Feeding protein supplements with maize silage - when and what type?

Date: 18 May 2004 Author: Brad Granzin

The low protein content of maize silage

Late autumn through to early spring is a time of the year when many producers in NSW feed maize silage to milking dairy cows due to limited pasture availability. A disadvantage of maize silage is its low protein content. It typically contains 8–10% protein, and a cow normally requires 15–17% protein in her diet.

When more than 15 kg (i.e. more than 5 kg dry matter (DM)) of maize silage is fed per cow per day, then some form of protein supplementation is normally required. For example, at Wollongbar Agricultural Institute, an early-winter daily diet for a milking cow comprises:

  • 5 kg DM of maize silage;
  • 6 kg of a maize-based concentrate;
  • 1–2 kg DM of kikuyu;
  • 4–5 kg DM of a combination of either ryegrass, prairie grass or brassicas;
  • 1 kg of cottonseed meal to balance this diet for protein — this protein supplement normally provides both rumen-degradable protein (i.e. protein available to rumen (first stomach) microbes) and bypass protein (available to the cow).

The issues

When is the best time of the day to feed protein supplements?

Producers have commented that when they feed protein meals in the dairy twice a day at milking, as opposed to feeding a protein meal with maize silage on a feedpad, cows tend to get fat and produce less milk. These are symptoms of a protein deficiency, and this suggests that by the time the microbes in the rumen of a cow need protein (i.e. rumen-degradable protein) to help digest maize silage, it’s all gone!

This problem is made worse when a high-protein pasture such as ryegrass is fed at night. The protein from this pasture (which may be two-thirds of a cow’s daily rumen-degradable protein intake) is therefore available at night, but the cow really needs it during the day when she is eating maize silage.

These factors would indicate that feeding a protein supplement with maize silage could be more beneficial than feeding it at other times of the day.

The cost of protein supplements

Protein meals range in price from $250 to $500 per tonne, making them an expensive input. In feedlot-type diets, urea can successfully replace part of this protein, while still maintaining milk production, at about one-sixth the cost of protein provided by protein meals. However, urea is only of benefit to rumen microbes to help them digest feed (i.e. it is only rumen-degradable protein).

To date, there have been few positive results obtained when grazing cows are supplemented with urea. This is probably because the protein in pastures is readily available in the rumen. However, when we modelled what happens in a typical ‘maize silage / pasture’ diet (at Wollongbar Agricultural Institute as described earlier), we predicted that if urea is fed along with maize silage during the early afternoon, it would potentially supply protein to rumen microbes to help them digest maize silage.

Research at Wollongbar

To examine these issues, two studies were undertaken at Wollongbar. In both studies, all cows:

  • grazed ryegrass or prairie grass pastures at night;
  • grazed kikuyu pastures in the morning;
  • were fed 5 kg DM of maize silage on a feedpad between noon and the afternoon milking;
  • received 3 kg of a grain-based concentrate in the dairy at both the morning and afternoon milking.

Study 1

In study 1, cows were also fed 0.8 kg cottonseed meal to balance the diet for protein. There were three groups of cows, with 12 cows in each group. Each group was fed in one of three different ways:

  1. With grain:
    0.4 kg cottonseed meal per cow was fed with the grain-based concentrate at both the morning and afternoon milking (a total of 0.8 kg per cow per day).
  2. With maize silage:
    0.8 kg cottonseed meal per cow was fed with maize silage daily.
  3. With a grain-based concentrate:
    0.8 kg cottonseed meal per cow per day was fed with a grain-based concentrate at the afternoon milking. We included this treatment to see if feeding all of this protein meal at the nutritionally worst time of the day (i.e. a couple hours after the maize silage was fed) would have a negative effect.

The experiment lasted for 42 days.

Study 2

In study 2, there were four groups of cows, also with 12 cows in each group. Four different treatments were examined in this study:

  1. With grain:
    Cows were fed 0.55 kg of cottonseed meal with grain fed at both the morning and afternoon milking (equals 1.1 kg of cottonseed meal per cow per day).
  2. With maize silage:
    Cows were fed 1.1 kg of cottonseed meal with maize silage.
  3. Urea:
    Instead of feeding 1.1 kg of cottonseed meal, this group was fed urea (140 g per cow per day) with maize silage (140 g of urea contains the same amount of protein equivalents as 1.1 kg of cottonseed meal). The urea was mixed with 20 L of water and poured over the silage. All of the other groups also had water poured over their silage.
  4. Control:
    No extra protein was fed to the cows in this group.

This experiment lasted for 40 days.

The results

Milk production and liveweight changes recorded during Studies 1 and 2 are given in Tables 1 and 2 respectively:

Table 1. Daily milk production and liveweight change per cow over the experimental period in Study 1
Parameter 0.8 kg of cottonseed meal per cow fed:
With grain at both milkings With maize silage on feedpad With grain at afternoon milking
Yield
    Milk (L) 23.4 22.7 21.8
    4% FCM(a)(L) 22.8 21.7 20.7
    Fat (g) 895 841 804
    Protein (g) 712 695 666
Composition (%)
    Fat 3.89 3.79 3.74
    Protein 3.07 3.12 3.07
 
LWC(b)(kg) 0.43 0.44 0.52

(a) 4% fat-corrected milk
(b)
Liveweight change (kg) per day

Table 2. Daily milk production and liveweight change per cow over the experimental period in Study 2
ParameterTreatment
Cottonseed meal Urea (140 g/cow) fed with maize silage No extra protein fed
With grain in dairy at each milking (i.e. 2 × 0.55 kg/cow) With maize silage on feedpad (1.1 kg/cow)
Yield
   Milk (L) 24.7 22.9 22.1 22.8
   4% FCM(a)(L) 24.2 23.3 23.3 23.1
   Fat (g) 955 940 958 932
   Protein (g) 793 750 727 751
Composition (%)
   Fat 3.90 4.20 4.38 4.12
   Protein 3.24 3.30 3.31 3.30
   Lactose 5.09 5.09 5.05 5.03
 
LWC(b) (kg) 0.18 0.13 –0.57 –0.26

(a) 4% fat-corrected milk
(b)
Liveweight change (kg) per day

Analysis of the results

Feeding a protein meal with grain twice a day at milking versus Feeding the protein meal with maize silage during the day

In both Studies 1 and 2, feeding a daily allocation of cottonseed meal in two equal feeds with grain at the morning and afternoon milking (e.g. either 0.4 kg (Study 1) or 0.55 kg (Study 2) per cow per milking) resulted in higher milk production than did feeding the same total amount of cottonseed meal with maize silage (e.g. 0.8 kg (Study 1) or 1.1 kg (Study 2) per cow per day).

The differences in daily 4% fat-corrected milk yield between these two treatments were consistent across studies (1.1 L (Study 1) and 0.9 L (Study 2) per cow per day). Exactly why this happened is unclear. It appears that although we predicted that rumen microbes would benefit when we fed cottonseed meal at the same time as maize silage, this wasn’t the case, as rumen ammonia levels indicated that rumen-degradable protein was not limiting the digestion of maize silage. We think that more of the cottonseed meal, when fed with grain, escaped digestion by microbes in the rumen (the cow’s first stomach) and was therefore more available to the cow via her intestines. Protein digested in the intestine is a source of energy (e.g. for milk production and liveweight gain) as well as providing the building blocks for milk protein.

Feeding urea versus Feeding cottonseed meal

In Study 2, cows that were fed urea instead of cottonseed meal produced less milk and lost more liveweight (see Table 2). Part of this effect was caused by the fact that the cows in this group ate 0.5 kg DM less silage per cow per day than the cows in the other groups. Poor palatability of the silage was probably the reason for this.

Rumen ammonia concentrations taken during Study 2 suggest that adequate levels of rumen-degradable protein were available at the time when cows were offered maize silage. Therefore urea was of no benefit, and probably cost the cows in terms of the energy required to excrete it.

Conclusions

These studies indicate the following:

  • When cows are fed a ‘maize silage / pasture’ diet which requires protein supplementation, the best way to feed this protein is to include it in the grain at the dairy twice a day, as opposed to feeding it with silage in one meal or in grain as one meal after the silage is fed.
  • Where the diet contains at least 40% good quality pasture, there is probably little benefit to feeding urea, as the cow is already receiving enough rumen-degradable protein.