Daily intake of lactating crossbred cows grazing elephant grass rotationally

 The goal of this trial was to estimate the total dry matter (TDMI) and daily pasture dry matter intakes (PDMI) by lactating crossbred Holstein  Zebu cows grazing elephant grass (Pennisetum purpureum Schum.) paddocks submitted to different rest periods. Three groups of 24 cows were used during two years. The paddocks were grazed during three days at the stocking rate of 4.5 cows/ha. Treatments consisted of resting periods of 30 days without concentrate and resting periods of 30, 37.5 and 45 days with 2 kg/cow/day of 20.6% crude protein concentrate. From July to October, pasture was supplemented with chopped sugarcane plus 1% urea. Total daily dry matter intake was estimated using the extrusa in vitro dry matter digestibility and the fecal output with chromium oxide. Regardless of the treatment the estimated average TDMI was 2.7, 2.9 and 2.9±0.03% and the mean PDMI was 1.9, 2.1 and 2.1±0.03% of body weight in the first, second and third grazing day, respectively (P<0.05). Only during the summer pasture quality was the same whichever the grazing day. Sugarcane effectively replaced grazing pasture, mainly in the first day when pasture dry matter intake was lowest.


Introduction
Pennisetum purpureum Schum.known as Elephant grass or Napier grass is a tall erect perennial tropical grass, with thick stems up to 4.5 m high, found on moist soils in areas with over 1,000 mm of annual rainfall.Elephant grass grows best on deep soils of moderate to fairly heavy texture and tolerates short droughts, but does not withstand waterlogging.It yields very large quantities of dry matter, but is low in protein content unless cut very young.It should not be cut closer than 10-15 cm from the ground.Under normal management, stands are invaded by weeds and run out after two or three years so that they have to be ploughed up and replanted (Tropical Feeds, 1999).
Although traditionally used as chopped grass in Brazil, there is evidence of the great potential of elephant grass as pasture.Results of milk production with crossbred cows grazing elephant grass, at Embrapa-Centro Nacional de Pesquisa de Gado de Leite, in Brazil, between 10,000 and 15,000 kg/ha, during the 180 days of rainy season, were reported by Deresz (1994) and Coser et al. (1996).However, elephant grass shows a high stocking rate only during the rainy season, since forage production is drastically reduced in the dry months.In South-eastern Brazil, elephant grass dry matter availability decreases from over 3,500 kg/ha in November, to less than 500 kg/ha in July (Aroeira et al., 1999).One of the alternatives to minimize this problem is the supplementation with chopped sugarcane.
Sugarcane is readily available as forage for dairy cattle and is used as a strategic supplementation particularly during the dry season.Both the sugar and fiber contents of sugarcane are high, but protein and lipid contents are very low (Valadares Filho et al., 1990;Aroeira et al., 1993).Milk productions over 15,000 kg/ha/year were reached with cows grazing elephant grass supplemented with sugarcane plus urea mixture during the dry season (Deresz & Mozzer, 1990).
The objective of this trial was to estimate the daily composition of extrusa, dry matter and elephant grass dry matter intakes of lactating crossbred Holstein Zebu cows grazing paddocks submitted to different resting intervals.

Material and Methods
The trial was carried out from April 1992 to October 1993 at Embrapa-Centro Nacional de Pesquisa de Gado de Leite, in Minas Gerais State, Brazil, at approximately 22 o latitude South and 43 o longitude West, at an average altitude of 426 m.In this region there are two well-defined seasons.The dry season lasting from April/May to September/October characterized by average temperatures of 17ºC and scarce rainfall (60 mm per month).The rainy season lasts from October to March, with average temperature of 24ºC and monthly rainfall of about 230 mm.
As in all South hemisphere, in Brazil the autumn months are March, April and May, the winter months are June, July and August, the spring months are September, October and November and the summer months are December, January and February.
An area of 5.3 ha planted with elephant grass (Pennisetum purpureum Schum.) was used.The pasture was fertilized with 200 kg of N, 200 kg of K 2 O, 40 kg of P 2 O 5 and 1,000 kg of dolomite limestone per ha/year.Nitrogen and K fertilizers were applied in November, January and March.The limestone and phosphorous were hand distributed in November.
Three groups of 24 crossbred Holstein Zebu cows each were used.These had an average of 45 to 60 days in milk, in their second or third lactation, and averaged 483±5.3kg of live weight.The first group of cows calved in February or March and entered the experimental area in April and remained until October (1992).The second group calved in August or September and stayed in the experimental area from October to April 1993.The third group calved in February or March and remained in the experimental area from April to October (1993).These animals were blocked taking into account the milk yield and live weight data from the pre-experimental period.A total of six cows per treatment (three per replication) were kept at all times.The cows were milked twice a day, at 7 a.m. and 2 p.m. and had free access to a mineral mixture and water before milking.
The area planted with elephant grass was divided into paddocks, alternately grazed for three consecutive days, and allowed different resting periods, making up the four experimental treatments, replicated twice, as follows: resting period of 30 days without concentrate supplementation (30W) and resting periods of 30 (30C), 37.5 (37.5C) and 45 (45C) days with concentrate supplementation.The number of paddocks and their areas varied according to the treatment in such a way that stocking rate was always about 4.5 cows/ha.Thus, for a resting period of 30 days, there were 11 paddocks of 607 m 2 each.For a resting period of 37.5 days, 13 paddocks of 494 m 2 and one paddock of 247 m 2 (grazed for one day and half) and for the 45 day resting periods, 16 paddocks measuring 417 m 2 each.
From October to June elephant grass was the only roughage available to the cows.During the driest months (July to October) the animals grazed elephant grass during the night and were confined between milking, in separate corrals, according to treatment, where they received chopped sugarcane plus 1% urea (9 parts of urea and 1 part of ammonium-sulfate) ad libtum.
The daily intake measurements were taken 16 times during the entire experimental period.Intake was estimated in April, May (autumn), July, August (winter) September, November (spring) and December (summer) of 1992 and January, February (summer), March, April, May (autumn), July, August (winter), September, October (spring) of 1993.Two esophageal fistulated dry cows, adapted to the grazing conditions, without fasting, were used to estimate quality of the forage intake.Material collection (extrusa) was carried out on three consecutive days, while the experimental cows grazed the same paddock.The fistulated cows were maintained in extra paddocks, for at least two weeks before the first collection period, managed at the same way of the experimental area, to be adapted to the forage.The two cows were moved from paddock to paddock after the amount of forage sampled was achieved (1.5 kg/cow).The samples were collected in the morning, between 9 and 12 a.m., weighed and oven-dried at 65 o C for 72 hours, before being analyzed for in vitro dry matter digestibility (IVDMD), following the method proposed by Tilley & Terry (1963).
The DM was determined at 100 o C and the CP by the Kjeldahl method (Association, 1990).The NDF and ADF were analyzed following the recommendations of Soest et al. (1991).
Fecal production was estimated using chromic-oxide (Cr 2 O 3 ) as an external marker, at a rate of 10 g/animal/ day, administered orally, wrapped in paper, with aid of a speculum, during 12 days, in two doses of 5 g each, immediately before milking times.Feces were collected fresh twice a day directly from the concrete floor or by rectal grabbing, from the 6 th to 12 th day while chromium was administered.Daily samples from the fecal collection period were pooled for analysis on an individual animal basis.Each sample was dried at 65 o C, ground through 1 mm sieves and submitted to digestion by nitro-chloric acid, following the methodology proposed by Kimura & Miller (1952).After chemical digestion, chromium (Cr) was determined by atomic absorption spectrophotometry.Fecal production (FP) was then calculated using the formula: FP = Cr administered (g/day)/Cr in the fecal DM (g/kg).
In the treatment where the diet was exclusively elephant grass, the formula above was applied directly.However, when the animals received the concentrate supplementation or when sugarcane plus urea was administered, the intake of this supplements was daily measured and the IVDMD of each feed was determined.The fecal output was separated by feed components and their fecal contribution was subtracted from the total fecal output.Elephant grass or herbage DMI was obtained by difference.Daily DMI was estimated with the extrusa IVDMD sampled at the first, second and third day of the occupation period and the fecal production was obtained 24 hours after that.
The experiment was conducted in a split-split plot design.Cows and treatments were arranged as main plots.Years were arranged as subplots and the seasons as splitsub-plots.The statistical analysis was carried out using the General Linear Model Procedures (GLM) of Statistical Analyses System (SAS Institute, 1993).

Results and Discussion
In both years elephant grass dry matter availability was different (P<0.05),according to the season (Table 1).The DM availabilities increased from winter to summer in both years.However, as the dry season (winter and spring) was more severe in 1993, daily herbage allowance in that period was lower (P<0.05)than in 1992.
Means DM availabilities of 2,423, 1,939 and 1,745±92 kg/ha of DM were observed for resting periods of 45, 37.5 and 30 days, respectively.No difference was detected (P>0.05) in average daily herbage allowances for the various resting periods, due to the different sizes and number of paddocks.The mean herbage allowance observed during the experimental period was 11.3±0.45kg/cow/day of DM (Aroeira et al., 1999).
Similar elephant grass DM availability have already been described in Southeastern Brazil.In the same experimental area Soares (1998) reported 2,354 kg/ha of DM in March.Benedetti & Colmanetti (1997) recorded mean availability of 4,248 kg/ha of DM during the rainy season.On the other hand, Olivo et al. (1992) and Coser et al. (1996) observed annual averages from 2,000 to 2,500 kg/ha of DM, respectively.
Regardless of the treatment, season or experimental year, the highest CP mean values (P<0.05) were observed in the first grazing day (13.9%),followed by data obtained at the second (11.8%) and finally at the third grazing day (10.5%).
Mean cell wall content as measured as NDF was higher (P<0.05) in summer than all other seasons although there was no difference in IVDMD among them (Table 2).Elephant grass growth rates are higher in wet summers with most of the dry matter being cell wall (Passos, 1994).Other higher neutral detergent fiber contents in elephant grass during the rainy season were described in the literature (Anindo & Potter, 1994;Soares, 1998).
In general, the mean IVDMD obtained at the first day were similar among treatments (P>0.05), and higher (P<0.05),than that observed at the second day, except during the summer.The data recorded at the third day were the lowest ones.Regardless of the season of the year, average chemical composition of extrusa varied (P<0.05) according to the grazing day, as shown in Tables 2 and 3.It is suggested that warmer temperatures (Soest, 1994) associated with selective grazing, resulted in increased levels of NDF and ADF, simultaneously as CP and IVDMD decreased.
Selective grass dry matter intake was the main factor since the changes were observed from the first through the third grazing days.During summer time extrusa IVDMD was similar during the second in relation to the first grazing day, probably because of more abundant rainfall.Nonetheless, other factors may have contributed to the chemical composition of elephant grass such as the amounts of rainfall and fertilizer applied, as well as soil composition and sampling methodology.Soares (1998) reported for the same area CP of 14.1 and 16.5, 14.1 and 19.1, 12.1 and 13.5 and 15.2 and 15.3% from an extrusa sample collected, respectively, during winter, spring, summer and autumn, from pastures fertilized with 300 and 700 kg/ha/year of nitrogen.In the present trial, extrusa composition varied also according to the grazing day and the experimental year, the IVDMD recorded in 1992 (59.5, 59.2 and 55.5%) were higher (P<0.05)than that observed in 1993 (53.7, 52.2 and 50.6%).

Autumn
(1) Values with different letters differ significantly within line (upper case) or within column (lower case) at 5% of probability by Tukey test.
For DM intake calculation, feces produced 24 hours after the extrusa sampling were always used.For example, for the calculation related to the first day DMI, data from extrusa collected at the first grazing day in a paddock and fecal output of the third day grazed at the previous paddock were used.For the second day DMI, data used came from extrusa collected at that same day as well as fecal output from the first grazing day.The same procedure was adopted for the third grazing day.Data related to the fecal output collected 48 hours after the collected extrusa were not tested.
The average total dry matter intake as percentage of body weight (BW), independent of treatment, season or experimental year, differed (P<0.05)relatively to grazing day.Values of 2.86, 3.02 and 3.10±0.33% of the BW, respectively, were observed at the first, second and third grazing days (Table 4).
Pasture dry matter intake did not differ among treatments, except during the summer (Aroeira et al., 1999).However, the daily intake was different ac-cording to the season of the year.Pasture dry matter intake (Table 4) was highest during summer, lowest in winter and intermediate in autumn and spring.In general, PDMI increased significantly from the first to second and third grazing day, being the last two values similar.In addition, spring PDMI values were similar at the first and second days.
Total and pasture dry matter intake were higher during the summer probably due to more pasture availability and decreased during autumn due to the reduction of elephantgrass DM availability (Table 4).Total dry matter intake was higher again during winter due to sugarcane plus urea supplementation and remained high during spring when elephant grass resumed its growth (Table 1).
Considering the dry matter intake in relation to the three grazing days/paddock, the means TDMI and PDMI increased from the first to the third grazing day (P<0.05).These data differ from the literature.Higher forage intake should be expected in the first grazing day based on the pasture availability and forage quality (Table 2).However, the probable reasons for the results obtained in this trial, could be due to the following factors: a) higher forage NDF content observed at the third grazing day (Table 3); b) lower rumen passage rate due to inferior forage quality; c) higher rumen fill in the third grazing day.
Cows body weight varied with the experimental treatments (P<0.05).Supplemented cows from the experimental treatments mean live body weight of 484±5.3kg were heavier than unsupplemented cows (472±5.3kg).

Conclusions
1. Forage intake in the first day is lower than that grazed in the third day.
2. Only in summer pasture quality is similar among grazing days.
3. During the dry season, sugarcane effectively replaces grazing pasture dry matter intake.
Table 4.Total dry matter intake (TDMI) and pasture dry matter intake (PDMI), according to the season of the year and grazing day as percent of the body weight (% BW) (1) .

Table 1 .
Elephant grass dry matter (DM) availability in 1992 and 1993, according to the season(1).

( 1 )
Values with different letters differ significantly within line (upper case) or within column (lower case) at 5% of probability by Tukey test.
Values with different letters differ significantly within line (upper case) or within column (lower case) at 5% of probability by Tukey test.