The effect of grazing management on the spatial heterogeneity of a perennial ryegrass sward and the utilisation by grazing dairy cows

Connell, Jan (2004) The effect of grazing management on the spatial heterogeneity of a perennial ryegrass sward and the utilisation by grazing dairy cows. PhD thesis, University of Glasgow.

Abstract

Swards in the mid season become a mosaic of frequently grazed shorter patches and taller infrequently grazed patches. The spatial heterogeneity of such swards can result in poo utilisation by grazing animals during the mid season. This study investigates how grazing pressure and topping, as grazing management tools affects the spatial heterogeneity of a continuously stocked perennial ryegrass (Lolium perenne) sward, together with the utilization of the infrequently grazed areas by grazing dairy cows. Increasing the grazing pressure in mid season significantly reduced the height, herbage mass and proportion of infrequently grazed patches within the sward by up to 4 cm, 1.5 t DM ha-1 and 10% respectively. This was only significant when the grazing pressure reduced the frequently grazed patch height to 6 cm or below. Frequently grazed patch height of 8 or 10 cm did not significantly affect the spatial heterogeneity of the sward. An asymptotic relationship was observed between frequently grazed patch height and infrequently grazed height and proportion. Grazing dairy cows utilised the infrequently grazed patches through reduced avoidance and significantly greater intake over a 2 to 3 week period. Grazing behaviour was also modified, with a trend for reduced bite rate and increased grazing time with greater utilisation of the infrequently grazed patches. The affect of utilising the infrequently grazed patches on the milk production per cow was negative, significantly reducing yield by 3 kg cow-1 d-1 without affecting fat and protein composition. The higher stocking rate, in order to maintain the grazing pressure, would be likely to increase milk yield on per hectare basis. Topping, as a management tool used from early season through to the mid season, enhanced the sward morphology of the infrequently grazed patch through increased tiller density, leaf content, reduced dead material, increased crude protein and digestibility compared to frequently grazed patch. The height and herbage mass of the infrequently grazed patches was significantly reduced. The proportion of infrequently grazed patches was significantly reduced by 10% only by topping at the 2 weekly interval and not by the 4 weekly interval. Topping at both frequencies initially reduced the total dry matter intake of cows but had no effect on milk production per cow. This may have been due to the greater ability to select a leafy diet within the infrequently grazed patches, which would be of higher digestibility, allowing for the maintenance of yield at lower dry matter intakes compared to cows on the non-topped swards. Grazing behaviour was altered through increased bite rate, grazing time and selection of infrequently grazed patches of cows on topped swards. The frequency of topping affected morphology and utilisation of infrequently grazed patches. The greater the frequency of topping the greater was the tiller density and leaf content of the infrequently grazed patches by mid season. Dairy cows utilised these patches through actively selecting to graze them, thereby significantly reducing the proportion within the sward by 10% compared to topping less frequently. Grazing management can affect the spatial heterogeneity of a continuously stocked sward in the mid season through morphology and dynamics of the patches, together with greater utilisation by grazing dairy cows.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Range management, animal sciences.
• Colleges/Schools: College of Medical Veterinary and Life Sciences
• Supervisor's Name: Offer, Dr. Nick, Thomas, Prof. Cled and Fisher, Dr. George
• Date of Award: 2004
• Depositing User: Enlighten Team
• Unique ID: glathesis:2004-71140
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 10 May 2019 10:49
• Last Modified: 15 Jun 2021 10:12
• URI: https://theses.gla.ac.uk/id/eprint/71140