In pasture-based automatic milking (AM) systems, cow traffic and behaviour are different and, generally, cows achieve lower milking frequencies (MF) and lower milk production levels compared with indoor AM systems. But what are the main factors influencing this? It is suggested that the three main factors are milking frequency, milking interval (MI) and nutritional status of the cow.

The profitability of an AM system depends on maximising the amount of milk output per day (kg milk/AM system per day), which is associated with increasing the utilisation rate (percentage of time spent milking cows).

Results to date (based on New Zealand work and current study at Moorepark) indicate that, in a pasture-based AM system, an increase in the utilisation rate can be achieved by lowering MF per cow to an average of 1.5 milkings per day (milk yield is not reduced at this milking frequency). This would potentially allow available capacity to perform additional milkings, which could occur from a combination of reduced milking intervals greater than 16 hours and an increased number of cows on the system. This would potentially increase the utilisation rate of the AM system and the milk volume produced per day.

But, for this to work, the best farm management factors specific to pasture-based farming systems that could potentially optimise milk production per day must be put in place. Thus, it is necessary to develop a framework of management procedures that will optimise milking frequency, milking interval and nutritional status of the cow.

Factors that are known:

  • A key factor in increasing the utilisation rate of the AM system is the movement of the cow within the system.
  • Efforts must be focused on reducing the occurrence of MI above 16 hours, because the milk secretion rate beyond that threshold tends to decrease.
  • At some milking frequency point between 1.5 and one per day, milk yield is significantly reduced.
  • Pre-grazing pasture cover must be appropriate.
  • Pasture allowance must be appropriate.
  • The critical question is, what is the best management system to achieve these criteria – is it the AB grazing system (two-way) or the ABC system (three-way).

    Grazing options

    There are two main grazing management options for pasture-based AM systems. With the AB system, cows are offered their daily pasture allocation in each of two equally sized portions of, for example, 9kg DM/cow. Cows can access each pasture allocation for 12 hours (24h/2). The allocations could open at 8am and 8pm. With this system, cows have a 12-hour access time window.

    The end of one access period coincides with the start of the access period for the subsequent allocation. But, once the cows enter the allocation, they can remain for a minimum of 11 hours. Thus, cows have a maximum 23 hours in which they are expected to voluntarily exit the allocation for, the last 11 hours of which no additional cows can gain access to this allocation. Cows are required to have exited the allocation one hour prior to the subsequent allocation closing for access.

    With the ABC system, cows are offered their daily pasture allocation in equally sized portions of, for example, 6kg DM/cow in each of three allocations. Cows can access each pasture allocation for eight hours (24h/3). For example, the allocations could open at 8am, 4pm and midnight. With this system, cows have an eight-hour access time window, but once they have entered the allocation, they can remain for a further seven hours. Thus, cows have a maximum of 14 hours in which they are expected to voluntarily exit the allocation for, the last hour of this, no additional cows can gain access to that allocation. Cows are required to have exited the allocation one hour prior to the subsequent allocation closing for access.

    Every time cows move from one allocation to the other, they must present at automatic drafting gates where milking permission is granted and cows are drafted to the dairy, or milking permission is denied and cows are drafted to pasture. It is reasonable to believe that an increase in the frequency of pasture allocations should encourage cow traffic in the system and increase MF and milk production.

    These two systems of pasture allocation were compared in terms of cow traffic, milk production and system utilisation in a pasture-based AM pilot study at FutureDairy AMS research farm (Elizabeth Macarthur Agricultural Institute, NSW DPI, Camden, New South Wales, Australia) in 2010.

    This study showed that offering cows in a pasture-based AM system three pasture allocations per day instead of two, was related to a reduction in MI, an increase in MF and an increase in daily yield output, together with a greater system utilisation.

    Interestingly, there were more late-lactation cows fetched from the paddock in the two-way system compared with the three-way system. The study involved the entire milking herd of 145 cows (with two robots = 72.5 cows per robot), at a mean of 121 days in milk and with a seven-day average milking frequency = 1.52 ± 0.41 milkings/cow per day and seven-day average milk yield = 21.3 ± 7.6kg/cow per day.

    In this study, cows had a concentrate allowance of 6kg of DM/day (with a maximum daily carryover of 50%) aimed to achieve a concentrate intake of 4kg DM/day. Cows were milked using two DeLaval VMS milking units.

    The provision of the third pasture allocation per day was associated with more milking sessions per hour, as well as a more even distribution of milking events throughout the day. There was an interaction between the number of pasture allocations and the time of the day. A higher number of milkings/hour were performed under the ABC system between 1am and 2am, between 10am and 11am, between 12pm and 1pm, and between 3pm and 6pm. The number of milkings/hour was similar between treatments for other times.

    Compared with AB, cows in the ABC system experienced a reduction in the occurrence of milking events with intervals above 4pm, from 64.4% to 27% and had a greater proportion of milking events occurring with intervals between four and nine hours. An interaction between the number of pasture allocations and stage of lactation (SOL) was found for likelihood of occurrence of extended milking interval (MI).

    Cows were more likely to have a milking event with extended MI in AB than ABC, and the effect was much higher for cows in late than mid and early SOL.

    Cows in the ABC system had 31% reduced MI, 40% greater MF and 20% greater daily milk production in comparison to the AB system. The increase in system utilisation with the ABC system was predominantly caused by an increase in the number of milking events and the resultant increase in milk yield harvested per day.

    As grass was the main incentive encouraging cows to move around, increasing the number of pasture allocations in a day reduced MI by reducing RT (the time taken for the cow to return to the milking unit). This is in agreement with previous reports from studies conducted in New Zealand which found that increasing the frequency of incentive allocation could increase cow traffic in an AMS. The reduction in average RT from the AB to the ABC systems, which ranged from 4.5 (early lactation cows) to eight (late lactation cows) hours, could be explained by two key factors.

    Feed

    Firstly, there was a reduction in the amount of feed made available under ABC (in total kg DM/allocation), which created the potential for feed depletion to occur more rapidly, thereby creating an incentive for cows to exit the allocation sooner in search of additional feed.

    Secondly, the maximum amount of time cows could spend in one allocation was reduced under ABC (22 hours for AB versus 14 hours for ABC). This was associated with a reduced access period to each allocation and a reduced voluntary exiting period, both of which likely affected mean RT.

    The access times or ‘‘gate change times’’ to the allocations at different times during the day has an effect on cow behaviour, which may also affect cow traffic. If pasture allocated during the day were to be depleted in the early hours of the night and another feed source became available at that time, it is possible that cow traffic could be encouraged and a higher number of milkings could be observed during the night and, particularly, during the early hours of the morning (2am to 6am). This hypothesis has not yet been tested, and it is important that any management put in place does not negatively compromise cow time budgets and animal welfare.

    Overall, the ABC system was associated with a higher utilisation of the system and higher output of milk per AM unit per day. These results support the recommendation that, wherever possible, farmers installing AMS should incorporate sufficient infrastructure to accommodate the ABC system, which provides additional flexibility. Not all farm layouts lend themselves to provision of more than two daily pasture allocations and it is important that farmers make informed decisions regarding the need to modify their farm layout prior to adoption of AMS.