Grazing crops: implications for reproducing sheep

Abstract

Integration of crops and livestock has been revitalised in Australia, initially as an opportunity to increase cropping within the high-rainfall grazing zones, and more recently, to improve enterprise diversification and profitability across the low-, medium- and high-rainfall, and mixed-farming zones. Young crops are highly digestible (>80% dry matter digestibility, DMD) with a high energy density (>12 MJ/kg DM) and, in much of southern Australia, fill a winter feed gap. The quality and time of feed availability also coincide with the high nutrient requirements of ewes in late pregnancy and lactation. In Western Australia and South Australia, young crops are available for lactating ewes and young growing lambs (autumn lambing). For the smaller proportion of growers who lamb later in winter, young crops are available for the last 1–2 months of pregnancy. In the later-lambing states of New South Wales and Victoria, crops may be grazed by ewes at any stage of pregnancy and lactation and/or by young lambs. In Tasmania, crops are more likely to be available during early–mid-gestation. Limited studies on feed budgeting with grazing crops have indicated that ewes can maintain or even increase liveweight, with a much lower level of feed on offer than would be required with traditional pastures (<500 kg DM/ha). This has the potential to increase whole-farm stocking rates and/or reduce fetal mortality, increase lamb birthweight and survival and improve lifetime production. Maintaining or increasing ewe liveweight during pregnancy and lactation may also result in heavier ewes the following year and higher ovulation rates. Pregnancy and lactation are also periods of increased susceptibility to metabolic disturbances. The composition of young crops increases this susceptibility. Pregnancy toxaemia, hypocalcaemia and hypomagnesaemia can influence ewe health and fetal survival. Chronic acidosis and excessive ammonia absorption from rapid introduction of pregnant ewes onto young crops may risk appetite loss and increase susceptibility to pregnancy toxaemia. Low magnesium and sodium combined with high potassium increases the risk of grass tetany. Most young crops (except canola) also have a tetany index >2.2, indicating a high risk of grass tetany. The elevated potassium also contributes to a high dietary cation–anion difference of approximately +49 mEq/100 g DM and this may cause metabolic alkalosis and hypocalcaemia. Pregnancy toxaemia, hypocalcaemia and grass tetany are all potential causes of increased ewe mortality. Pregnancy and/or lactation outcomes will also be influenced by a deficiency of trace elements. Grazing young crops in areas with a history of selenium, copper, iodine and cobalt deficiency will increase susceptibility to deficiency by increasing growth and feed intake. In conclusion, the grazing of young growing crops presents new opportunities for increased production and stocking rates in the mixed-farming zones. The value of this feed source is well recognised by some producers. While growing crops have a highly productive potential, they also come with an increased risk of a range metabolic disturbances and nutritional imbalances. These risks can be minimised by regular monitoring of livestock and crop biomass and the provision of mineral supplements.