Kidane A.1,2, Prestløkken E.1, and Steinshamn H.2
1Norwegian University of Life Sciences, Department of Animal and Aquacultural Sciences, Arboretveien 6, 1432 Aas, Norway; 2NIBIO, Norwegian Institute of Bioeconomy Research, Gunnars veg 6, 6630 Tingvoll, Norway; alemayes@nmbu.no
Abstract
We assessed milk production with 24 mid-lactation Norwegian Red dairy cows on a spring pasture dominated by timothy (Phleum pratense) for a period of 21 days using three pasture allocation techniques (n=8). Cows received weekly allowances at once (7 day-set-paddocking; 7SP), grazed 1/7 of 7SP allowance each day (daily-strip-grazing; 1SG), or grazed like 1SG but also had access to the previously grazed part of the paddock (daily-forward-grazing; 1FG). We hypothesized that 7SP would deteriorate sward quality and quantity over the grazing days whilst the other two treatments would provide balanced pasture quality and intake. These changes were expected to result in differences in milk yield and its components. However, changes in sward chemical composition (e.g. neutral detergent fiber, crude protein) over the grazing days in each week were not different among treatments (treatment × grazing day; P>0.05). Furthermore, no effect of treatments on milk yield and its components was observed.
Nonetheless, the effects of grazing days over a week on milk yield and components were different among treatments (treatment × grazing day; P<0.05). These treatment by grazing day interaction effects, in the absence main effect of treatment, could be due to fluctuations in daily DMI among treatments over the grazing days in each week.
Keywords: grazing, pasture allocation, dairy cow
Introduction
Grazed grass with dairy cows is a cheaper source of milk production (Wright, 2005) and grazing helps dairy cows to display their natural behaviour. However, there is a requirement for supplementation with concentrates to sustain high yielding dairy cows on pasture. This comes with extra cost due to the current competing demands for cereal grains. Therefore, looking for pasture allocation techniques that could result in an optimal feed intake with needed quality to support the current level of milk production, is vital for profitable dairy farming.
Comparisons of different grazing management systems under different conditions yielded differences in DM use efficiency, milk yield in dairy cows, as well as weight gain and methane emission with steers. It was evident from the works of Bryant et al. (1961) that such differences could be due to changes in the attributes of the grazed diet (e.g. proportions of morphological fractions, chemical composition of these fractions and physical architecture of the grazed sward).
Here, we conducted a short duration grazing experiment with mid-lactation Norwegian Red (NRF) dairy cows to assess the effects of three different pasture allocation techniques on milk yield and its chemical composition. We hypothesized that the quality of DM consumed and the quantity to consume in a grazed horizon will deteriorate with extended grazing days in a paddock for set paddocking whereas frequent allocation of a pasture would balance DMI and its quality.
Materials and methods
Twenty-four mid-lactation (DIM±SD; 124±37) NRF cows (mean BW±SD; 572±66) were randomly allocated into six groups (two replicates of size 4) and grazed a spring pasture dominated by timothy (Phleum pratense) for three 7 day periods (21 days in total) in one of the three pasture allocation techniques (7 day set-paddocking, 7SP; daily strip-grazing, 1SG; and daily forward-grazing, 1FG). The first week was used as an adaptation period. Milk yield was registered daily and milk samples were taken frequently for standard chemical analysis. Grazing was augmented with a supplementation of 5 kg d-1 cow-1 commercially available concentrate feed (FORMEL Favør 90, Felleskjøpet, Oslo).
Grazed sward samples were taken to represent three time points during each measurement week (start, middle and end of grazing) and dried at 60 °C for 48 hours and milled through a 1.0 mm sieve size for standard chemical analysis. Additional samples were also taken at the beginning of the adaptation week.
Dry matter availability was assessed using the raising plate meter.
Data collected over the measurement weeks were analyzed using SAS (SAS Institute Inc, Cary, NC, USA) mixed models as repeated measurement. Whenever they existed and contributed significantly to the model, day 0 measurements were used as covariates. Statistical significance was declared at P≤0.05.
Results and discussion
Data on chemical composition of grazed sward and milk yield are presented with Figure 1. Some of the measured and estimated chemical compositions were affected by grazing days in each week. As such, CP content declined in each measurement week (effect of day in a week; P<0.0001) regardless of the pasture allocation techniques. The decline in CP content was accompanied by an increasing NDF content in the
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Figure 1. Chemical composition of grazed sward, [(A) crude protein (g kg-1 DM), (B) neutral detergent fiber (g kg-1 DM), (C) estimated NEL (MJ kg-1 DM), (D) estimated AAT (g kg-1 DM)] and milk yield [(E) fresh milk yield (kg day-1) and (F) energy corrected milk yield (kg day-1)] over the grazing period. Adaptation week value for each of the parameters is indicated with black dots on day 1. Treatments: solid, long broken and short broken lines stand for 7SP, 1FG and 1SG groups (see text for description), respectively.
grazed sward (effect of day in week; P<0.001). However, the degree to which the NDF increased was dependent on the pasture allocation technique during week 1 of the measurement week (Treatment × grazing day effect P<0.05).
The estimated net energy lactation (NEL) and metabolisable protein supply (AAT) of the grazed sward declined with grazing days in each week (P<0.0001). However, the decline was uniform for all treatment groups and there was no treatment and treatment by grazing day interaction effects. This suggested that the fast spring pasture growth and associated sward morphological developments had stronger effects on sward quality than the effects of pasture allocation techniques. This is supported by some of the observed changes in chemical composition of the grazed sward; when day 1 values are compared to day 8 and day 15 (all pre-grazing values) with drops in CP and NEL, and abrupt increment in NDF content (Figure 1).
During the experiment, fresh milk yield (25.9, 25.3 and 25.3 kg/d; pooled st. dev. 4.64) and energy corrected milk yield (mean for milk sampling dates, 26.3, 27.2 and 25.6 kg day-1; pooled st. dev. = 4.49), for 7SP, 1SG and 1FG respectively, were not affected by the pasture allocation techniques (P>0.1).
However, both energy corrected milk and fresh milk yields were significantly (P<0.0001) affected by day of grazing in a week. In addition, we observed interaction effects (P<0.05) between treatments and grazing days for fresh milk yield, milk lactose, protein and milk urea contents in the absence of the main effect of treatment suggestive of the fluctuations in daily DMI among treatments over the grazing days in each week.
Conclusions
In summary, the lack of anticipated differences among the treatments in sward qualities over each week, behavioural adaptations by cows to adjust DMI subject to pasture allocation techniques and probably the resilience of dairy cows to accommodate changing nutritional conditions under such short experimental periods may explain the absence of treatment effects on milk yield and its chemical composition.
Acknowledgements
We are grateful to all people who helped in the field and laboratory work. This work was funded by National Research Council of Norway, Norsk senter for økologisk landbruk (NORSØK), Fylkesmannens landbruksavdeling i Møre og Romsdal (FMLA), Møre og Romsdal Fylkeskommune, TINE and Oikos.
References
Bryant H.T., Blaser R.E., Hammes R.C., and Hardison W.A. (1961) Method for Increased Milk Production with Rotational Grazing. Journal of Dairy Science 44, 1733-1741.
Wright, I.A. (2005) Future prospects for meat and milk from grass-based systems. In: Reynolds, S.G. and Frame G., (eds) Grasslands:
Developments, Opportunities, Perspectives. Science Publishers, Inc. Enfield (NH), USA, pp. 161-179.