Influence of multinutrient blocks on intake and rumen fermentation of dry cows fed basal diets of Trachypogon sp and Cynodon plectostachyus hays

Influence of multinutrient blocks on intake and rumen fermentation of dry cows fed basal diets of Trachypogon sp and Cynodon plectostachyus hays

Facultad de Agronomía, Universidad Central de Venezuela, Apartado 4579, Maracay, Venezuela

Two 3 x 3 Latin squares, each with the same three rumen fistulated dry cows, were carried out to evaluate the influence of multinutrient blocks (MB) on hay intake and some parameters of rumen fermentation. Trachypogon sp hay of 2.25% CP and 35.5% dry matter disappearance at 48h (DMD) was offered in Experiment 1 and Cynodon plectostachyus (star grass) hay of 6.6% CP and 57.6% DMD was used in Experiment 2. The treatments evaluated were a control diet based on hay and a mineral mixture and two different blocks containing mainly molasses, a mineral mixture, urea and either whole cotton seed or fish meal. Block intake was about one kg per day in Experiment 1 and approximately half of this amount in Experiment 2. MB appreciably increased hay intake and rumen ammonia N in Experiment 1 (P<0.05) and did not influence these measurements in Experiment 2 (P>0.05). MB slightly increased DMD between 1 and 3 percentage units in both experiments and did not influence other parameters evaluated, such as rumen pH and molar proportions and concentrations of volatile fatty acids. The results are discussed in relation to grazing feeding trials carried out at this Institute where a small influence of MB on production parameters has been observed.

Multinutrient blocks (MB) have been used with success to improve live-weight (LW) gain and milk yield in cattle feeding systems based on cereal straws in Asian countries (Preston and Leng 1987; Habib et al 1991; Saadullah 1991). However, animal responses to MB supplementation obtained in tropical America with cattle grazing high allowances of medium quality pastures or low allowances of low quality native grasses are smaller (Combellas 1992). The main purpose of these supplements is to supply degradable nitrogen (N) to satisfy the requirements of rumen microorganisms, with the consequent increases in microbial protein synthesis, roughage utilization and intake. To explain the results obtained some measurements of rumen fermentation parameters are required. The objective of this study was to evaluate the influence of MB on the intake and some rumen digestion characteristics of dry cows consuming two hays of contrasting qualities: Trachypogon sp, a native grass of low digestibility and protein content, and Cynodon plectostachyus (star grass), a cultivated grass of better quality.

Two consecutive trials were carried out using the same animals, the first one (Experiment 1) with Trachypogon sp. hay as the basal diet and the second (Experiment 2) with star grass hay. The experiments were conducted during the dry season in Maracay which has a range in average monthly temperatures from 23.4 to 26.5 °C.

A Latin square 3 x 3 with 17 day periods was used in each trial, with three Brahman x Holstein rumen fistulated dry cows of 453 kg average LW. The supplementation treatments were: (C) a control without blocks and two types of blocks containing either fish meal (F) or whole cotton seeds (CS). All animals received a mineral mixture ad libitum. The Trachypogon hay was harvested at the beginning of the dry season from an area with mature grass located in savannahs of this species where a simultaneous feeding trial with adult females was being carried out (Mata and Combellas 1992). The star grass hay was harvested from an unfertilized plot at the flowering stage. The mineral mixture had 35% calcinated bone meal, 39% common salt, 25% superphosphate, 0.73% zinc sulphate, 0.29 copper sulphate and 0.04 cobalt carbonate. The blocks had 30% molasses, 20% of the mineral mixture described before, 10% urea, 10% lime, 5% chopped hay and 25% of either fish meal in Treatment F or whole cotton seeds in treatment CS.

The animals were housed in partially covered individual pens of 4 x 8 m with 4 x 3 m of concrete floors. The hays were offered daily between 07:00 and 08:00h and approximately 50% refusals were allowed. The mineral mixture and the blocks were offered in separated metal troughs suspended from the railings. Drinking water was always available.

The intake of hays and blocks was determined during the last seven days of each period. Samples of hays offered were taken daily and accumulated in plastic bags. Mixed samples per period of hays and blocks were dried at 65 °C for 48h, ground through a 1mm screen and analyzed for N and ash (AOAC 1984), acid detergent fibre (Goering and Van Soest 1970), calcium (Fick et al 1979) and phosphorus (Harris and Popat 1954). Blocks were also analyzed for ether extract (AOAC 1984).

Rumen dry matter disappearance of trachypogon in Experiment 1 and trachypogon and star grass in Experiment 2 were estimated using the nylon bag technique (Orskov et al 1980) collecting two samples of each hay at 6, 12, 24, 48 and 72h during the last three days of each period. The DM rate of digestion was calculated as the half time (T2) using the procedure described by Kempton (1980). Rumen liquid samples were taken just before feeding and at 1, 2, 3, 4, 6, 9 and 12h after feeding of day 15 in each period and filtered through cheese cloth. pH was measured and two samples of 30 ml acidified with 8 drops of sulphuric acid 97% and stored in a freezer. The samples were analyzed for volatile fatty acids (VFA) and ammonia N following the procedures described by FAO (1986).

The chemical composition of hays and blocks is shown in Table 1. The trachypogon hay had a very low content of crude protein, calcium and phosphorus, whilst the star grass hay was of better quality but with a crude protein content within the range where consumption could be limited by this nutrient (Minson 1981).

Table 1: Chemical composition (% of DM) of hays and multinutrient blocks (MB).

	Hays		MB
	Trachypogon	Star grass	CS	F

Dry matter	94.2	91.6	87.0	84.9
Crude protein	2.25	6.6	41.6	53.5
Ether extract	-	-	6.2	1.2
Acid detergent fibre	50.4	44.1	19.2	2.7
Calcium	0.11	0.46	10.1	10.9
Phosphorous	0.03	0.13	2.48	3.70

The results of Experiment 1 are shown in Table 2. The supplementation with blocks appreciably increased ammonia N (P<0.05), with higher values obtained from the blocks containing whole cotton seeds. The differences between treatments were maintained throughout the day, as may be observed in the ammonia N concentrations of some selected times in Table 2. DM disappearances in nylon bags were somewhat higher, by about three percentage units, in treatments with blocks (P<0.05). The other fermentation parameters evaluated, pH, VFA and rate of digestion, were not affected by treatments (P>0.05). DM block consumption was approximately 1 kg/d (Table 2) in both treatments and hay intake was appreciably increased with blocks (P<0.05). Hay and total consumptions were slightly higher in the blocks with cotton seeds.

The block consumptions in Experiment 2 (Table 3) were about half of those obtained in the first trial and no significant effects were observed in any measurement (P>0.05). However, ammonia N, DM disappearance and hay consumption were slightly increased with the blocks, but small differences were observed between the two types. The DM disappearances at 48h of star grass hay was about 20 percentage units higher than with trachypogon hay and the ammonia N concentration was also appreciably smaller with the latter in control diets without N supplements (Tables 2 and 3).

Table 2: Rumen fermentation indices of dry cows offered trachypogon hay with or without blocks containing either whole cotton seeds (CS) or fish meal (F).

Without			Blocks
Blocks			sem
	CS	F

pH 6.7	6.7	6.6	0.04

Ammonia N (mg/l)
Daily average 7	142	79	25.7*
0h 9	109	90
3h 5	127	67
12h 2	123	87

VFA (mmol/100ml)
Acetic 5.05	6.77	6.38	0.209*
Propionic 1.19	1.64	1.47	0.058
Butyric 0.39	0.59	0.53	0.018*

VFA (molar proportions)
Acetic 76.2	75.2	76.1	1.88
Propionic 17.9	18.2	17.5	0.39
Butyric 5.9	6.6	6.3	0.07

DM disappearance at 48h (%) 35.5	38.9	38.4	0.52

Rate of DM digestion (T2,h) 131	131	131	7.7

Intake (kg DM/day)
Hay 4.25	6.77	5.94	1.077*
Block -	1.19	0.99	-
Total 4.25	7.96	6.93	0.192**

Intake (kgDM/100kgLW)
Hay 0.91	1.45	1.26	0.041*
Total 0.91	1.74	1.51	0.045**

The main effect of blocks offered to animals consuming trachypogon hay was a large increase in total and hay intake associated with high increases in rumen ammonia N. However, the influence of these supplements on forage DM disappearance in the rumen was small and this factor does not seem to explain the large effects of blocks on intake.

Table 3: Rumen fermentation indices of dry cows offered star grass hay with or without blocks containing either whole cotton seeds (CS) or fish meal (F).

	Without	Blocks
	Blocks			sem
		CS	F

pH	6.7	6.7	6.6	0.04

Ammonia N (mg/l)	40.6	71.2	78.1	9.87

VFA (mmol/100ml)
Acetic	8.68	7.74	8.19	2.03
Propionic	2.04	1.68	1.82	0.47
Butyric	0.82	0.67	0.78	0.24

VFA (molar proportions)
Acetic	75.2	76.7	75.9	0.54
Propionic	17.7	16.7	16.9	0.33
Butyric	7.1	6.6	7.2	0.39

DM disappearance at 48h (%)
Trachypogon	35.1	37.9	36.4	1.35
Star grass	57.6	59.4	61.3	0.89

Rate of DM digestion (T2,h)
Trachypogon	110.0	115.5	110.0	3.18
Star grass	53.3	56.2	56.3	0.89
Intake (kg DM/day)
Hay	8.05	8.57	8.14	0.439
Block	-	0.45	0.64	-
Total	8.05	9.02	8.78	0.320

Intake (kg DM/100 kg LW)
Hay	1.70	1.77	1.69	0.051
Total	1.70	1.88	1.85	0.035

The positive effects of duodenal N infusions on the intake of low quality roughage by sheep was observed in early studies by Egan (1965) and Egan and Moir (1965). More recent experiences with cattle have shown large increases in intake with small variations in roughage digestibility when protein supplements were given to animals consuming low quality native grass hays in North America (McCollum and Galyean 1985; Fleck et al 1988; Del Curto et al 1990; Hannah et al 1991) or when urea was infused into the rumen of bovines consuming rice straw (Perdok et al 1988).

Relevant practical recommendations may be offered, based on the apparent effect of the blocks on intake as opposed to roughage utilization efficiency. Sufficient amounts of the fibrous feed must be available to optimize intake and animal responses. Unfortunately, the availability of herbage and other roughages during the dry season is the main constraint to cattle production systems in tropical areas, and the main effect of degradable N supplementation cannot be achieved in these situations. When the same blocks used in these trials were used in a grazing experiment on trachypogon savannahs with adult beef cows during the dry season, small LW gain responses were observed (Mata and Combellas 1982). The absence of effects could be a consequence of the low herbage availability in these conditions, below 1000 kg DM/ha, precluding the increases in forage consumption.

The largest intake in Experiment 1 was observed in the treatment with blocks containing cotton seeds compared with those containing fish meal and was more related to the ammonia N in the rumen than to the amount of protein by-passing the rumen. The total consumption of crude protein in the blocks was similar in both treatments because the smaller consumption of block F was balanced by its larger content of crude protein. The information available does not permit an explanation of the differences in hay intake, because the DM disappearance from the rumen was very similar in both treatments, and the true protein derived from the blocks reaching the duodenum must have been greater in treatment F with by-pass protein.

The influence of blocks on intake in Experiment 2 with a roughage of better quality was very small. The block consumption was about half of that found in the first trial and could in part explain the smaller effect on ammonia N and intake. Nevertheless, block consumption in the second trial was still sufficiently high for any potential effect on forage intake to have become apparent. The crude protein content of the hay was relatively low and the ammonia N concentration was only 41 mg/litre in the control group. An increment of this metabolite to almost twice this concentration with the block treatments did not result in the intake responses observed in Experiment 1 with the lower quality forage. The ammonia N concentrations with the two blocks of Experiment 2 were very similar in spite of differences in N degradability, and may be related to the greater intake (about 50%) of the block with fish meal of lower degradability.

Leng (1990) has pointed out, from results obtained in Australia, that about 200 mg/litre of rumen ammonia N are required to achieve optimum intakes of forages which are low in N and of poor digestibility. The results of Experiment 1 are in line with this evidence. But in the second trial with a forage about 20 digestibility units higher but with a medium to low content of N, the ammonia N concentration to optimize intake seems to be lower. Similar results were obtained with sheep by Alvarez et al (1984) using diets consisting of approximately equal parts of Pennisetum purpureum and NaOH-treated maize cobs, and where the minimum ammonia N required for maximum intake was between 32 and 90 mg/litre.

The small production responses to blocks observed at this Institute with growing cattle (Arredondo and Combellas 1992) and dual purpose cows (Rojas et al 1992) grazing cultivated grasses could be explained, as in Experiment 2, by the absence of block effects on forage intake and DM utilization. More recent results (Torrealba and Combellas 1992) have shown that the rumen ammonia N concentration of animals grazing star and guinea grasses is over 180 mg/litre. The high levels achieved are related to the selectivity of the diet by animals under grazing conditions, and no responses to degradable N supplements on intake or production parameters are to be expected in this situation.

From the results it may be concluded that significant effects of blocks are only to be expected when they are offered to animals consuming fibrous feeds of low digestibility and protein content. Their main influence is through an increase in roughage intake and sufficient availability of this component is therefore essential for the effects of the blocks to become fully manifest.

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