes grazed dual-purpose wheat crops in these experiments with FOO at the commencement of grazing below industry recommendations (552 and 1092 kg DM/ha for Merino and cross bred ewes respectively, compared to recommendation of 1200 green kg DM/ha for twin-bearing ewes grazing pasture), and ewes did not lose condition during either experiment. In a third experiment (2012), lambs were used as a model for ewes. Liveweight gains of lambs peaked at a FOO of 1200-1350 kg DM/ha. The relationship between liveweight gain of lambs and FOO did not differ significantly for lambs grazing wheat crops sown at narrow or wide row spacing, or an annual ryegrass (Lolium multiflorum) pasture. The experiment was characterised by a decline in digestibility of the wheat pasture in the latter part of the experiment as the crop was grazed down, which significantly (P<0.05) reduced liveweight gains by 71 g/hd.day at a given FOO. Use of the height and mass data for ryegrass and wheat collected in this experiment in GrazFeed showed that late-pregnant ewes grazing dual-purpose wheat would be able to meet a higher proportion of their potential intake at a lower feed on offer compared to when grazing pasture. It was concluded that ewes can be grazed on dual-purpose wheat with a starting biomass as low as 500 kg DM/ha providing conservative stocking rates are used and the nutritive value of the crop remains high. The preceding research identified that with good management ewes could be safely grazed on dual-purpose wheat crops during late pregnancy and lactation. The hypotheses that when reproducing ewes were allowed to graze a dual-purpose crop the optimal lambing time (defined as the highest median gross margin returns over the long term) would be earlier in the season and the optimum stocking rate would be increased were tested using simulation modelling (AusFarm®). Modelling a portion of a mixed-farm in southern NSW with a lucerne feed base identified that allowing Merino ewes producing first-cross lambs to graze a dual-purpose wheat crop in winter would reduce the amount of grain supplement that needed to be fed, reduce inter-annual variation in returns and increase median gross margin returns. The optimal month of lambing for this enterprise was June and this was not altered when the dual-purpose crop was included in the system. Allowing autumn-lambing ewes to graze a dual-purpose wheat crop provided the opportunity to increase the number of ewes in the enterprise to the same level as lambing in June or July, although median gross margin returns remained lower than when lambing in June. The hypothesis that grazing reproducing ewes on dual-purpose wheat provides a strategy to manage the winter feed gap and can increase producer returns was supported. An increase in farm stocking rates only occurred for autumn lambing flocks when a dual-purpose wheat was included in the system. It was concluded that with good management ewes can safely be grazed on wheat; in particular it is recommended that ewes be provided ad libitum access to a mineral supplement that includes Mg, Ca and Na, and that grazing ewes in low BCS on wheat be avoided. Ewes can be grazed on wheat crops with a feed on offer of >500 kg DM/ha without adverse health implications.
|Qualification||Doctor of Philosophy|
|Award date||01 Aug 2014|
|Place of Publication||Australia|
|Publication status||Published - 2015|