Collectively, research results indicate that compared with good pasture management, pre-graze mowing during spring:
- does not increase cow intake or performance (milk solids production or body condition score gain)
- reduces pasture performance (tiller numbers, growth rates and density), which:
- reduces pasture surplus available for silage
- increases the requirement for imported feed.
Key results from a recent research experiment conducted at Lincoln University Research Dairy Farm (LURDF) are outlined below.
Pre-graze mowing trial
An experiment with four treatments (two farmlets per treatment) was carried out at Lincoln Farm for four months (October 2016 – February 2017). Stocking rate was 3.5 cows/ha.
- moderate pre-graze cover (2900 kg DM/ha) and grazing standing pastures
- moderate pre-graze cover and mowing before grazing
- high pre-graze cover (3500 kg DM/ha) and grazing standing pastures
- high pre-graze cover and mowing before grazing.
Farmlet decision rules included
- Target post-graze residuals of 8 clicks on rising plate meter (RPM).
- Mower was set to achieve residuals of 8 clicks on RPM.
- Paddocks were mowed once daily immediately prior to cows entering paddock.
- Rotation length was flexible to achieve different pre-graze covers.
- If rotation length could not be altered (based on feed wedge) and residuals were below target, then silage could be fed.
- If residuals were above target and feed wedge indicates a surplus, pasture can be harvested for silage.
- Weekly pre- and post-grazing pasture residuals.
- Remaining pasture in mown paddocks.
- Weekly pasture growth.
- Tillers (start and end of the trial).
- Pasture quality and botanical composition.
- Daily milk yields and weekly protein and fat yields.
- Fortnightly body condition score.
- Cow behaviour (grazing, ruminating, lying).
Key results from the trial
Mowing versus grazing
- There was no benefit of pre-graze mowing on cow intake, milksolids production or body condition score gain with both mown and grazed farmlets producing 1.8 kg MS/d throughout the experiment.
- Pasture disappearance (calculated from pre-grazing yield less post-grazing/mowing residual) was 2 kg DM/cow/day greater for mown farmlets BUT this was offset by refused pasture left in mown paddocks of 2 kg DM/cow/day.
- There was no difference in cow behaviour (lying, grazing, or ruminating times) between mown and grazed farmlets
- Pastures performance was reduced in the mown farmlets. Pastures were less dense (kg DM/ha) for a given height and contained less tillers
- Less silage was conserved from the mown farmlets
- More silage was fed to cows in the mown farmlets to maintain cow intakes.
High versus moderate pasture covers
- Cows offered the moderate pre-graze pasture covers (2900 vs. 3400) produced 6% more milksolids than high pre-graze pastures during the experiment.
- There was no consistent effect of pre-graze covers on pasture quality (e.g. digestibility, fibre or metabolisable energy) however, cows in the high-cover farmlets spent an extra 42 mins per day grazing which was possible an attempt to counter a reduced intake rate from grazing more mature pastures.
If target residuals are not met during spring, then mowing before grazing or topping after cows have left the paddock can be used as a tactical tool to meet target residuals. This will increase pasture quality in the following round, particularly in regions exposed to summer dry.
However, research indicates that post-graze topping is preferable to pre-graze mowing from a cow and pasture perspective. Cows can select higher quality feed, as smaller leaves (e.g. clover, new tillers) do not fall below the grazing horizon. It is also easier to identify a surplus if cows graze pastures first. Pasture wastage occurs with both post-graze topping and pre-graze mowing.
Effect of pre-graze mowing at different pre-graze masses on cow and pasture performance