Much of the forage produced in the subhumid areas of the USA is from cool-season species, which mature and become dormant in early summer. This results in minimal forage produced in mid- to late summer, a time referred to as a “summer slump.” ‘RedRiver’ crabgrass [Digitaria ciliaris (Retz) Koel.] is a productive warm-season annual that can fill this void, but optimum N fertilizer management for this species is unknown in much of the eastern plains of the USA. We tested N rates of 0, 50, 100, 150, and 200 lb N/acre from two sources, urea and ammonium nitrate, applied as single or split applications, on two cuttings during two years that contrasted in rainfall. A yield plateau was reached just above 100 lb N/acre both years, although yield was 2× higher in the moist year. However, forage fertilized with more than 100 lb N/acre generally had a higher N concentration than forage that received less N. Split application often resulted in less forage with a lower N concentration in the first cutting, but more in the second cutting than a single N application. Responses to N source were small, with application as ammonium nitrate yielding more than urea only in the second cutting at lower N rates.
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机译：在美国半湿润地区生产的大部分草料来自凉季物种，它们在初夏成熟并进入休眠状态。这导致在夏末至夏季，即所谓的“夏季不景气”期间产生的草料最少。 “红河蟹” [Digitaria ciliaris（Retz）Koel。]是一个有生产力的温暖季节，可以填补这一空白，但是在美国的许多东部平原上，对该物种的最佳氮肥管理尚不清楚。我们测试了两种来源（尿素和硝酸铵）的氮含量分别为0、50、100、150和200 lb N /英亩，这两种来源以单次施用或分次施用的形式，在两年内与降雨形成对比的情况下在两个插条上施用。尽管在潮湿的一年中产量要高出2倍，但这两个年份的产量都达到了略高于100 lb N /英亩的稳定水平。但是，施肥量大于100磅N /英亩的草料通常比接受较少N的草料具有更高的N浓度。分次施用通常会减少草料，N浓度较低的草料在第一次切割时会比第二次切割的草料多，而第二次切割的草料会比N含量高。单个N应用程序。对氮源的响应很小，因为硝酸铵仅在第二次切割时以较低的氮含量产生比尿素更高的产量。
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