Supplementation of Dairy Cows Grazing to Low and High Post Grazing Pasture Height

Abstract

Energy supplementation of pasture fed dairy cows has the potential to increase milk production while increasing nitrogen use efficiency (NUE) thus reducing the negative environmental effects of dairy farming in New Zealand. Urinary nitrogen (N) has an environmental impact due to its contribution to nitrate leaching and nitrous oxide emissions from dairy farms. It should therefore be beneficial to the dairy industry to explore methods of maximising milk production without losing focus on the negative effects of urinary N. This trial was designed to investigate the effects of concentrate supplementation and compressed post grazing pasture height, and thereafter called PGPH, on milk production and N partitioning in pasture fed dairy cows in New Zealand.

The objectives of this research were to measure the milk production and N partitioning responses of supplemented and unsupplemented dairy cows grazing at two different PGPH. It was predicted that supplementation would increase milk production while diluting N intake per kg of dry matter intake (DMI) thus reducing urinary N output per kg of milksolids (MS) produced. It was also predicted that high PGPH would increase milk production while potentially causing some deterioration of pasture quality versus low PGPH.

A total of 32 Friesian x Jersey lactating, spring calving dairy cows were divided into groups of 8 cows and allocated to four treatments; (1) low PGPH (3.5 cm) plus concentrate (LR+); (2) low PGPH (LR); (3) high PGPH (4.5 cm) plus concentrate (HR+); (4) high PGPH (HR). PGPH was recorded using a rising plate meter (RPM). Concentrate was consumed at a average rate of 3.5 kg DM per cow per day for the full length of the trial. Stocking Rate (SR) was 4.9 and 4.4 cows/ha for supplemented and unsupplemented groups respectively. Groups were allocated to 17 and 19 paddocks for supplemented and unsupplemented groups respectively. Paddocks were all of equal area. Cows were blocked on age, days in milk (DIM), liveweight (LW), breeding worth (BW) and previous MS production and grazed plots for 13 weeks from the 15 August to the 15 November 2013. Milk, pasture and concentrate samples were collected weekly. Faeces and urine samples were collected monthly. These data were statistically analysed within each rotation (weeks: 1-5 first, 6-9 second, and 10-13third rotation) using the residual maximum likelihood procedure of GenStat (REML, GenStat 12.2 VSN International).

PGPH remained constant throughout the 13 week period at 3.7 and 4.5 cm respectively. Mean MS production in the first, second and third rotations were 1.97, 2.13 and 1.97 kg MS/cow/day respectively. Mean milk yield in the first, second and third rotations were 22.57, 24.04 and 22.32 kg milk/d respectively. Increasing pasture height from 3.7 to 4.5 cm did not affect pasture quality, MS production or milk yield.

Concentrate supplementation significantly increased average milk yield (23.56, 25.26, 24.04 kg milk/d versus 21.57, 22.83, 20.59 kg milk/d) and average MS production (2.04, 2.20, 2.12 kg MS/d versus 1.90, 2.07, 1.82 kg/d) in rotations 1, 2 and 3 respectively. Average milk response (MR) to supplementation was 140 g MS/kg of dry matter (DM) or 9.96 g MS/ mega joule of metabolisable energy (MJ ME) for the first 13 weeks of lactation. Average milk protein percentage was higher (3.87% versus 3.65%) and average milk urea nitrogen (MUN) was lower (7 mmol/l versus 7.92 mmol/l) in rotation 3 for supplemented than unsupplemented groups. Average total N intake per day over 13 weeks was higher for supplemented (500 g/d) than unsupplemented (406 g/d) groups in rotation 3. Faecal and urinary N concentrations were higher for supplemented (HR+: 3.50%, 0.58%; LR+: 3.11%, 0.55%) than unsupplemented treatments (HR: 2.84%, 0.31%; LR: 2.83%, 0.55%) in rotation 3 but there was no significant effect of the percentage of N excreted in urine and faeces as a percentage of total N intake. Average body condition score (BCS) gain was higher for supplemented (+ 0.29) than unsupplemented (+ 0.13) groups over the 13 week period.

The implications of this experiment are that a MS response to additional DMI and higher SR in a supplemented farm system averaged 140 g MS/kg DM.