Pre-graze mowing

Mowing before grazing (pre-graze mowing) is a management strategy sometimes used to achieve target pasture residuals when there is a surplus of pasture.

Collectively, research results indicate that compared with good pasture management, pre-graze mowing during spring:

• does not increase cow intake or performance (milksolids 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.

Treatments were:

1. moderate pre-graze cover (2900 kg DM/ha) and grazing standing pastures
2. moderate pre-graze cover and mowing before grazing
3. high pre-graze cover (3500 kg DM/ha) and grazing standing pastures
4. 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.

Measurements/calculations included

Pasture

• 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.

Animal

• 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.

NOTE:

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.