Defoliation dynamics, pasture intake and milk production of dairy cows grazing lucerne pastures in a partial mixed-ration system

Abstract

The effect of lucerne pasture allocation on defoliation dynamics, pasture intake and animal production was investigated in a subtropical partial mixed-ration dairy system. The study took place at the Gatton Research Dairy, south-eastern Queensland, with a 28-day adaptation period followed by an 8-day treatment period during November and December 2016. Twenty-four multiparous Holstein Friesian dairy cows were offered 11 kg of dry matter (DM)/cow.day as partial mixed-ration dairy system, and four levels of daily pasture allocation measured to 5-cm residual pasture height (averaging 30.6, 20.5, 15.1 and 10.9 kg DM/cow.day). Cows with lower allocations were forced to graze further down the vertical plane and pasture intake and milk yield significantly (P < 0.001) declined. Cows grazed the top grazing stratum (TGS) across 80% of the pasture area before re-grazing another area of the paddock, regardless of the allocation level. Pasture intake (kg DM/ha) of the TGS was at least 2.9 times higher than that of the lower strata, regardless of allocation level. Therefore, the decline in pasture intake is explained by the transition from grazing the TGS to grazing lower strata. When the horizontal utilisation of the TGS approached 100%, the proportion of ungrazed, uncontaminated pasture approached 0% of the area, and intake and milk production declined. Grazing management strategies for lucerne should allocate pasture to lactating dairy cows to achieve horizontal utilisations approaching 0% for proportion of ungrazed, uncontaminated pasture to maximise intake and production. Secondary grazing herds or mechanical methods should be used to remove residual pasture to the ideal height for pasture regrowth.

Maximising Lucerne (Medicago sativa) Pasture Intake of Dairy Cows: 2—The Effect of Post-Grazing Pasture Height and Mixed Ration Level

“…Grazing management strategies that target high levels of utilisation have been shown to have negative impacts on pasture intake. Recently, Ison et al [6] found that low allocations resulted in high levels of pasture utilization and lower intake in cows grazing lucerne in partial mixed ration systems. The study identified that cows selectively grazed the top leafy portion of lucerne across the whole area of the paddock, before regrazing the residual 'stemmy' portion of pasture remaining in previously grazed areas of their current paddock [6].…”

“…Recently, Ison et al [6] found that low allocations resulted in high levels of pasture utilization and lower intake in cows grazing lucerne in partial mixed ration systems. The study identified that cows selectively grazed the top leafy portion of lucerne across the whole area of the paddock, before regrazing the residual 'stemmy' portion of pasture remaining in previously grazed areas of their current paddock [6]. When grazing the top leafy stratum, both intake and diet quality were maximised, as cows removed larger bites from the top leafy stratum with a high leaf content relative to stem [6].…”

“…The study identified that cows selectively grazed the top leafy portion of lucerne across the whole area of the paddock, before regrazing the residual 'stemmy' portion of pasture remaining in previously grazed areas of their current paddock [6]. When grazing the top leafy stratum, both intake and diet quality were maximised, as cows removed larger bites from the top leafy stratum with a high leaf content relative to stem [6]. Once the top leafy stratum was depleted across the whole allocation, cows began to regraze areas of the paddock.…”

“…Once the top leafy stratum was depleted across the whole allocation, cows began to regraze areas of the paddock. In doing so, bite depth decreased and consequently pasture intake rate and intake decreased [6]. Therefore, allocating pastures based on the horizontal utilisation of the top leafy stratum, rather than vertical utilisation of the pasture, maximises intake.…”

“…This occurs because allocations are large enough to ensure cattle only graze across the area of a paddock once and can selectively remove the top leafy stratum to achieve daily intake requirements. When PUP approaches 0%, intake declines as cows shift from grazing the top leafy stratum to subsequent lower strata, where the pasture mass removed declines [6]. Achieving horizontal utilisations where the pasture around faecal patches remains ungrazed ("PUP strategy"), ensures cows are only grazing the top leafy stratum, and intake and diet quality are maximised.…”