This review examines the potential for extended lactations on Victorian dairy farms in the context of (i)�the advantages and disadvantages of existing seasonal milk production systems; (ii) projected future changes to Victorian dairy farming; (iii) the mechanics of extended lactations; (iv) the change in costs associated with extended lactations; and (v) how extended lactations might be successfully implemented. Traditionally, Victorian dairying has been characterised by seasonal, low input, pasture-based milk production reliant on family labour. Successful implementation of this type of milk production system requires cows to calve in accordance with seasonal pasture supply. Recently a number of economic (static or falling milk prices), management (larger herds, increased labour expense, lower labour availability) and social pressures (animal welfare and environmental regulation) have emerged that question the viability of pasture-based dairying in a number of countries including Australia. Some of these threats to dairying may increase the attractiveness of systems that exploit the production potential of high yielding cows in larger herds, with increased supplementation of pasture-based diets over extended lactations. It is generally concluded that 12-month calving intervals are the most economic, based on the shape of the lactation curve and the perceived costs of extended lactations. However, these fundamentals have been changed by years of intensive selection for high milk production and the recent implementation of new management techniques such as Bovine somatotrophin, total mixed rations and increased milking frequency. Modern high yielding cows can maintain high daily yields for a much longer proportion of lactation than previously, but these cows are also largely unsuited to seasonal production systems due to a prolonged period of negative energy balance after calving and consequent low fertility. We have identified that calving and pregnancy incur substantial costs that are often hidden but need to be accounted for in valid economic comparisons. It is evident that seasonal pasture-based dairying requires disciplined management where certain tasks must be achieved in a timely manner if the system is to operate efficiently. Where the system does break down, often due to infertility for example, new management approaches are required. It is of concern that farmers may adopt a default position of extended lactations without realising or exploiting the potential benefits. Among the advantages of extended lactation management would be a more even spread in labour requirements, input costs and income across the year. In conclusion, extended lactations are a suitable option for some Victorian dairy enterprises. That suitability will depend on a number of factors; particularly, cow milk production potential, ability to grow pasture or feed supplements economically, management expertise, environmental constraints, herd size and labour availability.

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The aim was to examine the effect of dietary urea on yield and quality of embryos in superovulated donor ewes, and on embryo survival in recipient ewes. Ewes (25 donors and 34 recipients) were offered 1·25 kg of grass meal containing 50 g urea per ewe per day on a group basis; the remaining ewes (30 donors and 32 recipients) received 1·25 kg untreated grass meal per day. After 5 days, ewes were synchronized using a progestagen pessary for 14 days and stimulated with pregnant mare serum gonadotropin. Donors were naturally mated; recipients were detected in oestrus using vasectomized rams. On day 4 following the onset of oestrus, embryos were recovered from donors, graded morphologically and cell number counted. Two embryos of acceptable morphological grade (≥ eight cells on day 4) were then surgically transferred to recipient ewes. Recipients were slaughtered between days 34 and 36 post oestrus. The mean number of embryos recovered per ewe [3·68 (s.e. 0·41) v. 3·83 (s.e. 0·45)] was not different between untreated and urea-treated ewes, respectively. However, urea treatment of donor ewes reduced the mean cell number per embryo (10·27 (s.e. 0·27) v. 8·17 (s.e. 0·29) for untreated and urea-treated donor ewes, respectively, P < 0·001). Similarly, the percentage of embryos recovered that had ≥ 10 cells and their mean cell number was reduced following urea treatment of donor ewes (82/113 (73%) v. 39/93 (42%), P < 0·01 and 11·61 (s.e. 0·20) v. 10·41 (s.e. 0·29), P < 0·001) for untreated and urea-treated ewes, respectively. The diet offered to recipient ewes had no effect on the number of ewes confirmed pregnant (24/32 (75%) v. 28/34 (82%)) or on embryo survival following transfer (45/64 (70%) v. 51/68 (75%) for untreated and urea-treated ewes, respectively, P > 0·05). Excess nonprotein nitrogen in the form of dietary urea reduced embryo quality. However, embryo survival in recipient ewes was not affected when good quality embryos were transferred. This suggests that the deleterious effects of excess dietary urea are due to alterations to the oocyte and (or) the oviductal environment rather than changes in the uterine environment.

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