Measurement of Grassland Management Practice on Commercial Dairy Farms

Introduction

The simplest and most financially rewarding system of milk production for Irish dairy farmers is that based on making the maximum use of grazed grass. With good grazing management Irish dairy farms can have a long grazing season of high quality feed at low cost. The CAP reform proposals may result in lower milk prices and, as a result, may compel dairy farmers to use new technology to maintain their incomes. One such technology will be more efficient use of grazed grass. Grassland management practices were studied on 12 intensive dairy farms with a total of 1600 cows and an average stocking rate of 2.7 cows/ha in the grazing period autumn 1995 to winter 1998.

Grassland Measurement

Farm Grass Cover

Farm grass cover is the total supply of available grass (>4 cm) on all paddocks for grazing. A dairy farmer requires a farm map with the area of each paddock and a knowledge of the amount of grass on each paddock to measure farm grass cover.

Post-grazing sward height

Post-grazing sward height is the average height (cm) to which a paddock is grazed when the cows have finished grazing. The post grazing sward heights include both dung pads and non-dung pad areas. These give the farmer an independent measurement of how well his cows are being fed. They can also be used as an indication of the residual grass yield left after grazing which, in turn, will have an effect on pasture quality, in the subsequent grazing rotation. Table 2 shows a range of post-grazing severity scores, the associated grass height and a description of the grazing severity.

Table 2. Post grazing severity score

Pasture quality

Pasture quality is a measure of the digestible energy content of the feed. There are high correlation's between the quality of the pasture, as measured by either the proportion of live-leaf or organic matter digestibility (OMD), and its milk production potential. Factors which influence this are previous post-grazing heights, rotation length, pasture species and the degree of pasture topping being practiced. It was found that the most practical measurement of pasture quality is the proportion of live-leaf in the sward directly ahead of the cows. It is desirable that the proportion of live leaf should not be less than 65% (Table 3).

Table 3. The relationship between the proportion of live leaf and organic matter digestibility.

To convert (OMD) to dry matter digestibility (DMD) subtract 4 - 5 units from the OMD

Autumn/winter (late August to December)

The aim is to maximize the amount of grass utilized in the period September to December while finishing the grazing season with the desired farm grass cover. The farm grass covers obtained over this period depend on stocking rate, level of supplementation, autumn nitrogen application, calving pattern and milk yield potential. The following guidelines should be followed.

• Rotation length should be increased from 21 days in mid/late August to 30 - 35 days in late September.
• Aim for a pasture cover of 1000 -1100 kg DM/ha (at a stocking rate of 2.75 cows/ha) by late September.
• Pre-grazing yields should not exceed 3000 kg DM/ha and should be adjusted downwards for wetter type soils. Very high pre-grazing yields result in poor pasture quality and poor utilization of this pasture by the grazing animals.
• The final grazing rotation of the year should be 25 to 35 days in length and resting of paddocks should begin from October 10 to 15.
• Avoid grazing very high pre-grazing yields (>2500 Kg Dm/Ha) in the final rotation to avoid the problems outlined above. In addition, high pre-grazing yields in the final rotation were shown to have a detrimental effect on subsequent spring grass growth.
• The closing farm grass cover in late November/early December should be 400kg DM/ha, paddock cover should range between 200kg DM/ha (last closed paddock) to 900 kg DM/ha(first paddock closed).
• Pastures should be grazed well in the final rotation to encourage autumn/winter tillering. Aim for a residual grass yield of 200 kg DM/ha (5cm high) in paddocks as they are closed in the mid-October to mid-November period.

Spring (late February to late April)

The aim at this period is to maximize the amount of grazed grass in the cows diet while at the same time having a farm grass cover of >850 kg DM/ha by late April. The management factors which have the largest influence on the quantity of grazed grass consumed/cow over this period are overall stocking rate, calving pattern, autumn closing cover, silage ground availability and spring nitrogen application.

Appendix 2 shows how a feed budget may be used to reach the required targets on the farm. Weekly monitoring of grass cover is required with very variable spring grass growth rate. Appendix 3 shows the grass growth rates measured at five locations in the country.

The following guidelines should be followed:

• Aim for a farm grass cover of 550 to 600 kg DM/ha at turnout at a stocking rate of 2.75 cows/ha. Turnout at lower and higher cover is possible at lower and higher stocking rates, respectively.
• Pre-grazing yields at turnout should be greater than 1000kg DM/ha and daily grass allowance per cow should not be less than 5kg DM/cow/day.
• The available grass supply should be budgeted so that the first grazing rotation finishes at April 10 to 20.
• The target farm grass cover is 850 kg DM/ha in late April (at a stocking rate of 4.5 cows/ha on the grazing area).
• Excessively low post-grazing heights (<4.5 cm grass cover) should be avoided in this period as it will slow re-growths.
• Late turnout, with large farm grass cover, often leads to poor grass utilization and subsequent poor pasture quality.
• Good grazing management practices, such as, block grazing and a good farm road network reduce the risk of soil damage during this period.

Main Grazing Season (May to August)

The objective over this period is to achieve high cow performance from grass with minimal or no concentrates supplementation. High cow performance is achieved by maintaining a consistent grass supply for the herd on a daily basis. Weekly monitoring of farm grass cover will assist this. This will allow early alteration of grass supply, e.g., adjusting stocking rates and/or introducing supplements.

The key grassland management guidelines for this period are as follows;

• Farm grass cover should be maintained at 900-950 kg DM/ha or 200 to 240 kg DM/cow on the grazing area during the main grazing season.
• Under normal grass growth rate conditions, a stocking rate of 4.5 cows/ha from mid- April to early June permits adequate feeding of cows at pasture at a grass allowance of 20 -21 kg grass DM/cow/day. Supplementation is required at higher stocking rates. Appendices 4 & 5 give examples of how to calculate daily grass allowance, intakes per cow and kg DM available per cow.
• Pre-grazing yields should be maintained at 1800-2200 kg DM/ha.
• Pastures with high post-grazing residues (>350 kg DM/ha) or high post-grazing heights (>7.5 cm) should be topped. Pasture topping should take place early (mid-May) rather than late in the season.
• Grazing to a very low post-grazing height (< 5 cm) will result in inadequate grass intakes and will result in reduced milk yields and protein content.
• Aim to maintain >65% of live leaf in the sward immediately ahead of the cows. Appendix 6 gives the method of calculating the percentage live leaf in the sward.
• Use grass measurements to identify grass surpluses or deficits.

The following conditions indicate a grass surplus:

• Pasture covers greater than 1100 kg DM/ha on the grazing area in the period from late-April to early August
• An extremely steep grass supply wedge pattern (350 to 1/43000 kg DM/ha) on the grazing area.
• Grazing large covers (>2500kg DM/ha) will often result in reduced milk production while topping large residuals often reduces grass supply in the following rotation.

There is a grass deficit when the farm pasture cover is less than 750 kg DM/ha (at a stocking rate >3.5 cow/ha on the grazing area) and when pre-grazing yields are less than 1500kg DM/ha. Reducing grass covers to very low levels results in reduced grass growth rates on the total farm and generally results in underfeeding of the cows. Low farm grass cover can be alleviated by increasing the grazing area and/or introducing supplements while maintaining rotation length at 20 to 24 days.

Greater spring grass supply by improved autumn management

The study showed the importance of end of season grazing management and its large effect on spring grass supply. Figure 1 shows the increased grass covers at the end of the grazing seasons over the three years and its consequences for the following spring grass supply. The farmers in the study recognized this and changed their grazing management to obtain the benefit.

Figure 1: The average improvement in closing and opening covers over the three year period

Optimum utilisation of spring grass

Research results have shown the large benefit to be obtained from early spring grass. Weekly monitoring of farm grass supply from late February to late April allowed the dairy farmers to maximize this benefit although circumstances differed on the farms, eg., stocking rate, calving pattern, grass supply, etc.

Figure 2 shows the increase, in one spring calving herd, in the proportion of grazed grass in the diet from turnout until the end of April over the three years. The combination of better autumn grazing management and better feed budgeting in spring allowed the farmer to increase the amount of grazed grass by 1.0 tones over the three years.

Figure 2: The increase in the early spring allowance of grazed grass in the diet of one spring calving herd

Identification of grass surpluses and deficits

Frequent monitoring of grass supply during the grazing season allowed the dairy farmers to make earlier and better grazing management decisions. Allowing the grass supply to reduce to very low levels has been shown to reduce the total grass grown on a farm. Likewise, allowing very large grass covers to accumulate reduces milk production and increases the requirement for pasture topping. Excessive topping results in poor grass utilization and probably reduces grass supply in the following rotation. Figure 3 shows how frequent monitoring of grass supply allowed one dairy farmer to make the decisions required to match feed supply to feed demand. Figure 4 shows the grass cover profile of a poorly managed grazing system, pasture cover was in deficit both at turnout and during the main grazing season.

Achieving higher performance from grazed grass

One of the main reasons for low performance being obtained from grazed grass on some farms in the study was an inadequate daily grass allowance.

• This theory is supported by the very low post-grazing height, large reduction in milk yield, below optimum body condition score and reduced milk protein content of milk.
• A knowledge of pre-grazing yield and post-grazing height allowed dairy farmers to become more confident in achieving high performance from grazed grass. Figures 5 and 7 show daily feed allowance and milk production, respectively, over the grazing season for a well fed spring calving herd. Figures 6 and 8 show the daily feed allowance and the milk production, respectively, over the grazing season for a poorly fed spring calving herd.

Figure 3: Well managed grass cover over the grazing season

Grazing cover (Y axis on left of graph), stocking rate (Y axis on right of graph). Paddocks cut for silage: 3 paddocks on 27/5, 4 paddocks on 25/7 and 4 paddocks on 5/9.

Figure 4: Shows the grass cover profile of a poorly managed grasscover. Pasture cover is in deficit both at turnout and during the main grazing season

Grazing cover (Y axis on left of graph), stocking rate (Y axis on right of graph). Paddocks cut for silage: 4 paddocks cut for silage 26/8.

Figure 5: Feed allowance of an adequately fed spring calving

Figure 6: Feed allowance of an inadequately fed spring calving herd.

Figure 7: Milk yield and milk composition from the feed allowance shown in Figure 5

Figure 8: Milk yield and milk composition from the feed allowances shown in Figure 6