Feed and manure use in low-N-input and high-N-input dairy cattle production systems

A four-year field trail was conducted on a sandy, siliceous, isohyperthermic Psammentic Paleustalf (West et al 1984) at the International Crops Research Institute for the semi-arid tropics (ICRISAT) Sahelian Centre (ISC); 13°15'N, 2°18'E in the Republic of Niger, West Africa. Average annual rainfall at ISC is approximately 560 mm of which 95% falls during the May–September cropping season. Experimental treatments included a factorial combination of cattle or sheep dung only applications, the equivalent of one, two or three nights of dung production with or without urine, every one, two or three years. There were six replicates per treatment arranged in a completely randomized block design. During each study year, dung was applied during the two- to three-month dry period just prior to the rainy season and the planting of millet. Cattle and sheep were herded together on natural pasture for 10 h to 14 h daily. At night, the herd was split with half the cattle or sheep put into portable corrals centered over field plots (dung plus urine) for the designated number of nights (figures 1(a) and (b)). The remainder of the herd spent their nights in nearby stalls, and dung was collected each morning and spread on the 'dung only' plots. More detailed information on experimental design and statistical analyses, and the management of cattle, sheep, manure and millet yields can be found in Powell et al (1998).

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A four-year field trial was conducted on a fine-silty, mixed, superactive, mesic Typic Agriudoll (USDA-NRCS 2004) at the research farm of US Dairy Forage Research Center in Wisconsin USA; 43^o19'N, 89^o44'W. Average annual rainfall is approximately 790 mm of which 75% falls during the April–October cropping season. Two manure application methods were evaluated, each at two application levels: (1) the conventional method of keeping dairy heifers in a barn and the manure (dung plus small amounts of urine) produced during two (B2) or four (B4) days was scraped from barn floors and hauled to fields; and (2) the corralling method involved putting heifers in fields for two (C2) or four (C4) days to apply all dung and urine directly on soil (figures 1(c) and (d)). Twenty manure application periods (months) occurred during four seasons over a two year period. There were three replicates per treatment arranged in a completely randomized block design for each of the four seasons. Each manure application season was followed by a three-year crop rotation. A rotation of wheat-sudangrass-winter rye-corn-winter rye-corn was grown on plots that received manure during the two spring-summer seasons, and a rotation of corn-winter rye-corn-winter rye-corn was grown on plots that received manure during the two fall-winter seasons. More detailed information on experimental design and statistical analyses, and the management of heifers, feed, manure and crop yields can be found in Powell and Russelle (2009).

For both the Niger and Wisconsin experiments, two methods were used to evaluate impacts of manure collection and land application methods on crop production. To evaluate urine effects, yields in the corralled plot (dung plus urine) were divided by yields in dung only plot (or dung plus some urine in Wisconsin). To evaluate overall manure treatment (dung, with or without urine) effects, yields in control plots (no manure applications) were subtracted from yields in each manure treatment plot.

In Niger, millet grain yields were determined in each plot each year. This was designated as the food component of total millet dry matter (DM) production. Millet grain production (kg ha⁻¹) was divided by 165.5 kg, the annual per capita millet grain consumption in Niger during the period of the experiment (FAO and ICRISAT 1996). The quotient was the number of additional adults per hectare per year that the increase in millet grain due to manure applications would feed. Grain was subtracted from total millet DM and the remaining millet stover (the millet biomass that remains after grain harvest) was divided into two components: (1) forage DM consisting of chaff, immature panicles, upper stover (leaves plus stalks) and tillers, and, (2) biomass designated for soil conservation consisting of middle and lower stover portions (Powell and Fussell 1993). The number of cattle feed days obtained from the additional millet forage production due to manure applications was calculated by dividing forage DM by a daily forage intake requirement of 6.5 kg cattle⁻¹ (one cattle having 320 kg live weight consuming 2.0% of bodyweight in the form of millet stover, Powell et al 2013).

In Wisconsin, only total above-ground crop DM production was recorded in each treatment plot over the three-year rotation that followed manure applications. All crop DM in the rotations was designated as forage that could be used to feed dairy heifers. The number of heifer feed days obtained from the additional forage DM due to manure applications was calculated by dividing total forage DM by an assumed daily DMI of 9.5 kg heifer⁻¹ (one heifer having 450 kg live weight and consuming 2.1% of bodyweight, Hoffman and Kester 2013).

Dairy cattle herd compositions in SSA and WENAO differ considerably (table 1). Of the total animal units (one AU = 1000 kg animal live weight), 56% in SSA and 79% in WENAO are adult or replacement female animals. In SSA, lactating cows are much smaller and become pregnant much later and less often than adult females in WENAO. The feed DMI and NI of cattle in SSA are 85% and 60% of feed DMI and NI of dairy cows in WENAO, yet median milk production in SSA is only 10% of median milk production in WENAO. The amount of NI secreted as milk N (NUE-milk) is also 5 to 6 times lower in SSA than WENAO. The recent global analysis (Powell et al 2013) revealed great regional differences in distribution of dairy cattle in classes of NUE-milk. For the present analysis, the partitioning of all lactating cows into NUE-milk classes of <10%, <20% and <25% for SSA is 92%, 98% and 100% and for WENAO is 0%, 3% and 69%, respectively. As NUE-milk within the range of 20% to 35% are possible on commercial dairy farms (Flachowsky 2002, Flachowsky and Lebzien 2006, Chase 2003), these results indicate that substantial improvements in NUE-milk seem possible for both regions, especially SSA.

Because of low milk production and low NUE-milk, Nex per unit of milk is 7–8 times greater in SSA than in WENAO. The high proportion of non-lactating cows, the great variability and very low levels of milk production and NUE-milk in SSA reflect the multiple uses of cattle, the least which may be commercial milk production. Although commercial milk production is important in many peri-urban areas, in the mostly rural areas of SSA cattle are kept for milk, meat and many other purposes, such as social status, wealth store, and manure which is used for fuel, construction and as a precious soil fertility amendment. There appears to be a great need to better differentiate between dairy, beef and multi-purpose cattle production systems when making global assessments of environmental impacts of milk production (Powell et al 2013).

The provision of additional feed N to dairy cattle (as part of N-energy balanced rations) would seemingly be used much more efficiently in SSA than in WENAO (figure 2). The slope (3.2 g milk N per 100 g of feed N for each increase of 1000 kg milk AU⁻¹) of the linear regression for lactating cows in SSA is four times greater than the slope (0.8 g milk N per 100 g of feed N for each increase of 1000 kg milk AU⁻¹) of the linear regression for the higher production cows in WENAO. The increases in NUE-milk associated with increases in milk production are also more predictable in SSA (R² = 0.98) compared to WENAO (R² = 0. 66). There are many interrelated factors however that affect relationships between NI, milk production and NUE-milk that are beyond the scope of the present analysis. For example, animal breed, reproductive performance and health, and perhaps most importantly the balancing of feed N with digestible energy, amino acids, and minerals in rations fed to dairy cattle have great impacts on milk production and NUE-milk (Chase 2003, Broderick 2003, Flachowsky and Lebzien 2006, Powell et al 2014).

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The recent life cycle assessment (LCA) of the global dairy herd revealed that knowledge of feed N intake and milk production allow for accurate calculations of NUE-milk, Nex and the relative amounts of Nex in dung and urine (Powell et al 2013). In Niger average annual milk production is 560 kg AU⁻¹, NUE-milk is 2.8%, and Nex is 223 g N per kg of milk. In the USA annual milk production is 11 900 kg AU⁻¹, NUE-milk is 25.0%, and the amount of Nex is 14 g N per kg of milk. How dairy cattle are managed impacts the amount of dung N and urinary N captured and recycled through cropping systems and environmental N loss.

In Niger, corralling cattle (or sheep) on cropland overnight between cropping periods to apply both dung and urine (figures 1(a) and (b)) increased millet total DM by averages of 25% to 95% compared to the practice of housing the livestock overnight and applying only collected dung onto fields (figure 3). Whereas for cattle the greatest percentage yield increase (95%) was associated with corralling every year, for sheep similar yield increases (60% to 80%) were obtained by corralling every year or every 2nd year.

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The positive effects of UN on millet may be attributed not only to additional N but also the ability of UN to increases in soil pH and plant availability of phosphorus (P) in these sandy, P deficient soils (Powell et al 1998). In Niger the residual effects of UN (as measured in the every 2nd and every 3rd year plots, figure 3) were much greater than expected. Residual N availability may have been due to numerous factors, such as a greater than anticipated immobilization of applied N by the soil microbial pool, and the sequestration of N in soil minerals, which became available during subsequent years.

In Wisconsin, the effects of corralling (figures 1(c) and (d)) on crop yields were much less dramatic than what was observed in Niger (figure 3). Corralling dairy heifers on cropland for 2 or 4 days increased total forage DM yield by only 8%–11%, compared to the practice of housing heifers and applying only the manure scrapped from barn floors. The principal reasons for these low responses were likely associated with (1) the collection and application of some urine in the 'barn manure' plots, (2) overall manure N applications were 4–5 times greater than agronomic N requirements, and (3) the high fertility of loam soils in Wisconsin compared to the sandy soils of Niger. Related to the second point, an objective of the Wisconsin study was to determine manure N availability to crops in outside cattle holding areas (figure 1(c)) where 120–850 kg ha⁻¹ of manure N is deposited annually and unmanaged (Powell et al 2005). Related to the third point, at the experimental site approximately 115 kg N ha⁻¹ is mineralized from this silt loam soil annually, which makes determination of manure N effects on crop yields and crop N uptake difficult to detect (Powell et al 2011a).

Overall results of the Niger and Wisconsin experiments demonstrate the importance of herd and manure management on UN capture, recycling Nex through crops, especially in low-N-input dairy production systems. Although direct measurements of N loss were not made, partial N balances revealed substantial N loss likely occurred in both environments. In Niger, it was estimated that approximately 30% to 50% of UN deposited in corrals was lost as ammonia (Powell et al 1998). In Wisconsin, 20% to 30% of UN was estimated to have been lost during manure collection and transport to fields. Although manure N recovery by crops (% of Nex) was 30% to 50% higher in areas where dairy heifers had been corralled compared to application of barn manure, from 50% to 70% of the Nex deposited in corral plots could not be accounted for indicating large N loss (Powell and Russelle 2009).

As mentioned previously, the provision of more feed N to dairy cattle in SSA would not only likely be used efficiently to produce more milk (figure 2) but would also increase Nex, which could increase overall agricultural production. For example in Niger, applications of cattle dung alone every 1, 2 or 3 years increased total millet DM by approximately 2000 kg ha⁻¹ (figure 4(a)). The additional grain yield per hectare due to applications of cattle dung only would satisfy the annual millet grain requirements of approximately 2–3 persons; the additional forage per hectare would provide approximately 120–140 more feeding days for a typical head of cattle; and the remaining 840 kg of biomass per hectare could be used for soil conservation (which may also have long-term positive effects on soil fertility, crop and milk production). Millet yields were similar across the cattle dung application intervals of every 1, 2 or 3 years (figure 4(a)) indicating that most of dung's positive impacts on millet yield occur during the growing season that immediately follows dung application.

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In Niger, the impacts of corralling cattle (dung plus urine) on millet yields are dramatically greater and longer lasting than the application of dung alone (figures 3 and 4(a)). The additional grain yield per hectare due to corralling cattle would satisfy the annual millet grain requirements of about 3–5 persons; the additional forage would provide about 200–300 days of feed for a typical head of cattle; and the remaining 1610 kg ha⁻¹ of biomass could be used for soil conservation. Highest food and forage yields would be obtained in fields where cattle are corralled every year, followed by every 2nd and 3rd year.

In Wisconsin, the application of 2–4 days of barn manure (dung plus some urine N) increased total forage DM by approximately 3600–4750 kg ha⁻¹ per year. This would provide an additional 350–470 days of feed for a typical dairy heifer. The application of dung plus urine by corralling heifers directly in fields for 2–4 days increased total forage DM by approximately 4600–5500 kg ha⁻¹ per year. This would provide an additional 480 to 580 days of feed for a typical dairy heifer.

Substantial increases in milk production seem possible in SSA if more feed N (as part of a N-energy balanced ration) is made available (figure 2). In Niger, an increase in annual milk production from current median of 560 kg AU⁻¹ to 1320 kg AU⁻¹ would require approximately 114 g of additional NI per day. This calculation assumes that each additional 1.0 g NI would produce 4.9 g of milk (calculated by dividing the additional 2.6 g of milk N per 100 g NI associated with the milk production increase of 560 kg AU⁻¹ (figure 2) by an assumed milk N concentration of 2.6 g kg⁻¹). The required additional 114 g NI per AU could come from cowpea (Vigna unguiculata) hay, a protein supplement fed to cattle and sheep in semi-arid West Africa. A nutrition trial conducted in Niger (Schlecht et al 1998) reported that a daily supplement of 1.4 to 1.8 kg of cowpea hay per cow increased dung DM output by approximately 30% and urinary volume output by 100% to 400%. Using the reported N concentration in cowpea hay of 30.4 g kg⁻¹, a somewhat similar 1.2 kg of cowpea hay per cow (320 kg live weight, table 1) would provide the additional 114 g NI required to increase annual milk production 560 kg AU⁻¹, and at the same time increase Nex (especially UN) which could be used to greatly increase millet food, forage and biomass production (figures 3 and 4(a)).

In many WENAO settings, reductions in NI of dairy cattle would not greatly impact milk production (figure 2), but would reduce UN excretion and N loss. In Wisconsin, reductions in ration crude protein (CP = N × 6.25) concentrations from current state-wide average level of 175 g kg⁻¹ DM to 160 g kg⁻¹ DM (a level deemed sufficient for healthy, high production dairy cows, Broderick 2003) would decrease NI by 100 g AU⁻¹ per day (without reductions in milk production). This strategy would decrease urea N excretion by approximately 25% and ammonia and nitrous oxide emissions from dairy manure by 18% to 30% (Powell et al 2014).

Although reductions in feed N would reduce UN excretion and N emission, this practice may also decrease the fertilizer N value of manure, crop yield and manure N use efficiency. For example, the application of dung from cows fed a diet containing high CP diet (184 g CP kg⁻¹ DM) provided higher levels of crop N uptake and yield than dung from cows fed CP adequate diet (151 g CP kg⁻¹ DM, Powell et al 2006). Overall plant-available N levels are also higher in soils amended with slurry from cows fed a higher CP diet (168 g kg⁻¹ DM) than in soils amended with slurry from cows fed a lower CP diet (155 g kg⁻¹ DM, Powell et al 2011b). Reductions in manure N availability to crops due to less NI could be compensated easily however with fertilizer N, which is inexpensive, more easily applied, and generally used more efficiently by plants than manure N.