Comparison between continuous and rotational grazing for two grass-white clover mixtures in North Italian plain

Andrea Cavallero, Carlo Grignani and Amedeo Reyneri

Dipartimento di Agronomia, Selvicoltura e Gestione del territorio Via Michelangelo, 32-10126 Torino (Italy)

Introduction
Materials and methods
Results and discussion
Acknowledgments
References

Introduction

In previous research, Cavallero et al. (1991) pointed out the possibility of maintaining a high stocking rate (4.2 cows ha^-1) under continuous grazing in the irrigated Po plain. Those results were obtained with high N application (325 kg ha^-1) on grass-white clover mixtures with alternatively cocksfoot or perennial ryegrass, while tall fescue-white clover mixture was soon invaded by weeds.

After that, and always with the aim of verifying the possibility to de-intensify the maize dominant forage system of the Po plain, it seemed interesting to investigate the potentiality of grass-clover mixtures under a low N and K fertilization and with different management regimes.

Materials and methods

Management, sward structure, herbage production and quality, and diet composition under continuous and rotational grazing have been analyzed during three years (1987, 1990, 1991) in the western Po plain (North-West of Italy, 45° lat.). Two grass-white clover mixtures have been tested: Dactylis glomerata (cv. Cambria) -DT- and Lolium perenne (cv. Rathlin) -LT- both associated with Trifolium repens var. giganteum (cv. Regal).

Pastures have been utilized by heifers (Holstein Friesian) of 330 kg mean liveweight for a grazing season of 193 days. The continuous grazing system was organized by increasing the grazing surface twice during the season after the 1^st and the 2^nd cut; rotational grazing was organized on 5 plots, 2 of them grazed only after spring and midsummer cuts. The N mineral fertilization was on average 110 kg ha^-1 year^-1, applied during 2 distribution times.

Continuous grazed swards were maintained at an average height of 6-7 cm and 7-8 cm (patch areas excluded) for DT and LT mixtures respectively, while rotationally grazed swards were utilized from a pre-grazing height of 20 cm to a stubble height of 7-9 cm. Herbage height was measured by a falling plate of 30 cm diameter and 200 g m^-2 weight.

Vegetation composition was assessed by weight each month, cutting samples at ground level. Diet composition was estimated before and after grazing through "inclined point quadrats" by Warren Wilson; finally herbage intake was calculated by energy balance considering the animal liveweight gain and the nutritional value of the diet.

Results and discussion

Clover increased its presence through the years particularly in LT mixtures and under rotational grazing, while DT mixtures showed a more steady equilibrium under both utilization techniques (Figure 1). The vegetation evolution of the two mixtures seems to have been mainly influenced: a) by selective laminae utilization under continuous grazing, due to the clover leaf position on the canopy upper layer; b) by leaf turnover and competition for light under rotational grazing.

Dactylis glomerata demonstrated a good adaptability to very frequent defoliations under continuous grazing, by a strong modification of leaf size and stem angle (habitus); ryegrass, on the contrary, modified the tiller density rather than the leaf morphology (Table 1). Finally, white clover modified both leaf density and leaf size (Reyneri et al., 1993).

Cocksfoot leaf turnover was faster (130 °C leaf^-1) compared to ryegrass (159 °C leaf^-1), but slower than clover, particularly in summer (Grignani et al., 1993).

Figure 1 Evolution of sward composition

Table 1 Effects of grazing techniques on sward density and on species habitus

		Continuous grazing	Rotational grazing	Continuous grazing	Rotational grazing
Mixtures (1)		Stem density (n.m-2)		Clover leaf	density (n.m-2)
	DT	5000 c	3900 d	4400 a	4750 a
	LT	15400 a	6100 b	3350 b	3300 b
Species (2)		Area lamina- (cm2)		Stem angle (deg.)
Dactylis glomerata		4.1 cd	13.9 a	33° c	61° b
Lolium perenne		2.2 e	5.1 cd	68° a	69° a
Trifolium repens		6.1 c	12.8 b

(1) June records.
(2) June and July records

The grazing season was similar among utilization techniques and mixtures. Stocking rate was 14% lower for continuous grazing, moreover LT has carried a higher stocking rate (+ 5%) under continuous grazing and lower one (-3%) under rotational grazing (table 2). The higher stocking rate under rotational grazing was mainly due to the greater clover production, particularly in summer, with longer regrowth between defoliations.

The mean liveweight gain was 822 g d^-1 with non significant differences between treatments. The offered herbage quality under rotational grazing showed higher crude protein and lower NDF and ADF contents (Cavallero et al., 1993a).

Leaf blades and petioles were positively selected by heifers, while stems and senescent tissue were poorly ingested (Figure 2); hence, the nutritive value of the diet was higher than that of the offered herbage, particularly for crude protein (24% vs 20%). Under continuous grazing the amount of low nutritional value components in the diet, such as stems and senescent tissue, was greater, and fibre content higher (NDF 48% vs 41%). Little or no quality differences among mixtures were evident for both grazing techniques.

Figure 2 Intake of the main sward components under continous () and rotational () grazing.

According to Ridout and Robson (1991), the selection coefficient for clover was 1.60 and 2.06 for continuous and rotational grazing respectively; clover did not seem to be selected by active choice, but indeed because it was often dominant in the upper layer of the canopy, particularly under rotational grazing (Cavallero et al., 1993b).

Considering grazed plus harvested herbage by cutting, the continuous and the rotational grazing systems have produced 52 and 54 Mj ha^-1 of ME respectively for DT mixtures, and 54 and 52 for LT mixtures (Table 2).

In conclusion, in this research grass-clover mixtures have demonstrated the ability to maintain a high stocking rate with an acceptable per head animal production. Compared to previous experiments with higher N fertilization, stocking rate was 24% lower, as was found by Younie et al. (1986) and Sheldrick et al. (1987), but more difficulties were found to maintain a reasonable pasture composition due to the strong competition of white clover towards grass.

In the Po plain, grazing systems based on grass-clover mixtures can play a more significant role if de-intensification of farming takes place and the if maize-dominant forage system becomes less profitable.

Table 2 Effects of grazing techniques on stocking rate, animal performances and pasture production

			1987		1990		1991		AVERAGE
		Mixtures:	DT	LT	DT	LT	DT	LT	DT	LT
Grazing season (d)			190		212		178		193
Stoking rate (cows ha-1)
	- continuous grazing		3.1	3.4	2.4	2.6	3.1	3.0	2.9	3.0
	- rotational grazing		3.1	3.3	3.1	2.8	4.2	4.0	3.5	3.4
Liveweight gain (g head-1 d-1)			803		761		903		822
ME produced by the systems (Mj ha-1)
	- continuous grazing								52.1	53.6
	- rotational grazing								53.7	51.7

Acknowledgments

This study was supported by the MURST (Fondi 40% "Aspetti ecofisiologici di specie foraggere da prato e pascolo").

References

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CAVALLERO A., GRIGNANI C. and REYNERI A. (1993a) Pascolamento continuo intensivo e pascolamento turnato su associazioni bifite a base di Trifolium repens. Riv. di Agron., 2, 141-147.

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