Nitrogen and Italian ryegrass. 3. Growth up to 14 weeks: yields, proportions, digestibilities and nitrogen contents of crop fractions, and tiller populations

The regrowth of Italian ryegrass following a cut in late April was studied at weekly intervals up to 14 weeks. There were four levels of applied N: 28, 84, 140 and 196 kg ha^-1. The proportion of green leaf blade declined from 80% of DM yield after 2 or 3 weeks of regrowth to 5% after 13–14 weeks. About three-quarters of the increase in DM yield of total herbage from week 3 to week 11 was attributable to increased yield of ‘stem’. The level of applied N had little effect on the proportion of crop fractions except during the period immediately before and after the begiiming of ear emergence when the higher rates of N increased the proportion of ‘stem’ and reduced that of green leaf. The most digestible crop fractions were green leaf, inflorescence and immature ‘stem’. D-value of ‘stem’ declined by about 20 units from week 1 to week 14, while D-value of green and dead leaf declined at about half this rate and there was little or no decline in D-value of inflorescence. The N content of green leaf was consistently higher than that of the other fractions. The N content of ‘stem’declined markedly to around 0.5% by week 12. The N content of dead leaf and inflorescence was intermediate between that of green leaf and mature ‘stem’. The N content of all fractions was increased by applying more N. The tiller population increased during the flrst 3 weeks of regrowth and declined during the next 4. Applying more N increased both tiller population and weight per tiller.     

Cutting frequency and stubble height of reed canary grass (Phalaris arundinacea L.): influence on quality and quantity of biomass for biogas production

Abstract Reed canary grass (RCG) used for land treatment of waste water can serve as a substrate for biogas production. The aim of this experiment was to study the effects of two, three or four cuts per year to stubble heights of 5, 12·5 or 20 cm on the digestibility and yield of digestible organic matter (DOM) of RCG. Both dry‐matter yield and the yield of DOM decreased with more than two cuts per year. Height of cutting had no effect on the digestibility of the regrowth harvested from the different cutting regimes. Increased height of cutting resulted in increased dry‐matter production and therefore also of increased DOM yield in the regrowth harvested from the four‐cut regime. The total DOM yield from the four‐cut regime was, however, only 0·84 of the DOM yield from the two‐cut regime.     

The effects of forage preservation method and proportion of concentrate on digestion of cell wall carbohydrates and rumen digesta pool size in cattle

Six bulls with ruminal and duodenal cannulae were used in a 6 × 6 Latin square design with a 2 × 3 factorial arrangement of treatments. Barn-dried grass (G) and direct-cut silage (S) from the same sward were fed together with 250 (L), 500 (M) and 750 (H) g kg^-1 total dry matter (DM) of a barley-based concentrate (barley 875 and rape-seed meal 125 g kg^-1) at the level of 80 g DM kg^-1 live weight^0·75. Rumen and total digestibility of cell wall constituents were measured by a double marker and total collection method. Rumen pool sizes of dietary constituents were estimated by emptying the rumen. Particle-associated enzyme activities were measured from rumen particulate material and feed particles incubated in nylon bags in the rumen. Neutral detergent fibre (NDF) and hemicellulose digestibility were higher (P<0·05) for G diets than for S diets while no differences were observed between the forages in acid detergent fibre (ADF) and cellulose digestibility. Both rumen and total digestibility of cell wall constituents decreased with increasing level of concentrate. The proportion of total cell wall digestion in the rumen was unaffected by the forage preservation method and the proportion of concentrate. NDF, and especially cellulose digestibility, declined quadratically with increasing level of concentrate. Dietary effects on particle-associated carboxymethylcellulase and xylanase activity were consistent with those observed in cell wall digestion. There were no differences between the forages in rumen pool size of total ingesta or any dietary constituent. Rumen pool size of total ingesta decreased with increasing level of concentrate. On the other hand, DM content of ingesta increased with the level of concentrate, while the pool size of DM, NDF and ADF declined quadratically reaching a minimum on M level of concentrate. The differences in rumen NDF pool size were mainly in the digestible fraction. Calculation of digestion kinetic parameters showed that both the rate of passage and especially the rate of digestion were markedly depressed in animals fed on the highest level of concentrate. The results suggest that a reduced fate of digestion of NDF with high concentrate diets can be partially compensated for by an increased NDF retention time in the rumen of animals fed at a restricted level of feeding.     

Analysis of the nutritive value of timothy grown with varying N nutrition

Nitrogen fertilization affect growth and yield of timothy (Phleum Pratense L.), but there are conflicting report on its effect on nutritive value. Two experiments were conducted to describe changes in nutritive value of field-grown timothy fertilized with four rates of N fertilizer during spring and summer growth cycles, and to analyse relationships between parameters of nutritive value and the leaf to weight ratio (LWR). Early in the regrowth cycles when the harvestabe shoot biomass was approximately 200 g dry matter (DM) m^?2 and under non-limiting N and water conditions, the in vitro true digestibility (IVTD) of DM was greater in spring than in summer because the concentration of neutral-detergent fibre (NDF) was lower and its in vitro cell wall digestibility (IVCWD) was greater. However, the rates of decline in IVTD and IVCWD and the rate of increase in NDF were greater in spring than in summer. Nitrogen deficiency decresed IVTD and IVCWD, and increased NDF concentration early in the regrowth cycles. Nitrogen deficiency, however, reduced the rate of change in the three parameters of nutritive value. By the end of growth cycles, IVTD was 59 and 42 g kg^?1 DM greater when no N was applied than with non-limiting N conditions in spring and summer respectively. When no N was applied, the IVCWD was 76 and 52 g kg^?1 NDF greater, whereas the NDF concentration was 35 and 42 g kg^?1 DM less than under non-limiting N conditions in spring and summer respectively. The N deficiency increased the proportion of leaves in the biomass, which can explain for the most part the positve effect of the N deficiency on the nutritive value of timothy. This positive effect of the N deficiency on nutritive value though the proportion of leaves in the biomass, however, was reduced by the negative effect of N deficiency on the nutritive value of leaves or stems or both.     

The nutritional value of pressed potato vine silage

Potato (Solanum tuberoswn L.) vines were studied to determine if they could be successfully ensiled and fed to ruminant animals. Russet Burbank potato vine and alfalfa (Medicago sativa L.) silages were fed to goats (Capra hircus) to measure in vivo digestibility and to determine if potato glycoalkaloids were aversive or toxic. No differences in the feeding patterns between control and experimental animals were observed. Alfalfa silage was more digestible than the Russet Burbank potato vine silage due to a higher content of digestible cell wall materials. Digestibility differences between potato clones and harvest dates were found when in vivo digestibility tests were performed. W729R was the most digestible clone studied, especially for the early harvest dates. Most of the clones had vines that were more digestible, in vitro, than alfalfa. Vine glycoalkaloid contents of the experimental clones were similar to the values reported in the literature. There was no apparent correlation between vine glycoalkaloid content and percent digestible dry matter. A positive correlation of 0.38 between digestible dry matter and dry vine yield was found. Potato vine silage, produced after pressing the vines to remove water, was an acceptable feed for ruminant animals. Additional in vivo testing is necessary to determine if toxic effects due to glycoalkaloids or pesticides from long term feeding will be a problem.     

Influence of temperature on the digestibility of the tropical legume Macroptilium atropurpureum

The effect of temperature on the nutritive value of Macroptilium atropurpureum cv. Siratro and line 10–105 was determined in a controlled environment at day/night temperatures of 24/18, 30/24 and 36/30°C, and a 13-h daylength. Estimated dry matter digestibility, cell wall and lignin concentrations of leaf, stem and stubble were measured in two 5-week regrowth cuts.
Siratro and line 10–105 were similar in digestibility and responded similarly to temperature with whole tops, stem and stubble decreasing by an average of 0.0023, 0.0026 and 0.0056 units of digestibility for each 1°C rise in growth temperature. These changes reflect the increase in cell wall and lignin concentrations with higher temperature.
The digestibility of young, just-expanded leaves was increased by increased growth temperature. This effect may be explained by the faster rate of leaf expansion at high temperature which reduced cell wall and lignin concentrations at this stage of leaf development. However, for the bulk leaf fraction digestibility was unaffected by temperature. Leafiness declined at high temperatures.     

THE NUTRITIVE VALUE OF WEEPING LOVEGRASS DURING THE SPRING SEASON

A continuous digestibility trial, lasting 50 days with subperiods of 5 days each, was made with the spring regrowth of Eragrostis curvula grown in the semi-arid region of Argentina. Five sheep were used to measure the digestibility of DM, organic-matter, cell-wall contents and N. DM yield was estimated simultaneously. The fall in digestibility of all fractions examined was described by quadratic equations. Cell-wall digestibility explained 91% of the variation in organic-matter digestibility. The rate of decrease in crude-protein digestibility increased with time and was particularly noticeable after about 40 days of growth. Digestible DM yield increased at a decreasing rate as maturity advanced, but digestible crude-protein content decreased linearly throughout the trial. It is concluded that the optimum stage of maturity for weeping lovegrass utilization depends upon the performance required from the animals that are to utilize it.     

Age of regrowth as a factor affecting the nutritive value of hay of kikuyu grass (Pennisetum clandestinum) offered to lambs

The changes in chemical composition of hay of kikuyu grass ( Pennisetum clandestinum ) harvested at 50, 70 and 90 d of regrowth, and its effect on intake, digestibility, fermentation and microbial protein synthesis in the rumen (Experiment 1) and oxygen uptake by portal-drained viscera (PDV) (Experiment 2) were evaluated. The experiments were carried out with Polwarth × Texel crossbreed lambs with a mean live weight (s.e.m.) of 35(3·0) kg housed in metabolic cages. Organic matter (OM), neutral-detergent fibre (NDF) and nitrogen (N) intake, as well as rumen ammonia-N concentration, decreased linearly with age of regrowth ( P  <   0·05). Acid-detergent fibre (ADF) and indigestible NDF intakes were similar for all treatments. Apparent digestibility of organic matter (OM), NDF and N, as well as true digestibility of OM, microbial protein synthesis in the rumen, N retention, pH of rumen fluid and sugars, amino acids and peptide concentrations in rumen fluid were similar for all treatments. Age of regrowth also did not affect the kinetics of passage of the particulate phase through the digestive tract nor oxygen uptake by PDV. Heat production by PDV represented an average of 0·039 of digestible energy (DE) intake. Increasing the age of regrowth of kikuyu grass from 50 to 90 d did not affect digestibility nor the efficiency of DE use by PDV tissues of lambs but it reduced the nutritive value of the hay due to a lower intake of OM. Intake of hay appeared to be most limited by the ADF and indigestible NDF concentrations of the hay.     

STUDIES OF RESPONSE TO FERTILIZER NITROGEN

The effect of applying 62.8 or 125.6 kg/ha fertilizer N, 4 or 18 days before, 4 days after or immediately after harvesting primary growth at the hay stage on yield of DM, yield of digestible OM (DOM), uptake of N, % CP and % digestibility of OM (OMD) of the primary growth and regrowth from S24 perennial ryegrass were compared with harvesting the primary growth 4 or 18 days earlier. Pre–harvest application of N had no effect on yield of DM or DOM or on % OMD, but significantly increased uptake of N and % CP on a DM basis in the primary growth. The regrowth response was less than from equivalent amounts of N applied after harvest in proportion to the amount of N removed with primary growth. 20–40% N was taken up in the primary growth, even when applied 4 days before harvest. The proportion taken up was not influenced by the interval between application and harvest, but tbere was a marked increase in uptake wben rate of application rose to 125.6 kg N/ha. Harvesting 4 days earlier resulted in slight loss of bulk in the hay which was fully compensated by its extra quality, by improved regrowth and its more effective response to fertilizer N. The low yield of the primary growth harvested 18 days earlier was not compensated by the mass of the regrowth but it produced tbe highest quality primary growdi and top yield of regrowth.     

Initial evaluation of lupin for forage

Small-plot trials with Lupinus albus cv. Kievskij Mutant (sweet) and L. angustifolius cv. Kubesa (bitter) and Uniwhite (sweet) were sown in April 1977. Sequential whole-crop samples of the above ground dry matter (DM) were taken to assess the interaction of DM yield and forage quality during crop growth. Both species had similar DM yields, exceeding 11 t DM ha¹ in August. L. albus displayed superior whole-crop in vitro digestibility from late July, due to the large contribution to total DM yield from this time from its succulent, highly digestible pod shells. L. angustifolius showed higher N contents in June and early July, but N concentrations fell below those of L. albus from early August. The steady decline in digestibility of forage from L. angustifolius suggests this species should be harvested rather earlier than L. albus. Harvested in mid-August, L. albus should yield 11 t DM ha^-1, ata D-value of 64 and 23·5 g N per kg DM, whereas L. angustifolius harvested 10–14 d earlier should give the same DM yield, but at a D-value of 58 and 22·0 g N per kg DM. Lupins can thus be regarded as having good potential forage yields, but providing material of only moderate quality.     

Nutritional quality of semi-arid grassland in western Spain over a 10-year period: changes in chemical composition of grasses, legumes and forbs

From 1987 to 1996, the nutritional quality of the main botanical components (grasses, legumes and forbs) in semi-arid grasslands in the dehesa ecosystem in western Spain was analysed. Herbage samples were collected at the end of spring, in 30 locations, at two different topographic positions (upper and lower slope zones). Herbage mass over 2 cm and proportion of botanical components were estimated and samples were analysed for crude protein, neutral-detergent fibre (NDF), hemicellulose, cellulose, lignin and in vitro dry matter digestibility (DMD). Analysis of variance revealed a significant effect of sampling year on the herbage mass, proportion of botanical components and their nutritional quality. The three botanical groups, grasses, legumes and forbs, followed similar year-to-year trends in their crude protein, cellulose and lignin contents and in vitro DMD. Herbage mass was not significantly related to any meteorological variables, suggesting that interannual variation in biomass production of botanically complex pastures cannot be explained by a single factor. However, annual precipitation was significantly related to the proportion of the botanical group that was dominant at each slope zone: grasses in the lower zone and forbs in the upper zone. In the upper zone, spring precipitation explained part of the interannual variation in the NDF, cellulose, lignin contents and in vitro DMD of the botanical components.     

Production, survival and nutritive value of the perennial legumes Dorycnium hirsutum and D. rectum subjected to different cutting heights

Dorycnium hirsutum and D. rectum are perennial legumes which may have potential for use as pastures for the control of groundwater recharge in southern Australia. Little is known about the quality of the forage of Dorycnium species for grazing livestock or how these species respond to cutting. The effect of cutting height on plant survival, production of dry matter (DM), the proportion of leaf, edible stem (approximately <5 mm diameter) and woody stem in the DM and the nutritive value of the edible components was investigated. Biomass above five cutting‐height treatments (uncut, ground level, 5–8 cm, 10–15 cm and 15–30 cm above ground level) was removed at 8‐week intervals from plots of D. hirsutum and D. rectum from September 2002 to July 2003. In both species, plants subjected to lower cutting height treatments produced less DM above the height of the cut than those cut at higher heights. DM production declined over time in all treatments. Plants cut to ground level failed to regrow after the second harvest in D. hirsutum and the fourth harvest in D. rectum. Thus, these Dorycnium species were susceptible to high severity defoliations at 8‐week intervals. Negligible inedible woody stem was present in regrowth of both species after 8 weeks but D. hirsutum regrowth had a higher proportion of leaf (0·72) than D. rectum (0·56). Plants left uncut accumulated a large proportion of inedible woody stem in the DM (0·69 in both species) by July 2003, particularly at the base of the plant. Edible DM from regrowth of D. hirsutum and D. rectum had crude protein (CP) concentrations of 120 and 150 g kg^?1 DM; dry matter digestibility (DMD) values of 0·45 and 0·58; organic matter digestibility (OMD) values of 0·50 and 0·64; neutral‐detergent fibre (NDF) concentrations of 370 and 290 g kg^?1 DM; and acid‐detergent fibre (ADF) concentrations of 260 and 210 g kg^?1 DM, respectively. Medicago sativa, grown under similar conditions, had higher digestibility values (0·63 DMD and 0·66 OMD) and similar CP concentrations to D. rectum (140 g kg^?1 DM), but higher concentrations of NDF and ADF (410 and 290 g kg^?1 DM). Leaf material from both Dorycnium species had a higher nutritive value than edible stems, with DMD and OMD values of leaf of D. rectum being 0·68 and 0·74 respectively. Uncut plants had a much lower nutritive value of edible DM than the regrowth from cut treatments; older material was also of a lower nutritive value. The relatively low nutritive value of even the young regrowth of Dorycnium species suggests that forage quality is a major limitation to its use. Forage of Dorycnium species could be used during periods when other sources of forage are in short supply but infrequent grazing it is likely to produce forage of a low nutritive value.     

THE EFFECT OF ENSILING ON CROP DIGESTIBILITY

The digestibilities of 22 herbages, and of the silages made from them, were measured with sheep: in two experiments batches of the same herbage were ensiled by several different methods. In every case the digestibilities of herbage and of the corresponding silages were found to be very similar, and no significant effect of method of ensilage on digestibility was found. The importance of using the true dry-matter content of silages (i.e. corrected for losses of volatile constituents during drying) is emphasized; a depression in the digestibility of herbage during ensilage is likely to be estimated unless this correction is made. These experiments have shown that silage of high digestibility (>80%) can be made from highly digestible herbage; however the intakes, both of dry matter and of digestible dry matter, from such silages were found to be much lower than expected. If the potential for animal production of high digestibility in silage is to be exploited, intakes must be in-creased; two possible measures, viz. the wilting of herbage before ensilage, and the use of high-digestibility forages of high soluble carbohydrate contents, are discussed.     

The effect of physical form of a silage on its voluntary consumption and digestibility by sheep

Six castrated male sheep were used in a two-period crossover experiment to investigate the effect of mincing on the voluntary intake and digestibility of a silage. The silage was a high-quality material, containing 68 % digestihle organic matter in the dry matter, prepared from S24 perennial ryegrass cut with a precision chop forage harvester and ensiled with the addition of 2.3 1 t^?1 formic acid as preser vative. The median size of particles in the unprocessed silage was 10–20 mm and in the minced silage 2–3 mm. The intakes of dry matter, organic matter and nitrogen for the minced silage were greater than for the unminced material, and for dry matter and organic matter intakes the differences were signi ficant (P<0.01). Mincing was associated, however, with a depression in the digestibility of all three constituents which offset the increase in intake. The intakes of digestible organic matter were 817 g d^-1 for the minced silage and 729 g d^?1 for the unminced material (P>0.05). Corresponding figures for digestible nitrogen intakes were 208 gd^?1and23.0 gd^?1 (P>005). It is concluded that with high-quality silage diets there is a physical component in the mechanism for the control of voluntary food intake. It is suggested that for sheep, depressions in the digestibility of dietary constituents are unlikely to occur until silages are chopped to a median particle size of 5–10 mm.     

THE VOLUNTARY INTAKE OF DIPLOID (S22) AND TETRAPLOID (TETILA TETRONE) ITALIAN RYEGRASS, AND WHITE CLOVER BY SHEEP

Two experiments were conducted to measure the voluntary intake and digestibility of (a) primary growth and (b) regrowth of diploid (S22) and tetraploid (Tetila tetrone) varieties of Italian rye-grass (Lolium muitifiorum): Sl00 white clover (Trifolium repens) was included as an additional treatment in the second experiment with the re-growth herbage diets. Mature adult wether sheep, housed indoors in digesdhility crates, were fed individually on fresh herbage cut daily from late April to early June (primary growth) and in early Septemher (regrowth). The yield, digestihility, nitrogen and soluble–carhohydrate contents of the two ryegrass varieties were similar during the primary growth stage. Intake-digestibility relationships for the graces were positive, linear and different (P=0.05). The voluntary intake of diploid ryegrass was approximately 14% higher than that of tetraploid ryegrass of the same digestibility. With regrowth herbage the digestibility and voluntary intake of white clover was higher than both the Italian ryegrasses. The digestihility of the regrowth tetraploid was 6% units higher than the diploid variety, but there was no difference in the voluntary intake of digestible organic matter between the varieties at the regrowth stage. The intake of regrowth herbage was higher thaa that of primary growth herhage of similar digestihility.     

Effects of harvesting perennial ryegrass at different levels of herbage mass on voluntary intake and in vivo digestibility in sheep

The production performance of herbage‐fed animals is affected by herbage voluntary dry‐matter intake (VDMI) and organic matter digestibility. The objective of this study was to determine the effect of herbage mass (HM) on in vivo herbage voluntary intake and digestibility in sheep. The three HM treatments were as follows: 1,100 kg dry matter (DM)/ha (low, L), 2,300 kg DM/ha (medium, M) and 3,700 kg DM/ha (high, H). The study was a Latin square design, repeated on two occasions in 2012: 24 May to 20 July (summer) and 21 July to 5 October (autumn). Twelve Texel wether sheep (individually housed) were offered fresh cut perennial ryegrass herbage for ad libitum consumption. Using the total faecal collection method, the digestibility of dry matter, organic matter, neutral detergent fibre and acid detergent fibre was determined. In summer, L had a similar VDMI to M and both had a higher VDMI than H. In autumn, L had a higher VDMI than both M and H. For dry‐matter digestibility and organic matter digestibility, there was no significant difference between L and M, which were both higher than H. For every 1% increase in neutral detergent fibre digestibility, VDMI increased by 0.03 kg. In the zero‐grazing scenario examined, offering low (1,100 kg DM/ha) HM swards enabled animals to achieve high intakes of highly digestible herbage, which should ensure high animal production performance.     

Digestion in vitro of Italian and perennial ryegrasses, red clover, white clover and lucerne

Herbage at various stages of growth from three field experiments, one with Italian ryegrass, one with perennial ryegrass and one with white clover, red clover and lucerne, was incubated for 0, 24 or 48 h in rumen liquor followed by 48 h in acid-pepsin. The proportion of cell content OM in herbage (measured by pepsin) declined with advancing stage of growth more quickly in the legumes than in Italian ryegrass. At a high level of digestibility the legumes had a higher proportion of cell content OM than Italian ryegrass, but the position tended to be reversed at a low level of digestibility. Decline in proportion of cell content and decline in digestibility of cell wall were both major contributors to the decline in digestibility of Italian ryegrass with advancing maturity, whereas in white clover the decline in proportion of cell content was entirely responsible for the decline in digestibility. Lucerne and red clover occupied intermediate positions. At an early vegetative stage rate of digestion appeared to be higher in Italian than in perennial ryegrass. Rate of digestion was higher in the legumes than in the ryegrasses, the difference being largest at an advanced stage of growth.     

Variation among pregnant, non-lactating dairy cows in eating and ruminating behaviour, digestibility and voluntary intake of hay

The variation between non-lactating British Friesian cows in eating and ruminating pattern, digestibility and voluntary intake of hay were measured about 4 weeks before calving. Among fourteen of the cows the daily duration of eating ranged from 214 to 462 min and rumination from 410 to 599 min with CV of 20 and 14 respectively. Daily hay intake varied between cows from 5.93 to 11.18 kg dry matter (DM) with a CV of 14. The mean digestibility coefficient of organic matter measured in nineteen cows was 0.57 ± 0.014 s.d. and of cellulose 0.70 ± 0.013 s.d. Intake of hay and cellulose digestibility were correlated (r = 0.51, P <0.05). There was a positive correlation between hay intake and duration of eating (r=0.64, P <0.01) but not between intake of hay and rate of eating. Daily time spent ruminating per kg hay DM eaten was negatively correlated with hay intake (r = 0.66, P <0.01). It is concluded that variation between cows in chewing time and in digestibility of cellulose may be important factors related to variation between cows in voluntary intake of hay.     

Global environment change and simulated forage quality of wheat II. Water and nitrogen stress

Forage crops are frequently subjected to stress conditions resulting from inadequate supplies of water and N. Because forages grown under these stress conditions constitute an important resource in animal agriculture, this study was undertaken to assess possible changes in the nutritive value and productivity of forage crops as a consequence of global environment change. A relatively simple, mechanistic model of wheat was extended to simulate growth and important determinants of feed quality ([N], leaf:stem, dry matter digestibility) in an annual, temperate climate C₃ forage grass. Weather data for a semiarid region and different levels of applied N were used to examine the response of forage productivity to various levels of water and N availability. Not surprisingly, responses to global environment change were highly dependent on the availability of both water and N. When either resource was available at low levels, production of digestible dry matter was nearly unchanged by elevated [CO₂] or increased temperature. When compared at equivalent development stages, small increases in forage quality were simulated, mainly because higher temperature resulted in achievement of the initiation of grain fill at an earlier date. As N availability increased, differences in forage characteristics and productivity became more prominent. Elevated ambient [CO₂] increased vegetative mass, digestible dry matter, and concentration of digestible dry matter but decreased leaf:stem and [N]. Increased temperature generally had an effect on forage traits that was opposite to the elevated [CO₂] response. The combined effects of both factors sometimes cancelled each other, but usually one of the factors was dominant. Negative effects of temperature tended to be aggravated by dry conditions. At crop maturity, positive effects of elevated atmospheric [CO₂] on forage productivity and quality were severely decreased by nutrient and physiological constraints. These simulations indicate that when forage crops are grown under irrigation in semiarid regions, there may be substantial and complex changes in productivity and feed quality as a consequence of warmer temperature and elevated atmospheric [CO₂]. Under rainfed conditions, these differences could be quite erratic and virtually unpredictable within the current range of interannual variation in forage productivity and quality.