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Cow condition, milk yields and milk quality when grazing in a modified management intensive grazing system in Atlantic Canada

G. Carignan(1), N. McLean(1), A.H. Fredeen(1), J.P. Winter(1) and R.C. Martin(1,2)

Dept. of Plant and Animal Sciences, Nova Scotia Agricultural College
Organic Agriculture Centre of Canada, Nova Scotia Agricultural College

Introduction

Management intensive rotational grazing systems (MIG) are ordinarily managed to optimise vegetative growth because, as forage species mature the overall nutrient quality and digestibility of the forages decreases. The vegetative growth stage is maintained by controlling the frequency and intensity of defoliation events (grazing or clipping) and reducing opportunity for selective grazing. MIG usually creates relatively homogeneous pastures in terms of plant stage of maturity and canopy structure, however, landscape heterogeneity may be an important feature of bio-diverse agroecosystems.

This experiment examines how managing a pasture to promote biodiversity within a MIG system affects the productivity and health of lactating Holstein dairy cows.

Methods

Diverse, cool-season mixed species pastures were rotationally grazed by lactating dairy cows under a modified management intensive grazing (MIG) system. Cows were supplemented with grain concentrate at a rate of 25% of milk yield (w/w) to a maximum of 8 kg/d.

The experiment examined the effects of grazing intensity on cow weight and body condition score (BCS), milk yield and milk quality in a paired t-test design. Sixteen midlactation Holsteins, paired by milk production, age and stage of lactation, were randomly assigned to one of two grazing treatments (Fig. 1):

Intensive: grazing MIG treatments only
Extensive: grazing a combination of MIG treatments and those permitted to mature through longer recovery periods with no post-grazing clipping.

BCS was assessed on a 1-5 point scale with 0.25 point intervals and bodyweight was recorded approximately monthly. Milk yield was recorded twice daily and standardized to 4% milk fat. Milk quality was assessed by analysing the components: fat, protein, lactose, total solids (TS), milk urea nitrogen (MUN) and somatic cell count (SCC).


Figure 1. MIG and extensive (Ex) treatments (Left): May 2004 the pasture after the first grazing cycle. Cows were fenced out of Ex while MIG was grazed. (Right) : Sept. 2004 the pasture before the fifth and last grazing cycle. Note the accumulated stem material in Ex.

Results

Cow Condition:

Body weight (BW) and body condition score (BCS) were assessed for 2005 only. There was no difference in BW and the intensive group had a significantly higher BCS at one date (Table 1).

Table 1. Mean BCS of intensive and extensive grazing cow groups in 2005.
Date	Mean BSC		p
	Intensive	Extensive

05 / 27 / 05	2.65	2.58	ns
07 / 21 / 05	2.75	2.63	ns
08 / 03 / 05	2.77	2.62	0.09
08 / 30 / 05	2.73	2.49	0.05

Milk Yields:

Average milk yields, calculated when the cows were on the MIG and extensive treatments, were similar between groups on all but one date. The intensive group had higher milk yields than the extensive group in July 2004 (p< 0.05).

Table 2. Average daily milk yield (standardized to 4% fat) of intensive and extensive grazing cow groups in 2004 and 2005
Year	Grazing Cycle	4% Fat-Corrected Milk (kg/d)		p
		Intensive	Extensive

2004	2 June	28.9	27.5	ns
	3 July	26.2	23.9	0.05
	4 Aug.	28.7	25.6	ns
	5 Sept.	22.7	26.8	ns
	Season Average	25.4	25.9	n/a

2005	1 May	37.6	36.3	ns
	2 June	32.7	32.6	ns
	3 July	31.5	31.1	ns
	4 Aug.	32.5	30.4	ns
	Season Average	33.9	33.1	ns

Milk Quality:

Significant differences in milk components were observed on only two sampling dates from a total of sixteen (Table 3).

Table 3. Effect of grazing intensity on milk components.

Date	Grazing Cycle	Grazing Treatment	Fat (%)	TS (%)	SCC (cells/mL)	MUN (mg/dL)

14 July 04	3	Intensive	3.56	12.31	24.25
		Extensive	4.32	39.63	39.63
			*	*	*
23 June 05	2	Intensive	·	·	·	16.88
		Extensive	·	·	·	14.19
						*
TS: total solids; SCC: somatic cell count; MUN: milk urea nitrogen. * Significant difference (0.01 < P < 0.05) in grazing treatment means according to a two tailed t-test between intensive and extensive cow pairs.

Discussion

Two major factors may explain the few differences observed between the intensive and extensive grazing cows:

More available forage to the extensive grazing group due to the longer regrowth periods of a proportion of their pasture (the extensive pasture treatments). Therefore, though the forage in the extensive pasture treatments had lower quality, there was more of it. Cows may have had opportunity to increase their intake and/or select a higher quality diet.
Early in the grazing season when forage was lush and abundant, only fields maintained with the different management intensity treatments were grazed. Later in the season, when forage quality had started to decline, more fields were incorporated into the grazing rotation to allow sufficient time for pasture regrowth. These fields contained vegetative, high quality forage. Later still the cows were fed silage in the barn to allow for pasture regrowth. The extensive group of cows did not experience the low quality forage of the extensive treatments for a substantial amount of the grazing period, and so the effects of the extensive pasture treatments were greatly buffered.

Conclusions

Managing a pasture to promote heterogeneity, and therefore biodiversity, is possible within a MIG system.
Results show small losses in average condition and no loss of average weight or milk yield in the extensive compared to the intensive grazing group of lactating dairy cows.