DairyNZ senior scientist Kevin Macdonald explains a project set up to demonstrate the benefits of maintaining target post-grazing residuals.
- In spring it is important to achieve post-grazing residuals of 1500-1600kg DM/ha (7-8 clicks on the RPM; 3.5 to 4 cm).
- Failure to do so will result in poor pasture quality and lower milksolids (MS) production from pasture in summer/autumn and reduced profit.
During the last 20 years the use of supplementary feed in New Zealand dairy systems has increased1. When supplements are fed to grazing dairy cows, pasture dry matter intake declines which is known as substitution2. Feeding supplements when there is an adequate supply of pasture in spring (i.e. pasture growth is greater than herd demand) may lead to pasture wastage through substitution and higher than target post-grazing residuals which, in turn, results in reduced growth and pasture quality at subsequent grazings3,4.
In 2015, DairyNZ set up farmlets at Scott Farm (Hamilton) and WTARS (Hawera) to demonstrate the benefits of maintaining target post-grazing residuals (1500-1600kg DM/ha or 7-8 clicks on rising plate meter on the RPM; 3.5 to 4 cm) during spring.
Two farmlets were established at each site; cows on one farmlet grazed to the recommended target post-grazing residuals of 1500-1600kg DM/ha (Target Residual), while cows on the High Residual farmlet were offered up to 3kg PKE per day resulting in a higher post-grazing residual of 1800-2000kg DM/ha (Figure 1).
Cows in the High Residual group were supplemented from late August to mid-December at Scott Farm, and from mid-September to late December at WTARS. In mid-December at Scott Farm and late December at WTARS, the PKE was removed from the diet of the High Residual cows and both farmlets were managed similarly until the end of the project (late-March at Scott Farm and mid-April at WTARS). Supplements were removed to demonstrate the carry-over effect of high post-grazing residuals in spring on cow performance, and on pasture over summer and autumn at each location.
At both locations, milk production and pasture growth were measured, and at Scott Farm, pasture composition and nutritive value were also measured.
Preliminary results and discussion
From the start of the project until December (spring), the average post-grazing residual was 1660kg DM/ha (9.2 clicks RPM) and 1905 kgDM/ha (11.5 clicks RPM) for the Target Residual and High Residual farmlets, respectively. Once PKE supplementation was stopped (during summer-autumn), the cows grazed to 1850kg DM/ha (8.8 clicks RPM) and 1995kg DM/ha (9.7 clicks RPM) on the Target Residual and High Residual farmlets, respectively. The lower post-grazing height measured on the RPM but greater kg DM/ha in summer is normal as the pasture base gets denser, thus there is an increasing kg DM/ha for any given height due to a buildup of dead material5.
Total milksolids (MS) production at both sites was similar and the data has been averaged and is presented in Table 1.
Milk production and BCS
Although the MS profile differed at times, when cumulative production over the length of the project was measured, there was no benefit from cows peaking higher through feeding PKE and leaving a higher residual in spring, compared with cows grazing pastures only and achieving target residuals throughout the project (Figure 2).
The reduced MS production in summer-autumn of the High Residual cows can be attributed to the poorer pasture composition and lower quality arising from higher residuals in spring (Tables 2 and 3).
There was no difference in cow BCS at the end of the project at Scott Farm but at WTARS the High Residual cows were better – 4.2 compared with the Target Residual cows being 4.
Table 1: MS production per cow during spring, and summer-autumn, when PKE was removed from the system and target residuals were pursued on both farmlets and silage conserved. The results which also show BCS at the end of the project, are averages from combined Scott Farm and WTARS data.
|Target residual||High residual||Difference|
|MS (kg/cow) from start to end of PKE (spring)||187.9||198.7||+10.8|
|MS (kg/cow) from end of PKE to end of project (summer-autumn)||91.1||87.1||-4.0|
|MS (kg/cow) from start to end of project||279.0||285.9||+6.9|
|BCS at end of project||4.05||4.15||+0.10|
|Silage made (kg DM/cow)||338||393||+55|
Pasture composition and quality
While pasture composition and quality at Scott Farm was similar in spring, high post-grazing residuals in spring on the High Residual farmlet resulted in summer and autumn pastures with a lower perennial ryegrass content and higher proportion of dead material. This change in pasture composition resulted in reduced pasture quality in summer and autumn. (Table 2).
The pasture quality and composition data collected from the Scott Farm demonstration is consistent with expected trends. Research both in New Zealand and overseas has demonstrated that high post-grazing residuals in spring results in pastures with higher amounts of dead material and lower nutritive value at subsequent grazings3,4,6,7,8.
Table 2: Metabolisable energy (MJ ME/kg DM), crude protein and neutral detergent fibre (% of DM) of the Target Residual and High Residual pastures during spring and summer/autumn at Scott Farm.
|Target residual||High residual||Sig|
|PKE feeding (spring)|
|Metabolisable energy (MJ/kg DM)||12.6||12.5||ns|
|Crude protein (%)||19.2||18.5||ns|
|Post-PKE feeding (summer/autumn)|
|Metabolisable energy (MJ/kg DM)||10.8||10.0||***|
|Crude protein (%)||18.3||14.8||***|
Table 3: Botanical composition of the Target Residual and High Residual pastures during spring and summer/autumn at Scott Farm.
|Target residual||High residual||Sig|
|PKE feeding (spring)|
|Ryegrass leaf and pseudostem||69%||68%||ns|
|Ryegrass reproductive stem||17%||21%||ns|
|Post-PKE feeding (summer/autumn)|
|Ryegrass leaf and pseudostem||52%||42%||*|
|Ryegrass reproductive stem||4%||3%||ns|
A simple analysis using cost/return was conducted to determine the profitability of each farmlet (Table 4). The analysis took into account revenue from increased milk production, increased BCS gain, silage made and increased feed on farm at the end of the project. The costs accounted for were purchase and feeding of PKE, and making silage.
An economic loss of $42 and $19 per cow at Scott Farm and WTARS, respectively, resulted from leaving a higher post-grazing residual by supplementing the cows with PKE.
Table 4: A simple averaged cost analysis for both Scott Farm and WTARS from leaving a higher post-grazing residual by supplementing the cows with PKE during spring.
|Numerical increase in MS kg/cow||6.9||$26.91a|
|Silage conserved kg DM/cowb||55||$5.50|
|Difference in farm cover at end of project kg DM/cowd||13||$1.25|
|PKE fed kg/cowe||242||$67.79|
|Costs less return||-$30.37|
a Milkprice of $3.90/kg MS
b Extra silage conserved valued at 10c/kg DM over and above conservation costs
c Value of extra BCS at end of project based on reduced feed required during the dry period for BCS gain (where 125 kg PKE required for 1 BCS gain).
d Extra pasture valued at 10 c/kg DM
e PKE costed at $220/t wet weight on farm and extra $30/t DM for costs associated with feeding PKE in trailers. Total cost of feeding PKE = $280/tonne DM
This demonstration highlighted the carry-over consequences of not achieving target post-grazing residuals in spring. Feeding supplements when there was a surplus of pasture, and not achieving target residuals resulted in economic losses of between $20 and $40/cow at two sites. This was due to leaving valuable high energy feed in the paddock in spring and compromising subsequent pasture quality and production in summer and autumn.
To incorporate supplements into a pasture-based system profitably, they need to be added into the system in conjunction with good pasture management. Ensuring target pasture residuals are met throughout the season will minimise wastage of high energy feed during spring and result in good quality pasture throughout the remainder of the season.
- DairyNZ. 2015. DairyNZ Economic Survey, 2014-15. Occassional publication of DairyNZ, Private Bag 3221, Hamilton, New Zealand.
- Holmes, C.W., and J.R. Roche. 2007. Pastures and Supplements in Dairy Production Systems, in Pasture and Supplements for Grazing Animals. Occasional Publication No. 14, P.V. Rattray, I.M. Brookes, and A.M. Nicol, Editors. The New Zealand Society of Animal Production: Hamilton, New Zealand. p. 221-242.
- Hoogendoorn, C.J., C.W. Holmes, and A.C.P. Chu. 1992. Some effects of herbage composition, as influenced by previous grazing management, on milk production by cows grazing on ryegrass/white clover pastures. 2. Milk production in late spring/summer: effects of grazing intensity during the preceding spring period. Grass and Forage Science. 47: 316-325.
- Korte, C.J., B.R. Watkin, and W. Harris. 1984. Effects of the timing and intensity of spring grazings on reproductive development, tillering, and herbage production of perennial ryegrass dominat pasture. New Zealand Journal of Agricultural Research. 27: 135-179.
- Hoogendoorn, C.J., C.W. Holmes and A.C.P. Chu. 1992. Some effects of herbage composition, as influenced by previous grazing management, on milk production by cows grazing on ryegrass/white clover pastures. 2. Milk production in late spring/summer: effects of grazing intensity during the preceding spring period. Grass and Forage Science. 47:316-325.
- Stakelum, G., and P. Dillon. 2007. The effect of grazing pressure on rotationally grazed pastures in spring/early summer on subsequent sward characteristics. Irish journal of agricultural and food research. 46(1): 15-28.
- Stakelum, G., and P. Dillon. 2007. The effect of grazing pressure on rotationally grazed pastures in spring/early summer on the performance of dairy cows in the summer/autumn period. Irish journal of agricultural and food research. 46: 29-46.
- Michell, P., and W.J. Fulkerson. 1987. Effect of grazing intensity in spring on pasture growth, composition and digestibility, and on milk production by dairy cows. Australian Journal of Experimental Agriculture. 27: 35-40.
This article was originally published in Technical Series September 2016