The effect of fertilizer nitrogen input to grass-clover swards and calving date on the productivity of pasture-based dairy production.

Discipline: pastures & grazing; Keywords: white clover, nitrogen fertilizer, milk production, milk processability.

The environmental effects of mineral nitrogen are of global concern, and there is growing determination to mitigate its use for agricultural purposes. The replacement of synthetic fertilizer N with N fixed by bacteria (biological N fixation) in association with legumes such as white clover (Trifolium repens L.) offers the potential to lower fertilizer N use, nitrous oxide and ammonia emissions, and energy use. This also helps to reduce the carbon footprint and other environmental effects of milk production while not affecting the economics of the enterprise. The objective of the study by Dr K. M. Scully and co-workers was to examine a system of milk production with later than typical mean calving and turnout to pasture dates (mid-April in the northern hemisphere) with the end of lactation in the following January compared with systems with conventional calving and turnout dates (mid-February) with the end of lactation in mid-November. Aspects examined include the productivity of white clover in the pasture, biological N fixation, and pasture and milk production as well as indicators of the processability of late-lactation milk. The results of the study were published in the Journal of Dairy Science, Volume 104 of 2021, page 8870 to 8884; the title being: The effect of fertilizer nitrogen input to grass-clover swards and calving date on the productivity of pasture-based dairy production.

Fifty-four first-calf and multi-lactation Holstein-Friesian dairy cows were used in a one-factor study with three systems repeated over two years. The three systems were: early spring calving with annual fertilizer N input of 100 kg per ha applied in spring (ES100N; 2.1 cows per ha; grazing February to November), early spring calving without fertilizer N (ES0N; 1.6 cows per ha; grazing February to November) and late spring calving without fertilizer N (LS0N; 1.53 cows per ha; grazing April to January).

Annual pasture production was affected by an interaction between grazing system and year: Mean annual pasture yields for the two years were respectively ES100N; 10.35 and 9.88, ES0N; 8.88 and 8.63, LS0N; 9.18 and 10.31 ton of dry matter (DM) per ha. LS0N had higher pasture DM yield in the second year due to higher clover DM production and biological N fixation compared with the other systems. Clover stolon and root mass in the following February was correlated with stolon and root mass in the previous November with 64% of stolon mass present on LS0N in February (R2 = 0.84). There were no detectable differences in per-lactation milk yield (6,335 kg per cow), fat, protein and lactose yields (271, 226, 297 kg per cow, respectively), cow live weight (585 kg) or body condition score (3.02).

The zero-fertilizer N input systems had lower milk production per ha than ES100N because milk output was constrained by stocking rate. Pasture production on LS0N in the second year had the potential to carry a similar stocking rate of dairy cows to that of ES100N. Delaying the calving date until mid-April on LS0N resulted in inefficient use of pasture for milk production, whereas extending lactation into early winter lowered the quality of late-lactation milk for processing purposes. It is recommended that a mean calving date in mid-February under zero-fertilizer N input is a better option than a later calving date. It is possible that the beneficial effects of winter grazing on clover productivity could be achieved by grazing later and to a lower post-grazing height before housing for the winter. Such an approach could carry a higher stocking rate of dairy cows under zero-fertilizer N input; perhaps in the region of 2.0 to 2.5 cows per ha.

Comment the reader must remember that the study was done in the northern hemisphere and should adjust the months for the South African situation accordingly