Effect of Different Post-grazing Sward Height on Early Lactation Dairy Cow Performance

Research done in 2008 by McEvoy, et al.recommends a sward height of 4cm after grazing. However in the light of recent milk production targeting and the abolition of milk quotas in 2015 coming ever nearer Irish dairy farms are being urged to look to maximise milk yields per hectare which could be made easier by looking into other post grazed sward heights. A TEAGASC report has researched the effects of grazing pasture down to a height of 2.7 cm.
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With the upcoming abolition of milk quotas in 2015 and a 50% increase in milk production by 2020 targeted by the Food Harvest 2020. Irish grass-based dairy enterprises need to explore new strategies to increase milk production per hectare. The contribution of grazed grass in the diet of the lactating dairy cow is maximised through early spring turnout post-calving. This target is challenged by low spring grass supply. Grazing to a very low post-grazing sward height (PGSH; 2.7cm) during the two first grazing rotations improved pasture utilisation but substantially compromised dairy cow milk production during this period (Ganche et al., 2011). Low PGSH needs to be compared to current recommendations (4cm; McEvoy et al., 2008) to assess the differences in production performance. This study aimed to establish the implications of different PGSH in spring on dairy cow performance during early lactation.

Materials and Methods

Ninety (27 primiparous and 63 multiparous) spring calving Holstein Friesian dairy cows were balanced on calving date (13 February; s.d. 17.7 days), lactation number (2.1; s.d. 1.05), previous lactation (first 37 weeks) milk yield (4591; s.d. 682.7 kg), bodyweight (BW; 482, s.d. 57.8 kg) and body condition score (BCS; 2.92, s.d. 0.141) in a randomised block design, with animals randomly assigned pre-calving to one of three (n=30) PGSH treatments: 2.7 cm (severe - S), 3.5 cm (moderate – M) or 4.2 cm (lax – L) from February 14 to April 24, 2011. Fresh herbage was allocated daily. Herbage mass (HM; >2.7 cm) was calculated twice weekly by cutting two strips per grazing treatment. Pre and post-grazing heights were measured daily using a folding pasture plate meter.details The differences in PGSH were achieved by ensuring a 3 kg DM define /cow/d difference in daily herbage allowance (DHA) between treatments. All animals were supplemented with 3.4 kg of concentrate DM/d throughout the experiment. Grass DM intake (GDMI) was measured using the n-alkane technique (Dillon and Stakelum, 1989) during week 5 (March 12-18) and week 9 (April 11-16) of the experiment. Pasture utilisation was calculated for each grazing rotation from the pre-grazing yield seems an incorrect word? relative to the post-grazing yield. Milk yield was recorded daily; milk composition, BW and BCS were measured weekly. Data on Animal variables were analysed using covariate analysis and the PROC MIXED statement of SAS with terms for parity, treatment and the interaction of parity and treatment. Days in milk and pre-experimental values of what? were used as covariates in the model.

Results and Discussion

During the first two grazing rotations the L treatment had higher pre-grazing HM (+120 kg DM/ha; P<0.01) and pre-grazing sward height (+0.60 cm; P<0.001) when compared to the S and M treatments (956 kg DM/ha and 6.40 cm, respectively).Were S and M the same? Mean DHA (>2.7 cm) was 7.7, 10.0 and 12.1 kg DM/cow and mean PGSH was 2.7, 3.5 and 4.2 cm for the S, M and L treatments, respectively. Average total DMI was lowest for the S animals (13.3 kg DM/cow; P<0.001) when compared to the mean total DMI of the M and L animals, which did not differ significantly (15.1 kg DM/ha). Increasing PGSH from 2.7 to 3.5 to 4.2 cm resulted in a linear increase (P<0.001) in milk yield (22.5, 23.6 and 25.1 kg/cow/d, respectively data in Table) and protein yield (750, 807 and 857 g/d, respectively). As a result, milk solids yield increased with PGSH (P<0.001; Table 1). Severe grazing depressed (P<0.01) both cumulative milk yield (-160 kg/cow) and milk solids yield (-17 kg milk/cow), when compared to the M and L treatments which performed similarly (1538 kg milk/cow and 124 kg milk solids/cow). The severe decrease in production reflected the high level of restriction placed upon the animals in the S treatment as they were offered a very low DHA to achieve the desired PGSH. The quantity of grass utilised (>2.7 cm) was, however, maximised (P<0.001) by grazing to 2.7 cm (918 kg DM/ha) when compared to 3.5 cm (779 kg DM/ha) and 4.2 cm (764 kg DM/ha).

Effect of Post-grazing Height (PGSH) on Animal Performance During the Early Lactation Period PGSH Treatment1
Milk yield, kg/d 22.5a 23.6b 25.1c 0.51 0.001
Milk fat, g/kg 43.9a 46.8b 45.9b 0.91 0.015
Milk protein, g/kg 33.1a 34.1b 34.0b 0.35 0.022
Milk lactose, g/kg 46.8 46.9 47.0 0.22 0.636
Milk solids yield, kg/d 1.75a 1.91b 2.00c 0.046 0.001
End BW, kg 442a 451ab 464b 7.3 0.019
End BCS 2.71a 2.80b 2.87b 0.041 0.004
1PGSH: S (2.7 cm), M (3.5 cm), L (4.2 cm); a-c Means within a row with different superscripts differ


Grazing to 2.7 cm physically restricted cows from grazing further into the sward thereby lowering dry matter intake GDMI? which consequently resulted in substantial milk and milk solids production losses. Grazing cows to a lax PGSH (4.2 cm) in early spring resulted in reduced grass utilisation and no significant increase in cumulative milk and milk solids production when compared to grazing to 3.5 cm. This study concludes that grazing swards to 3.5 cm in early spring will satisfactorily achieve a balance between pasture utilisation and dairy cow production performance.

References Format incorrect

Dillon P. & Stakelum G. (1989). Ir. J. Agri. Res.. 8:104 (Abstract)
Ganche E., O’Donovan M., Delaby L., Boland T. and Kennedy E. (2011). Agricultural Research Forum 2011. p83
McEvoy M. other authors? (2008). Journal of Dairy Science, 91:1258-1269.

November 2012

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