Effects of postgrazing sward height on individual and per-hectare milk production of supplemented high-producing dairy cows.

Abstract

Residual sward height management is a key factor influencing milk production and grazed forage utilization per cow and per hectare in pasture-based dairy systems. This study evaluated the effects of postgrazing sward height on forage growth rate and DMI, milk production, and BCS in supplemented, high-producing dairy cows. The study captured cumulative treatment effects on animal performance across nearly an entire lactation, as well as sward dynamics over most of the tall fescue growing season. Thirty-two multiparous Holstein cows were assigned to 2 treatments: the control treatment, with a postgrazing sward height of 5.0 to 7.0 cm (CT), and the high treatment, with a postgrazing sward height of 12.0 to 15.0 cm (HI). The study spanned 215 d of grazing on a Lolium arundinaceum sward in a randomized complete block design, with 4 blocks, each measuring 3.2 ha, and consisted of 2 paddocks, each measuring 1.6 ha. Cows had one grazing session between milkings and were supplemented with a mixed ration in the opposite session. Rotational grazing was employed, with new strips allocated once the postgrazing sward height for each treatment was achieved. A new grazing cycle was initiated when plants reached the 2.5 to 3.0-leaf phenological state. Any extra or unused areas within a grazing cycle were always recorded. The postgrazing sward heights were 7.7 and 14.4 cm for the CT and HI treatment, respectively. Forage growth rate tended to be 21% higher in the HI treatment (40 vs. 33 kg DM/ha per day), and cows stayed longer per strip (3.9 vs. 2.9 d) and had 2.5 times more grazing cycles (8.3 vs. 3.3). Total forage disappearance was similar between treatments. Cows in the HI treatment had greater individual grazed forage DMI (+2.0 kg/cow) and BCS throughout lactation compared with CT cows. The HI treatment also increased the individual milk production by 10% (31.5 vs. 34.5 kg milk/d) while reducing concentrate intake per unit of milk by 10%, resulting in a tendency for higher income over feed costs at cow level. However, although we observed a tendency for a higher forage growth rate, this approach required a larger grazing area (1.18 vs. 1.60 ha/4 cows), leading to a trend toward 21% lower milk output per hectare (23,598 vs. 18,528 kg milk/ha). In contrast, the CT treatment supported a higher stocking rate but resulted in lower grazed forage DMI, higher supplemental feed input per hectare and slower BCS recovery. These findings highlight the importance of aligning grazing strategies with system-specific goals. Farmers seeking to maximize land productivity may favor lower postgrazing sward heights, although these must be managed carefully to prevent adverse effects on animal health, reproduction, and long-term productivity due to slower BCS recovery during lactation and lower BCS at the onset of dry off. Conversely, systems aiming to reduce feed costs may benefit from a higher residual sward height, which enhances individual cow performance and BCS while decreasing reliance on supplemental feed.