The effects of high stocking rates on milk production from dryland and irrigated Mediterranean pastures

Abstract

An experiment using herds of ~20 cows (farmlets) assessed the effects of high stocking rates on production and profitability of feeding systems based on dryland and irrigated perennial ryegrass-based pastures in a Mediterranean environment in South Australia over 4 years. A target level of milk production of 7000 L/cow.year was set, based on predicted intakes of 2.7 t DM/cow.year as concentrates, pasture intakes from 1.5 to 2.7 t/cow.year and purchased fodder. In years 1 and 2, up to 1.5 t DM/cow.year of purchased fodder was used and in years 3 and 4 the amounts were increased if necessary to enable levels of milk production per cow to be maintained at target levels. Cows in dryland farmlets calved in March to May inclusive and were stocked at 2.5, 2.9, 3.3, 3.6 and 4.1 cows/ha, while those in irrigated farmlets calved in August to October inclusive and were stocked at 4.1, 5.2, 6.3 and 7.4 cows/ha. In the first 2 years, when inputs of purchased fodder were limited, milk production per cow was reduced with higher stocking rates (P < 0.01), but in years 3 and 4 there were no differences. Mean production was 7149 kg/cow.year in years 1 and 2, and 8162 kg/cow.year in years 3 and 4. Production per hectare was very closely related to stocking rate in all years (P < 0.01), increasing from 18 to 34 t milk/ha.year for dryland farmlets (1300 to 2200 kg milk solids/ha) and from 30 to 60 t milk/ha.year for irrigated farmlets (2200 to 4100 kg milk solids/ha). Almost all of these increases were attributed to the increases in grain and purchased fodder inputs associated with the increases in stocking rate. Net pasture accumulation rates and pasture harvest were generally not altered with stocking rate, though as stocking rate increased there was a change to more of the pasture being grazed and less conserved in both dryland and irrigated farmlets. Total pasture harvest averaged ~8 and 14 t DM/ha.year for dryland and irrigated pastures, respectively. An exception was at the highest stocking rate under irrigation, where pugging during winter was associated with a 14% reduction in annual pasture growth. There were several indications that these high stocking rates may not be sustainable without substantial changes in management practice. There were large and positive nutrient balances and associated increases in soil mineral content (P < 0.01), especially for phosphorus and nitrate nitrogen, with both stocking rate and succeeding years. Levels under irrigation were considerably higher (up to 90 and 240 mg/kg of soil for nitrate nitrogen and phosphorus, respectively) than under dryland pastures (60 and 140 mg/kg, respectively). Soil organic carbon levels did not change with stocking rate, indicating a high level of utilisation of forage grown. Weed ingress was also high (to 22% DM) in all treatments and especially in heavily stocked irrigated pastures during winter. It was concluded the higher stocking rates used exceeded those that are feasible for Mediterranean pastures in this environment and upper levels of stocking are suggested to be 2.5 cows/ha for dryland pastures and 5.2 cows/ha for irrigated pastures. To sustain these suggested stocking rates will require further development of management practices to avoid large increases in soil minerals and weed invasion of pastures.

Effects of stocking rate, supplementation, genotype and their interactions on grazing dairy systems: a review

“…The regression equation derived from these data indicates that as comparative SR increases from 60 to 100 kg LW/t DM, annual pasture utilisation increases from 66 to 85%, with an average increment rate of 4.6% for every 10 kg LW/t DM of increment in comparative SR. In contrast, systems that fed concentrates in farmlet studies in the USA (Fales et al 1995) and Australia (Valentine et al 2009) showed low or nil reduction in pasture intake and pasture utilisation at higher SR because SR and supplementation per hectare were both increased simultaneously in these studies. However, even though comparative SR was not reported in these two studies, it can be inferred that comparative SR was similar across treatments in both studies.…”

“…In this experiment, the grazing pressure at the higher SR was not so extreme as to cause detrimental effects on pasture accumulation, and may have allowed pastures to remain close to their optimum herbage mass. In contrast, in an experiment exploring the effect of SR (2.5, 2.9, 3.3, 3.6 and 4.1 cows/ha) on the dairy system, Valentine et al (2009) reported no effect of SR on pasture production. However, in the latter experiment, cows were fed supplements and pasture consumed represented less than 34% of the diet in all SR treatments in dryland farmlets.…”

“…For example, herbage utilisation was kept in a relatively narrow range in the experiment of Jensen et al (2005) where supplements were introduced in a range from 0 to 20 t DM/ha and SR was increased from 3.0 to 6.9 cows/ha (Table 4), and in the experiment of Valentine et al (2009) where SR increased from 2.5 to 4.1 cows/ha and supplements fed increased from 11.2 to 17.8 t DM/ha from the lower to the higher SR.…”

The effect of stocking rate on soil solution nitrate concentrations beneath a free-draining dairy production system in Ireland

“…The biological effects of increased overall farm SR on milk production per hectare observed within our study are consistent with previous findings and further substantiate the potential for increased biological efficiency within higher-SR grazing systems. Similar to both Valentine et al (2009) andFariña et al (2011), SR had no significant effect on net herbage accumulation but, as a consequence of the increased efficiency of herbage removal and increased grazing severity at higher SR, resulted in increased grazed grass utilization. Both Baudracco et al (2010) and McCarthy et al (2011) have previously likened an increase in SR to a reduction in daily herbage allowance.…”

Planning for whole-farm systems research at a credible scale: subdividing land into farmlets with equivalent initial conditions

“…Other examples of mostly small farmlets used in research, especially in the dairy industry, have been reported for: a replicated experiment exploring dairy cow stocking rates over two grazing seasons in North America (Fales et al 1995); investigations of nitrogen cycling on four unreplicated, selfcontained farmlets of 6.5 ha with each containing 16 randomly allocated paddocks in New Zealand (Ledgard et al 1999); a study of automated milking on a 9.2-ha farmlet for which paddock layout details have been provided (Jago et al 2007); unreplicated systems studies in Queensland dairy research where five different 20-cow systems were run under research station conditions (Lowe 2007); two 6-ha farmlets studying the effects of hay or silage conservation on the Ellinbank research farm in Victoria (Thomas and Mathews 1991); and dryland and irrigated farmlets at the Flaxley Agriculture Centre in South Australia where the areas of each were~30 and 16 ha each, respectively, at a wide range of stocking rates (Valentine et al 2009).…”