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How do I manage my grass during a drought?

Soil moisture deficits have begun to impact on daily grass growth regionally, with growth rates on some farms falling below 45kg DM per day (PastureBase May 2020), writes Joe Patten, Teagasc.

As a result, average farm covers are beginning to drop below the target 160-170kg DM per cow.

Given the risk of sustained dry weather, it is important that prompt actions are taken to manage the situation.

Grazing management decision rules

The main priority now is to reduce daily grass demand to below daily growth rate. This will help to hold grass cover on the farm, protecting current growth and speeding up recovery when rain arrives.

Rotation length must be maintained at 24-25 days Effectively this means grazing no more than 4% of the grazing platform daily. Assess the grass available on this area and supplement with forage/ concentrate to balance herd demand.

For example, a 140-cow herd is grazing 45ha (3.1 stocking rate). Max daily area allowed should be 1.8ha (4% of 45). If there is 1100kg DM per ha available, then the paddock has 1.8*1100= 1980kg available. Herd demand is 2520kg per day, therefore 540kg of total supplement is required per day.

In this example, holding total grass allowance to 1980kg equates to 44kg daily demand per ha day (1980/45). This will hold grass cover per ha reasonably well if growth rates are within 5-7kg daily. Larger deficits will rapidly reduce the average farm cover.

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If there is a larger deficit between growth and demand it will be necessary to temporarily reduce demand further by reducing grazing stocking rate and feeding extra silage.

Increasing rotation length beyond 30 days may lead to much reduced grass quality in current conditions

Post grazing residuals of 4 to 4.5cm must be maintained, otherwise feed is being wasted.

Maintain fertilizer N applications after grazing. Risk of N losses is low at present. However, if drought conditions persist to >60mm soil moisture deficit it is advised delay N until rain is forecast.

Hints and tips on feeding out forage supplements in dry weather

Dry conditions should make the task of feeding out much easier compared to spring. Each farm will have its own preference (based on facilities/machinery/labour) but the main aim remains to reduce total daily grass intake to the level of daily growth or below. Once available daily grass is known, some options for feeding are:

  • Separate a portion of the herd and place on 100% silage plus meal in the shed or a convenient paddock. This may be a paddock marked for reseeding later in the year.
    • A small area of fresh grass can be allocated to this group daily. Some farms have used a double temporary wire feeding rail to good effect. This approach simplifies grazing management of the main group.
  • 2. Offer silage to all cows in the grazing paddock, placing silage along perimeter fencing. This works best where feed can be allocated with a diet feeder. Silage allocation should be calculated to balance available grass on the paddock daily. Forage should be spread along a long linear distance (1m per cow) to reduce bullying.
  • 3. Hold part/all of the herd in the yard for silage feeding after milking and turn out after about 2 hours. This can simplify feeding out silage but there is a risk of injury due to slippery concrete floors. Adequate space needed.
  • 4. High fibre straights can be offered PKE/hulls/pulp at a rate of 3 kg per Some farms choose to feed these in mobile feed troughs in the field.
    • Note that citrus pulp does not work well in this situation due to its lower NDF fibre content. Ensure full access to clean water.

Whichever actions are chosen, it is vital to act now to ensure that grass supply is rationed out as early as possible. Plan to supplement until 3-4 days after growth exceeds demand.

Concentrate feeding guidelines:

Parlour-fed concentrate will form a major part of daily feed allowance in drought conditions. Some decisions rules are:

  • Feed up to 5-6kg of parlour concentrate per day as part of an overall feed This is a relatively safe level provided adequate forage and water are provided. A further 2-3kg of high fibre straights can be fed out-of-parlour.
  • Purchase concentrate based on UFL value, targeting a value of >0.94 UFL on a fresh weight basis
  • Ration crude protein should be decided based on overall composition of the diet. In normal circumstances a 14% high energy ration would be adequate at grass.
    • However, during a drought it is likely that lower protein ingredients will form a significant part of the diet.
    • Also, where grass is drought stressed and lacking N uptake, it is possible that sward protein content could be lower than normal.
    • Therefore, it is recommended that a 16% ration be used if grass intake is around 7 to 10kg per day. If the herd is placed on silage full-time than a high energy ration of 18+% will be needed in the short term. These targets are for parlour rations fed at 4-6kg.
  • Be careful not to overfeed magnesium. A rule of thumb is that cows will tolerate up to twice the recommended allowance of CalMag over a short period (100-120g per day);
  • Above this level, there may be issues with scouring as Mg has a laxative effect. Therefore, if concentrate is formulated for a 2kg feeding rate then max feeding rate should be limited to 4kg.

Decision rules on grazing silage crops

Areas closed for silage and accessible for grazing with <2200kg DM covers may be grazed as a ‘standing supplement’. Pre-mowing does not confer any advantage in this situation;

Recent work on zero-grazing in NI showed a significant drop in milk yield where heavy swards (2500 kg DM) were cut and fed, relative to cutting or directly grazing lower mass swards (<1600kg DM). The decision to zero graze should be based on pre-grazing yield;

Overall, if silage swards have surpassed ideal pre-grazing herbage masses and are nearing cutting stage then it is preferable to leave for silage cutting at this stage;

Maincrop silage yields are quite good in many areas– take the opportunity to make first cut silage before quality begins to drop in early June. Second cut yields will be much better as a result.

Other management issues

If drought conditions persist into June consider offloading problem cows that are already in line for culling;

Once-daily milking (OAD) is known to help cows retain body condition at a cost of reduced milk solids output (15-20%). However, this assumes that cows’ intake is maintained relative to 2x daily milking;

Where OAD milking is imposed in tandem with reduced feed intake, milk output may be reduced by >30%. OAD is an option to manage severe situations but at this point, the preferred option is to supplement required feed instead;

Do not neglect young Total dry matter intake requirements are small relative to the milking herd but nonetheless adequate feed dry matter (2.0 to 2.2% of liveweight) must be offered daily.

If silage must be fed for a few weeks in summer, complete an early fodder budget. This will allow plenty of time to take action if there is a risk of feed shortages later in the year.

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