The objectives of this study were to determine: 1) ammonia emission rates from stored scraped and flushed manure from dairy cows fed either normal or low N diet; and 2) seasonal effects on ammonia emission rates from stored scraped and flushed dairy manure. Four pilot-scale tanks were used for manure storage with different treatments - scraped manure for normal diet (NS), flushed manure for normal diet (NF), scraped manure for low N diet (LS), and flushed manure for low N diet (LF). The first part of the study lasted for 1 month and four treatments were all investigated; the second part of the study lasted for 12 months and two tanks with treatments NS and NF were investigated. Dynamic flux chambers and a photoacoustic gas analyzer were used to measure ammonia emission rates.

There was no significant change of the N content of manure as the dietary N content is reduced (from 17.8% to 15.9% crude protein). However, ammonia emission rates from manure storage tanks were reduced by 33% (from 27.4 ± 38.1 to 18.4 ± 21.9 mg m⁻²h⁻¹; P<0.0001 based on paired t-test). Flushing manure reduced emission rates by 72% compared to scraping manure (from 35.6 ± 39.6 to 10.1 ± 8.2 mg m⁻²h⁻¹; P<0.0001 based on paired t-test). Ammonia emission rates for NS, NF, LS and LF were 43.9 ± 48.0, 10.9 ± 8.7, 27.4 ± 27.3, and 9.3 ± 7.8 mg m-2 h-1, respectively. The chamber headspace temperature for NS, NF, LS and LF were 26.0 ± 6.9, 25.8 ± 6.8, 26.6 ± 6.5, and 27.2 ± 6.7 °C, respectively. The manure pH for NS, NF, LS, and LF were 6.3 ± 0.1, 6.4 ± 0.3, 6.4 ± 0.1, and 6.1 ± 0.1, respectively. Both dietary N reduction and manure flushing are recommended to reduce ammonia emission rates from dairy manure storage tanks.

Ammonia emission rates were higher in summer and fall, due to higher air temperature and higher manure pH. The pH of scraped manure was 7.2 ± 0.6, 6.7 ± 0.2, 6.5 ± 0.3 and 7.0 ± 0.3 for fall, winter, spring and summer, respectively. The pH of flushed manure was 6.8 ± 0.4, 6.7 ± 0.4, 6.4 ± 0.3 and 6.8 ± 0.4 for fall, winter, spring and summer, respectively. Ammonia emission rates from scraped manure for fall, winter, spring, and summer were 7.4 ± 8.6, -0.5 ± 1.2, 1.1 ± 1.9, and 5.8 ± 2.7 mg m⁻²h⁻¹, respectively. Ammonia emission rates from flushed manure for fall, winter, spring, and summer were 3.9 ± 4.2, -0.5 ± 0.9, 0.8 ± 1.4, and 4.4 ± 1.2 mg m⁻²h⁻¹, respectively. Seasonal changes of air temperature and manure pH were key factors affecting ammonia emissions from manure storage in this study. Seasonal climate conditions including precipitations (rainstorms and snows) and icing can cause reduction of ammonia emissions from manure storage in open air. More attention should be paid to reduce ammonia emissions in warmer seasons, e.g., by covering the storage facilities.